Plausible Sources of Membrane-Forming Fatty Acids on the Early Earth: A Review of the Literature and an Estimation of Amounts (original) (raw)

Introduction

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Cells use bilayer membranes to separate themselves from their environment. In modern cells, these membranes are composed of phospholipids. During the origin of cells on Earth, more primitive membranes likely played a similar role, (1) sequestering cellular building blocks (2,3) and polymers. (4) The hydrocarbons in modern phospholipids are tails of fatty acids connected by an ester linkage to the glycerol backbone. Fatty acids themselves can assemble into membranes. Fatty acids were likely more abundant than phospholipids on the early Earth, leading to a common hypothesis that the membranes of the first cells were composed of fatty acids. (1)

Fatty acids consist of a hydrocarbon tail and a carboxylic acid headgroup (Figure 1). Saturated fatty acids with eight or more carbons in a linear chain can assemble into membranes. (5) Fatty acids with unsaturated (6) or branched (7) chains can also assemble into membranes, although in these cases it is unknown whether the minimum number of carbons for membrane assembly is greater or less than eight. Fatty acids with carboxylic acids at both ends of a carbon chain do not assemble into membranes on their own, although these dicarboxylic acids can incorporate into membranes when additional types of amphiphiles are present. (8)

Figure 1

Figure 1. Fatty acid assembly depends on the pH of the surrounding solution. When the pH is below the effective p_K_a of the fatty acids in a bilayer, fatty acids form an oil that is immiscible with the surrounding aqueous solution (bottom). When the pH is near the effective p_K_a, fatty acids assemble into bilayers in a membrane (middle). Vesicles, spherical shells of these membranes, may have served as the membrane compartments for the first cells on Earth. When the pH is above the p_K_a of the fatty acids in a bilayer, the fatty acids assemble into micelles, which cannot encapsulate aqueous solutes (top).

To form membranes, the solution pH must be within about half a unit of the effective p_K_a of the fatty acid in a bilayer (5) (where p_K_a = −log10 of the equilibrium constant, _K_a, for the dissociation of the fatty acid into a proton and the negatively charged amphiphile). If the solution pH is high enough such that the vast majority of headgroups are charged or if the fatty acids have fewer than eight carbons, then fatty acids assemble into nanoscale micelles that cannot encapsulate aqueous solutes (Figure 1). On the other hand, if the solution pH is low enough that the vast majority of headgroups are uncharged, then fatty acids separate into an oil phase. (9)

Fatty acid membranes provide some advantages for early cell replication compared to modern phospholipid membranes. (9) For example, fatty acid membranes are moderately permeable to salts and small organic molecules such as nucleotides, allowing internal replication of nucleic acids, which would have been critical for developing cells. (10) The surface area of these vesicles increases when they incorporate additional fatty acids from the environment into the membrane. (11,12) A growing vesicle can be supplied with fatty acids from micelles (11,13) or from other vesicles. (14) Vesicles that retain fatty acids grow while others shrink, which could have enabled competition between primitive cells for a limited supply of fatty acids. (12) After acquiring excess membrane surface area, primitive cells could have divided when exposed to modest shear stress. (13,15) Vesicles of phospholipids do not grow or divide as readily because aqueous solubility of a phospholipid with two hydrophobic tails is much lower than the solubility of a single-tailed fatty acid, so transfer through aqueous solution is slower. (16,17)

Here, we review how membrane-forming fatty acids could have been supplied on the early Earth. We identify three relatively well-characterized sources of abiotic fatty acids: delivery by meteorites, synthesis on the surface of metal catalysts, and synthesis by electrochemistry (Figure 2). There are also reports of fatty acid syntheses that fall outside these categories, but these have been less robustly investigated. We summarize the important details of the experiments that have been carried out and discuss whether similar reactions could have plausibly occurred on the early Earth. Finally, we estimate the total mass of fatty acids produced from each of the three relatively well-characterized sources during the Hadean eon when the first cells are hypothesized to have formed. (18)

Figure 2

Figure 2. There are three well-characterized sources that could have provided fatty acids to the early Earth. (A) Carbonaceous meteorites can deliver fatty acids. (19−22) (B) Metal surfaces can catalyze fatty acid synthesis. As one example, Nooner and Oro mixed filings of the Canyon Diablo meteorite (containing iron and nickel) with deuterium and carbon monoxide gases, and the mixture was heated to 400 °C to produce fatty acids. (23) Similar experiments have used pure Fe, Ni, or Fe- and Ni-containing minerals as catalysts and a variety of carbon and hydrogen sources to synthesize fatty acids (Table 1). (C) Fatty acids can also be synthesized during electrical sparking (Table 2). As one example, Yuen et al. used an electric discharge to synthesize fatty acids from methane. (24)

Carbonaceous Meteorites Deliver Fatty Acids to Earth

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Fatty acids have been detected in a variety of carbonaceous meteorites that have landed on Earth, (19−22) and molecules extracted from at least one such meteorite assemble into membranes. (25) At least 18 different carbonaceous meteorites have been analyzed, and both linear-chain and branched-chain fatty acids have been identified. (26) Depending on the type of carbonaceous meteorite, the abundance of membrane-forming fatty acids can range from 1 ppb to 100 ppm by mass. (26) Recent reviews provide detailed analyses of meteoritic fatty acids. (26,27) Importantly, it remains unclear which types of reactions are responsible for synthesizing meteoritic fatty acids in space. (26)

Could fatty acids delivered by carbonaceous meteorites have dissolved into water and assembled into membranes? Meteorites can fragment in airbursts during passage through the atmosphere, allowing some fatty acids to remain intact. (28,29) Meteorites with radii less than 100 m tend to fragment when differential pressure across the small body exceeds the material strength. (28) The Chelyabinsk ordinary chondrite meteorite (radius ∼ 10 m) that fell in Russia in 2013 fragmented at an altitude above 25 km, and fragments were spread into an area 250–300 km2 around the trajectory. (30) The largest fragment was ∼0.7 m mean diameter and fell into a lake. Fatty acids in carbonaceous chondrites would likely be similarly dispersed over a wide spatial area.

When meteorite fragments disperse into water on Earth’s surface, fatty acids can dissolve. Numerical models suggest that nucleobases leach out of 20 cm meteorite fragments and mix into the surrounding water within about three years. (29) At moderately alkaline pH, fatty acids can be even more soluble because of their charged headgroup. However, in order for fatty acids to assemble into membranes, the fatty acids must accumulate in solution above the critical vesicle concentration (∼15 mM for decanoic acid (3)). If fatty acids are present at concentrations below the critical vesicle concentration, membrane assembly does not occur.

Here, we use the measured abundance of decanoic acid (a 10-carbon fatty acid) in carbonaceous meteorites (26) to calculate the volume of a meteorite fragment that would be required to deliver enough decanoic acid into water so that the concentration of decanoic acid equals the critical vesicle concentration. Although it is not known precisely how meteorite size, initial velocity, or impact angle influences the fraction of fatty acids that survive impact, we note that a large portion of the meteorite’s initial mass (and thus a large portion of its fatty acids) may be destroyed by ablation and heating during travel through the atmosphere. (31−33) Given that our estimates rely on the average mass fraction of decanoic acid that has been recovered from natural carbonaceous meteorites and that we do not know a priori the mass, velocity, and impact angle of each meteorite, we assume the measured fatty acid abundances represent the average survival over the entire population of possible impacts. Although this assumption introduces potential errors, a more precise calculation is beyond the scope of this study.

The volume of a meteorite fragment (_V_meteor, expressed in m3) that would be required to deliver enough decanoic acid to reach the critical vesicle concentration as a function of water volume (_V_water, expressed in m3) is given by eq 1:

Vmeteor=CcvcVwaterwaρ≈61.5Vwater

(1)

where _C_cvc is the critical vesicle concentration for decanoic acid (15 mol/m3 of water, equivalent to 15 mM), w is the molar mass of decanoic acid (0.1723 kg/mol), a is the dimensionless fraction of the meteorite’s mass that is decanoic acid (2 × 10–5 for CM2 type meteorites), and ρ is the meteorite density (2100 kg/m3 for CM type meteorites).

The result of our estimate is shown in Figure 3. To deliver enough decanoic acid to form membranes, the volume of the meteorite fragment would have to exceed the volume of the waterbody. If a meteorite fragment were to land in a large enough waterbody to submerge the fragment, the decanoic acid that subsequently dissolved in the water would be too dilute to form membranes. However, this does not rule out membrane formation. The concentration of fatty acids could increase during dry periods, accompanied by a net loss of water due to evaporation. (34) Although we limited our calculation to decanoic acid because the critical vesicle concentration has been measured, a range of fatty acids from 8 to 12 carbons can be delivered simultaneously during a meteorite impact. The presence of these additional fatty acids could enable membrane formation at lower concentrations of decanoic acid. (35,36) We do not consider that CM type meteorites can contain significant water (up to ∼9% by mass (37)). If all meteoritic fatty acids were to somehow dissolve in only this meteoritic water, the concentration of decanoic acid could exceed the critical vesicle concentration; this can be interpreted as an upper limit for the concentration of fatty acids delivered by a meteorite.

Figure 3

Figure 3. A single fragment of a carbonaceous meteorite cannot directly deliver enough decanoic acid to a body of water to form membranes. To exceed the critical vesicle concentration (∼15 mM), (3) the volume of the meteorite (red line) would exceed the volume of the water. However, subsequent evaporation of the water could concentrate decanoic acid and enable membrane formation. A CM2 type meteorite is assumed because it contains the most decanoic acid on average (20 ppm by mass (26)). The density of CM-type meteorites is 2100 kg/m3. (38) Only meteorites with radii less than 100 m (∼106 m3 for a spherical meteorite) can fragment and impact the Earth’s surface with low enough energy to preserve fatty acids. (28,29) See eq 1 for details of the calculation.

To conclude our section on carbonaceous meteorites, we find that meteorite delivery was unlikely to directly yield high enough aqueous concentrations of fatty acids to form membranes on the early Earth. Additional processes would have been necessary to further concentrate fatty acids above the critical vesicle concentration, which we cannot rule out.

Catalysis of Fatty Acid Synthesis by Metal Surfaces

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The most commonly reported abiotic synthesis of fatty acids involves catalytic metal surfaces. Within this class of syntheses, Fischer–Tropsch reactions are the most thoroughly investigated. Fischer–Tropsch reactions occur when H2 and either CO or CO2 adsorb onto a metal surface. (39,40) Surfaces of solid iron or nickel are most commonly tested, although FeS, NiS, and Fe3O4 minerals have also been used to produce fatty acids. (41−44) In most experiments, metal surfaces must be heated above 150 °C to produce fatty acids. Catalysts contain metals in their reduced form; the synthesis of membrane-forming fatty acids (at least 8 carbons) has not been demonstrated on oxidized metal surfaces. (23,45−48) The synthesis of fatty acids seems to occur at the gas–solid interface (Figure 4). Even in experiments designed to eliminate gaseous headspace, reactions are suggested to proceed in gaseous bubbles within the aqueous solution. (48) Whether or not an explicit gaseous headspace is present, the carbon-containing precursors are generally supplied as gases. However, for experimental convenience in hydrothermal experiments, both formic acid and oxalic acid have been used as aqueous starting materials for fatty acid synthesis because both compounds decompose into H2 and CO2 at temperatures above 150 °C. (49)

Figure 4

Figure 4. Fatty acid synthesis occurs at the interfaces between reducing metal surfaces and a gaseous headspace. (A) Nooner and Oro showed that deuterium and carbon monoxide gases react together on the surface of hot (400 °C) meteorite filings to produce membrane-forming fatty acids. (23,50) When the meteorite filings were artificially oxidized, fatty acid synthesis was not observed. (23) (B) In hydrothermal experiments, McCollum et al. report that the synthesis of membrane-forming fatty acids occurs within gaseous bubbles adsorbed onto oxidation-resistant stainless steel surfaces. (49) When oxidized metal surfaces are present instead of stainless steel, only short-chain (<5 carbons) fatty acids are formed. (23,45−48) In these hydrothermal experiments, aqueous formic acid or oxalic acid is used for experimental convenience as a source of H2 and CO2.

Metal-catalyzed reactions generally create a diverse mixture of product types, including hydrocarbons and fatty alcohols in addition to fatty acids. (48,49,51−54) For each type of product, molecules with more carbons are less abundant. (40) Many experiments have produced only short-chain carboxylic acids containing fewer than the 8 carbons required for membrane assembly (5) (Figure 5). The carbon chain of a fatty acid could be elongated upon further reaction with a metal catalyst, (40) although additional experiments are required to validate this hypothesis. Table 1 summarizes experiments in the literature that have produced fatty acids using metal catalysts.

Figure 5

Figure 5. Length of fatty acids delivered by meteorites (labeled “Meteor.”) or produced in abiotic synthesis experiments. Fatty acids with at least eight carbons, indicated by the dashed red line, can assemble into membranes. All the fatty acids produced are saturated and unbranched (except in Scheidler et al. 2016, where experiments also produced unsaturated fatty acids). Note that detection of a fatty acid with a certain length may not have been attempted during every experiment.

Table 1. Summary of Experiments That Used Metal Surfaces to Catalyze the Synthesis of Fatty Acidsa

Metal-catalyzed reactions could have occurred on the early Earth after meteorite impacts, which delivered iron, nickel, and heat. Meteoritic metals could have been exposed to atmospheric H2, CO2, and CO, enabling surface-catalyzed synthesis of fatty acids. Extremely large impactors, about the size of the asteroid Vesta (∼1020 kg), could have transformed the Earth into a global Fischer–Tropsch reactor with surface temperatures >100 °C and high partial pressures of H2, CO2, and CO for thousands of years. (55) Catalytic metal surfaces would be required to produce fatty acids, so future research in this area will be especially valuable if it constrains the location and quantity of reduced metals after such impacts. (56) Although a large impact would have been catastrophic for any life that was already present on Earth, it could have potentially seeded Earth’s postimpact surface with fatty acids and other necessary biomolecules, enabling life to subsequently emerge. (57)

Ground-breaking experiments by Nooner and Oro modeled a postimpact scenario for fatty acid synthesis. (23,50) It remains uncertain how the yield of fatty acids depends on experimental parameters such as partial pressure and temperature. Kinetic models for the Fischer–Tropsch process have been developed in industrial settings, which generally do not mimic plausible early Earth conditions, and the quantitative form of the models depends on the design of the reactor. (58) Until experiments are conducted to understand how fatty acid production depends on reaction parameters more relevant to the early Earth, there will be substantial uncertainty in estimates of production of fatty acids by metal catalysts on the early Earth.

Another natural setting in which the metal catalyzed synthesis of fatty acids might occur is in a hydrothermal environment, where the conversion of CO2 and H2 into fatty acids is thermodynamically favorable. (59) Fatty acids have been detected in natural hydrothermal systems; however, it is unclear what fraction of those fatty acids were produced by modern cells. (60) Ultramafic rocks (relatively Fe- and Mg-rich and Si-poor) are common at some deep sea hydrothermal vents. (61) Could metal-rich minerals within these rocks serve as catalysts for fatty acid synthesis? To address this question, we consider the oxidation state of the mineral surface. Ultramafic mineral surfaces become oxidized by reacting with seawater and generating H2 in serpentinization reactions. (62) As noted above, to date, there have been no reports of the synthesis of membrane-forming fatty acids (at least 8 carbons) on oxidized metal surfaces. (23,45−48) Laboratory experiments failed to produce fatty acids with more than 2 carbons when oxidized olivine (ultramafic mineral with general composition (Fe,Mg)SiO4) was the sole catalyst. (45) In these experiments, the olivine was heated in water for 96 days to allow ample time for oxidation by serpentinization, and formic acid was included as an additional source of H2. (45) It is unknown whether olivine surfaces could catalyze fatty acid synthesis before becoming oxidized during serpentinization.

Hydrothermal experiments with oxidation-resistant stainless steel surfaces, clearly not natural settings, do produce fatty acids from formic acid with up to 22 carbons. (49) However, these experiments also permitted vapor-phase reactions, which may have enabled production of longer fatty acids regardless of the oxidation state of the surface. Thus, in natural hydrothermal settings, it remains unclear if catalytically active, reduced mineral surfaces could persist or be replenished quickly enough to catalyze fatty acid synthesis. Without a suitable catalyst, fatty acid synthesis would likely be slow or nonexistent in hydrothermal environments. (62) In conclusion, additional experiments are necessary to determine whether fatty acids could be synthesized in natural hydrothermal settings.

Electrochemical Synthesis of Fatty Acids

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Electrochemical reactions can generate diverse organic compounds, including amino acids, (68,69) nucleobases, (70) and fatty acids. For example, spark discharges between an electrode in the gaseous headspace and another electrode either in the same headspace (71) or in solution (24,72) can produce fatty acids. In the experiments summarized in Table 2, CH4 in the gaseous headspace served as the source of carbon in three of the four experiments generating linear fatty acids of varying lengths with only one experiment reporting chains long enough (at least 8 carbons) to form membranes. (71) An additional electric discharge experiment produced carbon chains of 2–6 carbons with carboxylic acids on both ends. (73)

Table 2. Summary of Electrochemical Experiments That Have Synthesized Fatty Acidsa

The goal of most electrochemical experiments is to simulate lightning strikes through a methane-rich atmosphere on the early Earth. However, attributes of natural lightning strikes are challenging to reproduce in the laboratory. Criado-Reyes et al. used a Tesla coil that generated a 3 × 104 V potential for 7 days at room temperature. (71) In contrast, a natural lightning strike generates a potential of about 108 V for <1 s, (74) and temperatures of the air surrounding a lightning strike can reach 30 000 °C. (75) Unfortunately, experimental evidence is lacking to describe how fatty acid yields depend on the voltage, duration, or total energy dissipated by laboratory sparking. Chyba and Sagan assumed that electrochemical production of organic molecules on early Earth should depend on the total amount of electrical energy dissipated by lightning strikes and coronal discharges. (76) Additional experiments are needed to validate this assumption.

Only slightly more is known about the role of the atmosphere and solid surfaces during electrochemical synthesis. Experiments by Schlesinger and Miller have shown that the yield of amino acids during sparking increases with the partial pressure of CH4; (77) similar experiments with fatty acids are still needed. Borosilicate glass as a substrate has also been shown to increase the yields of electrochemical fatty acid synthesis. (71) Although borosilicate does not occur naturally, silicates would have been ubiquitous on the early Earth because they are common rock-forming minerals. Additional experiments are necessary to better understand how the yields of electrochemical fatty acid production depend on the gaseous, solid, aqueous, and electrical environments. Nevertheless, the available experimental data suggest that fatty acid synthesis is possible under certain electrochemical conditions.

Alternative Types of Fatty Acid Synthesis

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In addition to the three main sources reviewed above, there are references in the literature to four alternative types of fatty acid synthesis (Figure 6, Table 3). These include photochemical reactions, (54,79) irradiation by massive particles, (80,81) gas-phase ion–molecule reactions, (82) and redox reactions in aqueous solution. (83−85)

Figure 6

Figure 6. Alternative reaction types that have demonstrated fatty acid synthesis. (A) Groth and Weyssenhoff used UV photochemistry to convert ethane into fatty acids. (79) (B) Kaiser et al. irradiated an ultracold (10 K) mixture of ∼99% CH4 and ∼1% O2 with 9 MeV alpha particles to produce fatty acids. (80) (C) Blagojevic et al. report reactions between gas-phase ions (CH2+ and C2H4+) and CO to produce fatty acids. (82) (D) Novotný et al. report the decomposition of monosaccharides into fatty acids under mild alkaline conditions (50 mM NaOH). (84)

Table 3. Summary of Experiments That Have Synthesized Fatty Acids by the Mechanisms Illustrated in Figure 6a

There have been two reports of fatty acid synthesis by UV photochemistry. By irradiating a gaseous mixture of ethane and ammonia above boiling water (Figure 6A) at 185 and 254 nm, Groth and Weyssenhoff generated fatty acids 1–5 carbons long. (79) Bonfio et al. used a multistep procedure to synthesize longer-chain fatty acids capable of membrane assembly. (54) UV irradiation at 254 nm was used in one step, and catalysis by a nickel surface was used in a subsequent step. Photochemistry could provide a plausible explanation for the presence of fatty acids on the surface of the early Earth because UV radiation down to ∼200 nm could have penetrated prebiotic atmospheres. (86) The experiments by Groth and Weyssenhoff did not generate fatty acids when methane was used instead of ethane; (79) further investigation is needed to determine whether radiation from realistic early Earth solar spectra could have enabled photochemical conversion of methane into fatty acids. In contrast, Bonfio et al. used starting materials that are more plausibly prebiotic such as formaldehyde, HCN, NaH2PO4, Na2CO3, and NaSH, (54) but these reagents impose a constraint on the geochemical scenario for the early Earth. In a separate experiment by Dworkin et al., membrane-forming amphiphiles were synthesized by irradiating ultracold ices composed of 100:50:1:1 H2O:CH3OH:NH3:CO with 121.6 and ∼160 nm UV photons, although the identity of these amphiphiles was not determined. (87)

The second type of “alternative” fatty acid synthesis is irradiation by particles. Experiments of this type were designed to simulate chemistry in the outer solar system, rather than the early Earth. By irradiating an ultracold (10 K) mixture of ∼99% CH4 and ∼1% O2 with 9 MeV alpha particles, Kaiser et al. produced linear fatty acids with 13, 15, or 17 carbons (80) (Figure 6B). In a subsequent experiment by Kim and Kaiser, an ultracold (10 K) mixture of CO2 and hydrocarbons (1–6 carbons) was irradiated with 5 keV electrons. The presence of carboxylic acids was confirmed by Fourier-transform infrared spectroscopy, but specific fatty acids were not identified. (81)

A third alternative fatty acid synthesis is discussed by Blagojevic et al. (82) In their experiments, reactions between gas-phase ions (CH2+ and C2H4+) and molecules (CO) resulted in carboxylic acids with 2 or 3 carbons (Figure 6C). Additional gas-phase ion–molecule reactions to produce formic acid (e.g., CH4 + O2+ → HCOOH2+ + H) have been suggested but not validated experimentally. (88) These reactions were suggested to occur in the interstellar medium, and the relevance to early Earth conditions has not been explored.

The fourth group of lesser studied fatty acid syntheses are redox reactions. Three sets of redox reactions have been shown to produce fatty acids in solution. The first experiments from Sato et al. use hydrogen peroxide to oxidize fatty aldehydes (7–8 carbons) to produce fatty acids with the same carbon-chain length. (85) It is unlikely that sufficient hydrogen peroxide would have been present on the early Earth. (89) In the second set of experiments, Novotný et al. report the decomposition of monosaccharides (3–6 carbons) into linear carboxylic acids (1–3 carbons) under mild alkaline conditions (50 mM NaOH, Figure 6D). (84) Diverse monosaccharides are obtained in low yield during the formose reaction, (90) and alkaline lakes on early Earth might have been sites for decomposition into short-chain fatty acids. (91,92) Finally, Lowe et al. reported production of formic acid and potentially other carboxylic acids by heating a mixture of ammonia and hydrogen cyanide to 90 °C. (83) Hydrogen cyanide is considered a prebiotic reagent that can be sequestered and concentrated as ferrocyanide within early Earth environments. (91,93)

There are numerous additional pathways for the synthesis of short-chain (1–3 carbons) linear fatty acids, (94) but because membrane assembly requires fatty acids with at least 8 carbons, we have generally omitted them. We are unaware of any abiotic mechanisms by which carbon chains of existing fatty acids are elongated. Modern cells synthesize fatty acids from acetyl-CoA using the sophisticated enzyme complex fatty acid synthetase. (95) Because the intermediate compounds that are produced during this process are unstable, it is believed that fatty acid synthesis via these reactions would have occurred at a negligible rate on the early Earth before the emergence of enzymes. (96) Finally, thermodynamic calculations suggest that fatty acids can be synthesized from polyaromatic hydrocarbons, (97) but to our knowledge, this synthesis has never been demonstrated experimentally.

Critical Analysis of Analytical Techniques

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Many of the reactions discussed above produce a wide variety of fatty acids and other products. Uniquely identifying products can be challenging, and determining the concentration of each product is even more difficult. In Table 4, we summarize the reported concentrations of fatty acids from each experiment and comment on potential limitations of the analyses. In general, the papers in Table 4 convincingly identify fatty acids of various lengths but do not provide substantial evidence for the fatty acid concentrations that they report. Moreover, many papers report only relative concentrations of fatty acids (e.g., X% of all products by mass or moles) instead of absolute concentrations (e.g., X moles or X grams), so it is difficult to compare product yields between experiment types. An ideal strategy for the characterization and absolute quantitation of fatty acids was employed by Yuen et al., which involved chromatography and tandem mass spectrometry with isotope-labeled internal standards to eliminate matrix effects. (24) Additional synthesis experiments that determine the absolute concentration of fatty acids would be valuable.

Table 4. Summary of Analytical Techniques in Each Reporta

Estimating the Contribution of Each Source to the Early Earth Fatty Acid Inventory

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To gauge the relative importance of each fatty acid source for the origin of cells, we go beyond purely reviewing the literature with a goal of estimating how much fatty acid could be supplied to the early Earth from three sources: delivery by carbonaceous chondrites, catalysis by metal surfaces, and electrochemistry. Many relevant parameters for these estimates lack experimental constraints, so assumptions are needed. Our first assumption is that cells formed during the latter part of the Hadean eon; the full eon was ∼4.6 to 4.0 billion years ago (Gya). (18) During the early Hadean, Earth was potentially hit by impactors that were large enough to sterilize the planet’s surface and reset the fatty acid inventory, so life must have originated after the last such event. (98) The median age of estimates for the last ocean-vaporizing impact is ∼4.3 Gya, (99) and we assume that life had originated by 4.0 Gya. (18) In the subsections below, we construct back-of-the-envelope estimates for the total mass of fatty acids supplied to Earth during this interval. Our estimates are based on empirical data from the literature, and we indicate when existing models from the literature are applied. We articulate our assumptions and suggest experiments that will help to refine these estimates. Our estimates cannot distinguish fatty acids that form membranes (more than 8 carbons) from those that do not (less than 8 carbons) because the empirical data that one of our estimates relies on does not do so.

Meteorite Delivery

We estimate the total mass of 2–12 carbon fatty acids delivered to Earth by carbonaceous meteorites, _M_s, using the following equation:

In eq 2, f L is the dimensionless fraction of a meteorite’s mass that comprises fatty acids of length L, where the length denotes the number of carbons in the fatty acid chain. Values for f L depend on the type of carbonaceous meteorite, and average values can range from 10–9 for 10-carbon fatty acids to 10–3 for 2-carbon fatty acids. (26) Parameter M is the time-integrated total mass of carbonaceous meteorites with radii of 1–100 m that would impact Earth from 4.3 Gya to 4.0. To estimate M, we integrate the following equation for the mass flux (kg/year) at time t (years) in the past from 4.3 to 4.0 Gya:

m˙=8.9C(1+1.6×10−10et/τ)(mmax0.46−mmin0.46)q

(3)

This equation is adapted from Chyba and Sagan. (100) In eq 3, ṁ is the mass flux (mass/year) of the total mass of carbonaceous meteorites with mass from _m_min to _m_max that would impact Earth per year; C is the frequency of carbonaceous meteorites relative to all types of meteorites (∼4%) observed in the meteorite fall record; (101) τ is the decay constant of 144 million years for the impactor population; _m_min is taken as the mass of a meteorite with a 1 m radius; _m_max is taken as the mass of a meteorite with a 100 m radius; q is 1 kg0.54/year. We assume that all meteorites are spherical with a uniform density (2.1 g/cm3 for CM-type meteorites (38) or 3.2 g/cm3 for C2-type meteorites (102)), so we can calculate the mass of a meteorite in kilograms from its radius in meters. Meteorites with radii less than 100 m can fragment into pieces that impact the Earth’s surface with low enough energy that fatty acids are preserved. (28,29) Using eq 3, we estimate that ∼1015 kg of carbonaceous meteorites with radii between 1 and 100 m would impact Earth from 4.3 to 4.0 Gya.

Assuming that all carbonaceous meteorites are either CM1-type or C2-type, we use eqs 2 and 3 to calculate bounds for the total mass of fatty acids with a length of 2–12 carbons that could have been delivered to Earth from 4.3 to 4.0 Gya. We find that 1010–1013 kg of fatty acids with a length of 2–12 carbons could have been delivered to Earth from 4.3 to 4.0 Gya by carbonaceous meteorites. Our estimate varies over 3 orders of magnitude because the abundance of fatty acids on each type of meteorite (f L) varies considerably between different meteorite types. We consider only carbonaceous chondrites because they are the most well-characterized type of meteorite that can deliver fatty acids. We do not quantify additional uncertainties in the meteorite flux, nor do we quantify the influence of meteorite size, impact velocity, and impact angle on the fraction of fatty acids that survive impact. (103)

Catalysis by Metal Surfaces after Vesta-Sized Impact

Next, we consider fatty acid synthesis on metal surfaces in the wake of a large Vesta-sized (∼1020 kg) asteroid impact on the early Earth. Such an impact would deliver iron to Earth’s surface, which could both act as a catalyst for fatty acid synthesis and generate a H2-rich atmosphere by reactions with steam from the vaporized ocean, (57) as in eq 4:

An H2-rich atmosphere appears to be required for fatty acid synthesis by metal catalysts (Table 1). Such a massive asteroid impact would generate a high enough surface temperature to destroy most organic molecules on Earth; then, as the Earth cooled, the synthesis of fatty acids could occur in the H2-rich atmosphere. We consider only the last impactor that would reset Earth’s fatty acid inventory, which most likely hit Earth between 4.4 and 4.1 Gya. (99) Significantly smaller meteorite impacts would not generate high partial pressures of H2 in the atmosphere because H2 escapes to space on time scales of ∼106–107 years. Additionally, small impactors would not generate global surface temperatures above 200 °C, which we assume is the minimum temperature required for fatty acid synthesis. Therefore, smaller impactors would not produce many fatty acids, and we do not consider their contribution here.

To estimate the total mass of fatty acids synthesized by metal catalysts after such an impact, _M_c, we assume that the rate of fatty acid synthesis depends linearly on gas pressures according to the following equation:

Mc=∫t400t200kcmcatPH2(tc)PCO(tc)dtc

(5)

Here, _k_c is an empirical rate constant for fatty acid synthesis, calculated from the results of experiments at 400 °C by Nooner and Oro. (23) They provide the only measurement to date of the absolute concentration of fatty acids produced on a metal catalyst in the absence of an aqueous phase. The value of _k_c is 7.4 × 10–6 kilograms of 6–18 carbon fatty acids (excluding 12-carbon fatty acids, for which data are not available) per bar of H2, per bar of CO, per hour of reaction time, per kilogram of catalytic surface available. (23) Only the summed mass of all fatty acids is reported by Nooner and Oro; (23) there is no information given about the number of moles of individual fatty acids. Here, _m_cat is the mass of available metal catalyst, in kilograms. _P_H2 and _P_CO are the atmospheric partial pressures of hydrogen and carbon monoxide, respectively, in bar; both are functions of time, _t_c. The product is integrated over time from _t_400 to _t_200. _t_400 is the number of hours after the impact until the surface temperature reaches 400 °C, and _t_200 is the number of hours after the impact until the surface temperature cools to 200 °C. We assume that fatty acid synthesis occurs at a constant rate _k_c when the temperature is within this range and that the reaction does not occur when the temperature is outside this range.

We assume that 33% of the asteroid’s mass is iron and that 7% of this iron (i.e., 2.3% of the total asteroid mass) remains available on Earth’s surface to catalyze fatty acid synthesis (giving _m_cat = 2.3 × 1018 kg) based on a linear extrapolation of the impact simulation performed by Citron and Stewart. (56) The assumption of ∼33% iron mass follows Zahnle et al., (55) which is the total iron in high iron enstatite (EH-type) meteorites (104) and also the fraction of Earth’s mass in its iron core. (105) Bodies with enstatite composition are candidates for impactors that hit the Earth after the Moon-forming impact and Earth’s core formation, although the contribution of carbonaceous versus enstatite compositions is debated. (106−108) In EH enstatites, which are highly reduced, most of the iron is metallic with a smaller fraction of iron sulfide. (109) In a postimpact vapor plume, the iron is vaporized into atoms, which condensation sequences show condenses to form metallic iron with subsequent cooling. (110)

We assume that another fraction of the asteroid’s iron (between 1% and 90%) is used to generate H2 from excess H2O according to eq 4. We adapted a thermochemical model previously developed by Zahnle et al. to compute _P_H2, _P_CO, and temperature as a function of time after the impact. (55) Depending on the fraction of the asteroid’s iron that reacts with water to produce H2 gas, we find that _P_H2 ranges from 10–3 to 10–1 bar and _P_CO ranges from 10–3 to 10–7 bar. We calculate that the Earth’s postimpact temperature would be between 200 and 400 °C for ∼1000 years, and we assume that fatty acid synthesis occurs at a constant rate during this time. Equation 5 estimates that between 1011 and 1015 kg of fatty acids with a length 6–18 of carbons (excluding 12 carbons, for which data are not available) would be synthesized using metal catalysts in the wake of a Vesta-size impactor.

Our estimate for the total mass of fatty acids synthesized by metal catalysts is uncertain because many factors in our analysis are poorly constrained. First, it is not known whether reduced metals would remain exposed to the atmosphere on Earth’s surface after an extremely massive impact. (56) Additionally, it is unclear how a H2O steam atmosphere would influence the rate of fatty acid synthesis; steam can affect the yield of Fischer–Tropsch reactions in different ways, depending on the type of catalyst and the type of reaction product. (111) Even without steam in the atmosphere, it is uncertain whether the rate of fatty acid synthesis depends linearly on gas partial pressures. Kinetic models for the Fischer–Tropsch process have been developed in industrial settings, which generally do not mimic plausible early Earth conditions; functions for each partial pressure depend on the engineered design of the catalyst. (58) Additionally, the effect of temperature on the final product distribution also appears to depend on the catalyst design. (51,112) Separate experiments indicate that the yield of fatty acids depends linearly on the amount of catalyst surface area that is available. (64) In general, further experiments are necessary to constrain these reaction parameters and the precise functional form for fatty acid production in plausible conditions on the early Earth.

Electrochemical Synthesis

We also construct an estimate for the total mass of fatty acids that could be synthesized by electrochemistry from 4.3 to 4.0 Gya,

where R is an empirical rate constant for the synthesis of 2–7 carbon fatty acids, calculated from electrochemical experiments by Yuen et al. (24) The value of R is 1.1 × 10–17 kilograms of 2–7 carbon fatty acids per bar of CH4, per joule of electrical energy dissipated by sparking, per hour of reaction time. (24) Although there are no electrochemical experiments that provide absolute concentrations for membrane-forming (longer than 8 carbons) fatty acids, Yuen et al. (24) provide absolute concentrations for the largest number of fatty acids. _P_CH4 is the partial pressure of methane on the early Earth, which we estimate to be between 10–15 to 10–1 bar after the last Vesta-sized (∼1020 kg) asteroid impact. This value depends on the initial preimpact abundance of atmospheric CO2, the fraction of the impactor’s iron that becomes oxidized (1–100%), and the importance of methane-forming catalysts, which may reduce the quench temperature of methane, thereby increasing its abundance. (55)B is the amount of electrical energy dissipated by lightning and corona discharges on the Hadean Earth during a year. We assume that B is 1.5 × 1018 joules per year, which is the electrical energy dissipated per year on the modern Earth. (76)_t_E is the reaction time in years. We assume _t_E is 105 years, which is an estimate for the lifetime of a methane-rich atmosphere after a Vesta-sized (∼1020 kg) asteroid impact. (55)

By applying these assumptions, we calculate that 10–4 to 1010 kg of fatty acids with a length of 2–7 carbons could be synthesized on Earth from 4.3 to 4.0 Gya by electrochemical reactions. Although separate experiments have shown that electrochemical production of membrane forming fatty acids with more than 8 carbons is possible, (71) membrane-forming fatty acids were not detected in the experiments by Yuen et al., (24) so our estimate is not directly informative about membrane formation on early Earth. More data are needed about the absolute concentration of membrane-forming fatty acids produced during electrochemical experiments.

There are uncertainties in our electrochemistry estimate. First, there is insufficient information available to estimate the total energy dissipated by the Tesla coil during the experiments by Yuen et al. (24) If we assume that their Tesla coil used 30 000 V potential (71) and 15 A, we can estimate that ∼4 × 1010 joules of energy were dissipated during their 24 h experiment. Even if we had perfect information about the Tesla coil voltage and current, it might not be straightforward to estimate the amount of energy that is usable for chemical synthesis. (113) Furthermore, we assumed that the yield of fatty acids depends only linearly on the amount of available electrical energy. (76) Further experiments are necessary to validate this assumption. Experiments by Schlesinger and Miller suggest that the yield of amino acids during sparking does increase linearly with increasing CH4 partial pressure (below ∼ 0.06 bar), (77) but further experiments are needed to validate this assumption for fatty acids.

Summary of Estimates

We have compared the total mass of fatty acids produced by 3 different sources during the Hadean eon, following the last extremely massive impactor that would have reset Earth’s fatty acid inventory. We estimate that 1011 to 1015 kg of 6–18 carbon fatty acids could have been synthesized by metal catalysts derived from the massive impactor. The total mass of fatty acids that could have been delivered by carbonaceous meteorites is 1010 to 1013 kg of 2–12 carbon fatty acids. The yield of 2–7 carbon fatty acids from electrochemical processes is potentially smaller, between 10–4 and 1010 kg. Consequently, an integrated supply of fatty acids to the Earth’s surface from all sources (dominated by metal surface production) between ∼10–4 and ∼100 kg/m2 is possible, given the Earth’s surface area of 5.1 × 1014 m2.

Ultimately, the local concentration of fatty acids determines whether or not membranes form, so the possible sources should be evaluated by this criterion. Although meteorites could have delivered a significant mass of fatty acids across the Earth’s surface, the aqueous concentration of fatty acids in a single waterbody would not have been high enough to form membranes without evaporating a significant volume of water (Figure 3). In contrast, a local stockpile of fatty acids could have been produced on atmosphere-exposed metal surfaces after an extremely massive impact, and subsequent dissolution into water could have allowed membrane formation. Although little is known about the electrochemical synthesis rate for membrane-forming fatty acids, repeated lightning strikes into the same small waterbody seem unlikely, so it is unclear whether a high enough local concentration of membrane-forming fatty acids could have formed via electrochemistry. Fatty acids in aqueous solution can be degraded via photochemistry, (114) so fatty acids that are slowly synthesized by electrochemistry may not have attained high enough concentrations to form membranes, whereas a large stockpile of fatty acids dissolving off metal surfaces may have been less sensitive to photochemical degradation.

Although the estimates above have many uncertainties, they are valuable as a first attempt to quantitatively compare fatty acid sources on the early Earth. We hope that future experiments can further constrain these estimates.

Alternative Amphiphiles

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In addition to fatty acids, alternative types of amphiphiles may have been synthesized on the early Earth, (115−117) and these amphiphiles might have incorporated into the membranes of the earliest cells. For example, alcohols are commonly produced along with fatty acids in many experiments, (48,49,51−54) and long-chain fatty alcohols are known to stabilize fatty acid membranes. (5,118) An excess of long-chain fatty alcohols form oil droplets, and the oil may disrupt membranes. In addition, phase-separated coacervates could have served as another type of prebiotic compartment, (119,120) and fatty acid membranes may have even assembled around such coacervate compartments. (121)

Conclusions

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Fatty acids can assemble into membranes and could have formed the boundaries for the first cells. Our Review highlights multiple potential sources of fatty acids on the early Earth. The three most well-characterized sources are meteorite delivery, synthesis on metal surfaces, and synthesis by electrochemistry. Other reactions involving photochemistry, irradiation by massive particles, ion–molecule reactions, and diverse redox reactions in aqueous solution may have also produced fatty acids in natural environments. To refine quantitative estimates for the relative importance of each fatty acid source, more detailed constraints are needed. We highlight the following questions to help any future experiments have the widest possible impact:

• How do the yields of fatty acids from metal-catalyzed reactions depend on the temperature and the partial pressures of gases such as H2, CO2, and H2O? Data would help to constrain estimates for fatty acid production following ocean-vaporizing impacts.
• Can membrane-forming fatty acids (>8 carbons) form in hydrothermal experiments with nonoxidized, ultramafic minerals as the sole catalyst? Data would elucidate the potential for fatty acid production at hydrothermal vents.
• How do the yields of fatty acids depend on the voltage, duration, or total energy dissipated during electrical sparking? Data would help to constrain estimates for fatty acid production during lightning strikes.
• What are the absolute concentrations of fatty acids in these types of experiments?

In summary, our analysis suggests that fatty acids could have been available on the early Earth. We have not assessed whether those fatty acids would have been sufficiently concentrated to assemble into membranes except in the limited case of small meteorite fragments delivered into aqueous environments. For fatty acids supplied via alternative sources, further data are required to assess the potential for membrane formation. By investigating possible sources of fatty acids on the early Earth, we hope to constrain the environmental setting for the origin of cells.

Author Information

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• • David C. Catling - Department of Earth and Space Sciences and Astrobiology Program, , and , University of Washington, Seattle, Washington 98195, United States; https://orcid.org/0000-0001-5646-120X; Email: [email protected]
• • Zoe R. Todd - Department of Earth and Space Sciences, , and , University of Washington, Seattle, Washington 98195, United States; https://orcid.org/0000-0001-6116-4285
  • Nicholas Wogan - Department of Earth and Space Sciences and Astrobiology Program, , and , University of Washington, Seattle, Washington 98195, United States
  • Roy A. Black - Department of Chemistry, , and , University of Washington, Seattle, Washington 98195, United States; https://orcid.org/0000-0002-3914-5747
  • Sarah L. Keller - Department of Chemistry and Astrobiology Program, , and , University of Washington, Seattle, Washington 98195, United States; https://orcid.org/0000-0001-5919-0185
• Z.R.C. conducted the literature review. Z.R.C., Z.R.T., and N.W. calculated fatty acid abundances. Z.R.C., Z.R.T., N.W., R.A.B, S.L.K, and D.C.C. wrote the paper.
• The authors declare no competing financial interest.

Acknowledgments

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We thank Milomir Suvira for helpful discussions of electrochemical sparking, Martin Sadilek for helpful discussions of analytical methods, Kevin Zahnle for helpful comments on all portions of the manuscript, and Ben K. D. Pearce for sharing raw data on meteoritic fatty acid abundances that he and his colleagues compiled. (26) This work was supported in part by grant NNX17AK86G (Exobiology) from NASA to S.L.K., by a grant (MCB 1925731) from the NSF to S.L.K., and by a grant (511570FY20, DCC) from the Simons Foundation to D.C.C. Z.R.C. was funded by an NSF fellowship (NSF GRFP DGE 1762114).

Abbreviations

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    Biophysical Journal (2004),87 (2),988-998CODEN:BIOJAU; ISSN:0006-3495. (Biophysical Society)
    Membrane vesicles composed of fatty acids can be made to grow and divide under lab. conditions, and thus provide a model system relevant to the emergence of cellular life. Fatty acid vesicles grow spontaneously when alk. micelles are added to buffered vesicles. To investigate the mechanism of this process, we used stopped-flow kinetics to analyze the diln. of non-exchanging FRET probes incorporated into preformed vesicles during growth. Oleate vesicle growth occurs in two phases (fast and slow), indicating two pathways for the incorporation of fatty acid into preformed vesicles. We propose that the fast phase, which is stoichiometrically limited by the preformed vesicles, results from the formation of a "shell" of fatty acid around a vesicle, followed by rapid transfer of this fatty acid into the preformed vesicle. The slower phase may result from incorporation of fatty acid which had been trapped in an intermediate state. We provide independent evidence for the rapid transformation of micelles into an aggregated intermediate form after transfer from high to low pH. Our results show that the most efficient incorporation of added oleate into oleic acid/oleate vesicles occurs under conditions that avoid a large transient increase in the micelle/vesicle ratio.
12. 12
    Chen, I. A.; Roberts, R. W.; Szostak, J. W. The Emergence of Competition Between Model Protocells. Science 2004, 305 (5689), 1474– 1476, DOI: 10.1126/science.1100757
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    The Emergence of Competition Between Model Protocells
    Chen, Irene A.; Roberts, Richard W.; Szostak, Jack W.
    Science (Washington, DC, United States) (2004),305 (5689),1474-1476CODEN:SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The transition from independent mol. entities to cellular structures with integrated behaviors was a crucial aspect of the origin of life. We show that simple phys. principles can mediate a coordinated interaction between genome and compartment boundary, independent of any genomic functions beyond self-replication. RNA, encapsulated in fatty acid vesicles, exerts an osmotic pressure on the vesicle membrane that drives the uptake of addnl. membrane components, leading to membrane growth at the expense of relaxed vesicles, which shrink. Thus, more efficient RNA replication could cause faster cell growth, leading to the emergence of Darwinian evolution at the cellular level.
13. 13
    Zhu, T. F.; Szostak, J. W. Coupled Growth and Division of Model Protocell Membranes. J. Am. Chem. Soc. 2009, 131 (15), 5705– 5713, DOI: 10.1021/ja900919c
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    Coupled Growth and Division of Model Protocell Membranes
    Zhu, Ting F.; Szostak, Jack W.
    Journal of the American Chemical Society (2009),131 (15),5705-5713CODEN:JACSAT; ISSN:0002-7863. (American Chemical Society)
    The generation of synthetic forms of cellular life requires solns. to the problem of how biol. processes such as cyclic growth and division could emerge from purely phys. and chem. systems. Small unilamellar fatty acid vesicles grow when fed with fatty acid micelles and can be forced to divide by extrusion, but this artificial division process results in significant loss of protocell contents during each division cycle. Here we describe a simple and efficient pathway for model protocell membrane growth and division. The growth of large multilamellar fatty acid vesicles fed with fatty acid micelles, in a soln. where solute permeation across the membranes is slow, results in the transformation of initially spherical vesicles into long thread-like vesicles, a process driven by the transient imbalance between surface area and vol. growth. Modest shear forces are then sufficient to cause the thread-like vesicles to divide into multiple daughter vesicles without loss of internal contents. In an environment of gentle shear, protocell growth and division are thus coupled processes. We show that model protocells can proceed through multiple cycles of reprodn. Encapsulated RNA mols., representing a primitive genome, are distributed to the daughter vesicles. Our observations bring us closer to the lab. synthesis of a complete protocell consisting of a self-replicating genome and a self-replicating membrane compartment. In addn., the robustness and simplicity of this pathway suggests that similar processes might have occurred under the prebiotic conditions of the early Earth.
14. 14
    Toparlak, Ö. D.; Wang, A.; Mansy, S. S. Population-Level Membrane Diversity Triggers Growth and Division of Protocells. JACS Au 2021, 1 (5), 560– 568, DOI: 10.1021/jacsau.0c00079
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    Population-Level Membrane Diversity Triggers Growth and Division of Protocells
    Toparlak, O. Duhan; Wang, Anna; Mansy, Sheref S.
    JACS Au (2021),1 (5),560-568CODEN:JAAUCR; ISSN:2691-3704. (American Chemical Society)
    To date, multiple mechanisms have been described for the growth and division of model protocells, all of which exploit the lipid dynamics of fatty acids. In some examples, the more heterogeneous aggregate consisting of fatty acid and diacyl phospholipid or fatty acid and peptide grows at the expense of the more homogeneous aggregate contg. a restricted set of lipids with similar dynamics. Imbalances between surface area and vol. during growth can generate filamentous vesicles, which are typically divided by shear forces. Here, we describe another pathway for growth and division that depends simply on differences in the compns. of fatty acid membranes without addnl. components. Growth is driven by the thermodynamically favorable mixing of lipids between two populations, i.e., the system as a whole proceeds toward equil. Division is the result of growth-induced curvature. Importantly, growth and division do not require a specific compn. of lipids. For example, vesicles made from one type of lipid, e.g., short-chain fatty acids, grow and divide when fed with vesicles consisting of another type of lipid, e.g., long-chain fatty acids, and vice versa. After equilibration, addnl. rounds of growth and division could potentially proceed by the introduction of compositionally distinct aggregates. Since prebiotic synthesis likely gave rise to mixts. of lipids, the data are consistent with the presence of growing and dividing protocells on the prebiotic Earth.
15. 15
    Hanczyc, M. M.; Fujikawa, S. M.; Szostak, J. W. Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Division. Science 2003, 302 (5645), 618– 622, DOI: 10.1126/science.1089904
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    Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Division
    Hanczyc, Martin M.; Fujikawa, Shelly M.; Szostak, Jack W.
    Science (Washington, DC, United States) (2003),302 (5645),618-622CODEN:SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The clay montmorillonite is known to catalyze the polymn. of RNA from activated ribonucleotides. Here we report that montmorillonite accelerates the spontaneous conversion of fatty acid micelles into vesicles. Clay particles often become encapsulated in these vesicles, thus providing a pathway for the prebiotic encapsulation of catalytically active surfaces within membrane vesicles. In addn., RNA adsorbed to clay can be encapsulated within vesicles. Once formed, such vesicles can grow by incorporating fatty acid supplied as micelles and can divide without diln. of their contents by extrusion through small pores. These processes mediate vesicle replication through cycles of growth and division. The formation, growth, and division of the earliest cells may have occurred in response to similar interactions with mineral particles and inputs of material and energy.
16. 16
    Coreta-Gomes, F. M.; Vaz, W. L. C.; Moreno, M. J. Effect of Acyl Chain Length on the Rate of Phospholipid Flip-Flop and Intermembrane Transfer. J. Membr. Biol. 2018, 251 (3), 431– 442, DOI: 10.1007/s00232-017-0009-4
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    Effect of Acyl Chain Length on the Rate of Phospholipid Flip-Flop and Intermembrane Transfer
    Coreta-Gomes, Filipe M.; Vaz, Winchil L. C.; Moreno, Maria J.
    Journal of Membrane Biology (2018),251 (3),431-442CODEN:JMBBBO; ISSN:0022-2631. (Springer)
    The rate at which phospholipids equilibrate between different membranes and between the non-polar environments in biol. fluids is of high importance in the understanding of biomembrane diversity, as well as in the development of liposomes for drug delivery. In this work, we characterize the rate of insertion into and desorption from POPC bilayers for a homologous series of amphiphiles with the fluorescent NBD group attached to phosphoethanolamines of different acyl chain lengths, NBD-diCn-PE with n = 6, 8, 10, and 12. The rate of translocation between bilayer leaflets was also characterized, providing all the relevant parameters for their interaction with lipid bilayers. The results are complemented with data for NBD-diC14-PE obtained from literature (Abreu et al. Biophys J 87:353-365, 2004; Moreno et al. Biophys J 91:873-881, 2006). The rate of translocation between the POPC leaflets is not dependent on the length of the acyl chains, while this affects strongly the rate of desorption from the bilayer. Insertion in the POPC bilayer is not diffusion controlled showing a significant dependence on the acyl chain length and on temp. The results obtained are compared with those previously reported for NBD-LysoC14-PE (Sampaio et al. Biophys J 88:4064-4071, 2005), and with the homologous series of single chain amphiphiles NBD-Cn (Cardoso et al. J Phys Chem B 114:16337-16346, 2010; J Phys Chem B 115:10098-10108, 2011). This allows the establishment of important relations between the rate consts. for interaction with the lipid bilayers and the structural properties of the amphiphiles, namely the total surface and the cross-section of their non-polar region.
17. 17
    Budin, I.; Szostak, J. W. Physical Effects Underlying the Transition from Primitive to Modern Cell Membranes. Proc. Natl. Acad. Sci. USA 2011, 108 (13), 5249– 5254, DOI: 10.1073/pnas.1100498108
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    Physical effects underlying the transition from primitive to modern cell membranes
    Budin, Itay; Szostak, Jack W.
    Proceedings of the National Academy of Sciences of the United States of America (2011),108 (13),5249-5254, S5249/1-S5249/13CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    To understand the emergence of Darwinian evolution, it is necessary to identify phys. mechanisms that enabled primitive cells to compete with one another. Whereas all modern cell membranes are composed primarily of diacyl or dialkyl glycerol phospholipids, the first cell membranes are thought to have self-assembled from simple, single-chain lipids synthesized in the environment. We asked what selective advantage could have driven the transition from primitive to modern membranes, esp. during early stages characterized by low levels of membrane phospholipid. Here we demonstrate that surprisingly low levels of phospholipids can drive proto cell membrane growth during competition for single-chain lipids. Growth results from the decreasing fatty acid efflux from membranes with increasing phospholipid content. The ability to synthesize phospholipids from single-chain substrates would have therefore been highly advantageous for early cells competing for a limited supply of lipids. We show that the resulting increase in membrane phospholipid content would have led to a cascade of new selective pressures for the evolution of metabolic and transport machinery to overcome the reduced membrane permeability of diacyl lipid membranes. The evolution of phospholipid membranes could thus have been a deterministic. outcome of intrinsic phys.
18. 18
    Bell, E. A.; Boehnke, P.; Harrison, T. M.; Mao, W. L. Potentially Biogenic Carbon Preserved in a 4.1 Billion-Year-Old Zircon. Proc. Natl. Acad. Sci. USA 2015, 112 (47), 14518, DOI: 10.1073/pnas.1517557112
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    Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon
    Bell, Elizabeth A.; Boehnke, Patrick; Harrison, T. Mark; Mao, Wendy L.
    Proceedings of the National Academy of Sciences of the United States of America (2015),112 (47),14518-14521CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Evidence of life on Earth is manifestly preserved in the rock record. However, the microfossil record only extends to ∼3.5 billion years (Ga), the chemofossil record arguably to ∼3.8 Ga, and the rock record to 4.0 Ga. Detrital zircons from Jack Hills, Western Australia range in age up to nearly 4.4 Ga. From a population of over 10,000 Jack Hills zircons, we identified one >3.8-Ga zircon that contains primary graphite inclusions. The authors report carbon isotopic measurements on these inclusions in a concordant, 4.10 ± 0.01-Ga zircon. The authors interpret these inclusions as primary due to their enclosure in a crack-free host as shown by transmission X-ray microscopy and their crystal habit. Their δ13CPDB of -24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga, or ∼300 My earlier than has been previously proposed.
19. 19
    Lawless, J. G.; Yuen, G. U. Quantification of Monocarboxylic Acids in the Murchison Carbonaceous Meteorite. Nature 1979, 282 (5737), 396– 398, DOI: 10.1038/282396a0
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    Quantification of monocarboxylic acids in the Murchison carbonaceous meteorite
    Lawless, James G.; Yuen, George U.
    Nature (London, United Kingdom) (1979),282 (5737),396-8CODEN:NATUAS; ISSN:0028-0836.
    The monocarboxylic acids in the Murchison chondrite were identified and quantified using mass spectroscopy and gas chromatog. The acid concn. decreases as the no. of C atoms in the acid increases, which supports an extraterrestrial abiotic synthesis. The acids show no predominance of straight- over branched-chain isomers. Possible processes responsible for producing the obsd. isomeric ratios are discussed.
20. 20
    Naraoka, H.; Shimoyama, A.; Harada, K. Molecular Distribution of Monocarboxylic Acids in Asuka Carbonaceous Chondrites from Antarctica. Orig Life Evol Biosph 1999, 29 (2), 187– 201, DOI: 10.1023/A:1006547127028
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    Molecular distribution of monocarboxylic acids in Asuka carbonaceous chondrites from Antarctica
    Naraoka, Hiroshi; Shimoyama, Akira; Harada, Kaoru
    Origins of Life and Evolution of the Biosphere (1999),29 (2),187-201CODEN:OLEBEM; ISSN:0169-6149. (Kluwer Academic Publishers)
    Mol. distribution of low-mol.-wt. monocarboxylic acids was studied in three CM2 Asuka carbonaceous chondrites (A-881280, A-881334 and A-881458), which were recovered from Antarctica by the 29th Japanese Antarctic Research Expedition in 1988. GC and GC/MS analyses identified more than 30 monocarboxylic acids in A-881458, including aliph. and arom. acids with various structural isomers. Isomeric phenolic compds. were also identified. The aliph. carboxylic acids have straight-chain structures having 2 to 12 carbon atoms (C2 to C12), and branched-chain structures (C4 to C9). The quantities of straight-chain acids decrease logarithmically with increasing carbon no. At the same carbon no., a straight-chain isomer is always predominant compared to branched-chain isomers. All of the 14 possible C4, C5 and C6 aliph. monocarboxylic acids (not including optical isomers) have been identified, although all the isomers were not reported in Murchison and Y-791198 meteorites. Of the 17 possible isomeric C7 acids, at least 14 isomers were tentatively identified by mass spectra (EI and CI mode). At C8 or above, peaks of branched-chain isomers become obscure, probably due to the large no. of isomers and small concns. Branched-chain monocarboxylic acids over C6 have never been reported in Murchison. Although occurrence of aliph. acids are similar between A-881458 and Murchison at C4, C5 and C6 acids, a major difference is that A-881458 as well as Y-791198 have straight-chain predominance among isomers in contrast to Murchison being branched-chain predominant. In the case of isomeric arom. compds. such as toluic acids and cresols, m-toluic acid and p-cresol are more abundant among their isomers, resp. The mol. distribution may not reflect thermodn. equil. but rather a kinetically controlled process for their formation mechanism. The other two CM2 chondrites (A-881280 and A-881334) were depleted in carboxylic acids in spite of similar carbon contents. The depletion is not due to weathering on ice, because the degrees of weathering are small and similar among the three chondrites. Probably those latter two chondrites may have been subjected to aq. alteration or metamorphism on their meteorite parent bodies.
21. 21
    Aponte, J. C.; Alexandre, M. R.; Wang, Y.; Brearley, A. J.; Alexander, C. M. O.; Huang, Y. Effects of Secondary Alteration on the Composition of Free and IOM-Derived Monocarboxylic Acids in Carbonaceous Chondrites. Geochim. Cosmochim. Acta 2011, 75 (9), 2309– 2323, DOI: 10.1016/j.gca.2011.01.040
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    Effects of secondary alteration on the composition of free and IOM-derived monocarboxylic acids in carbonaceous chondrites
    Aponte, Jose C.; Alexandre, Marcelo R.; Wang, Yi; Brearley, Adrian J.; Alexander, Conel M. O'D.; Huang, Yongsong
    Geochimica et Cosmochimica Acta (2011),75 (9),2309-2323CODEN:GCACAK; ISSN:0016-7037. (Elsevier B.V.)
    Monocarboxylic acids (MCAs) are important astrobiol. because they are often the most abundant sol. compds. in carbonaceous chondrites (CCs) and are potential synthetic end products for many biol. important compds. However, there has been no systematic study on the effect of parent body alteration on mol. and isotopic variability of MCAs. Since MCAs in meteorites are dominated by low mol. wt. (C1-C8), highly volatile compds., their distributions are likely to be particularly sensitive to secondary alteration processes. In contrast, the aliph. side chains of insol. org. matter (IOM) in CCs, whose compn. has been shown to be closely related to the MCAs, may be far more resistant to secondary alteration. The authors detd. the distributions and H- and C-isotopic ratios of free and IOM-derived MCAs in six carbonaceous chondrites with a range of classifications: Murchison (CM2), EET 87770 (CR2), ALH 83034 (CM1), ALH 83033 (CM2), MET 00430 (CV3) and WIS 91600 (C2). Mineralogical and petrol. characteristics to the MCAs distributions are compared to better define the processes leading to the synthesis and alteration of meteoritic MCAs. Results show that aq. and esp. thermal alteration in the parent bodies led to major loss of free MCAs and depletion of straight relative to branched chain compds. However, the MCAs derived from aliph. side chains of IOM are well preserved despite of secondary alterations. The mol. and isotopic similarities of IOM-derived MCAs in different chondrite samples indicate very similar synthetic histories for org. matter in different meteorites.
22. 22
    Hilts, R. W.; Herd, C. D. K.; Simkus, D. N.; Slater, G. F. Soluble Organic Compounds in the Tagish Lake Meteorite. Meteoritics & Planetary Science 2014, 49 (4), 526– 549, DOI: 10.1111/maps.12272
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    Soluble organic compounds in the Tagish Lake meteorite
    Hilts, Robert W.; Herd, Christopher D. K.; Simkus, Danielle N.; Slater, Greg F.
    Meteoritics & Planetary Science (2014),49 (4),526-549CODEN:MPSCFY; ISSN:1945-5100. (Wiley-Blackwell)
    The C2 ungrouped Tagish Lake meteorite preserves a range of lithologies, reflecting variable degrees of parent-body aq. alteration. Here, we report on sol. org. compds., including aliph. and arom. hydrocarbons, monocarboxylic acids, and amino acids, found within specimens representative of the range of aq. alteration. We find that differences in sol. org. compds. among the lithologies may be explained by oxidative, fluid-assisted alteration, primarily involving the derivation of sol. org. compds. from macromol. material. In contrast, amino acids probably evolved from precursor mols., albeit in parallel with other sol. org. compds. Our results demonstrate the role of parent-body alteration in the modification of org. matter and generation of prebiotic compds. in the early solar system, and have implications for interpretation of the complement of sol. org. compds. in carbonaceous chondrites.
23. 23
    Nooner, D. W.; Oro, J. Synthesis of Fatty Acids by a Closed System Fischer–Tropsch Process. In Hydrocarbon Synthesis from Carbon Monoxide and Hydrogen; Advances in Chemistry; American Chemical Society, 1979; Vol. 178, pp 159– 171.
24. 24
    Yuen, G. U.; Lawless, J. G.; Edelson, E. H. Quantification of Monocarboxylic Acids from a Spark Discharge Synthesis. J. Mol. Evol 1981, 17 (1), 43– 47, DOI: 10.1007/BF01792423
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    Quantification of monocarboxylic acids from a spark discharge synthesis
    Yuen, George U.; Lawless, James G.; Edelson, Edward H.
    Journal of Molecular Evolution (1981),17 (1),43-7CODEN:JMEVAU; ISSN:0022-2844.
    Some 16 monocarboxylic acids (C2-C7), formed by the Miller-Urey spark discharge process, were identified and quantified by gas chromatog. and mass fragmentog. using a deuterium spiking technique. The molar concn. and isomeric distribution of these lab.-synthesized monocarboxylic acids are compared with those previously reported for the Murchison meteorite. They show similar trends: molar concn. decreases with increasing mol. wt. and the ratio of normal/branched isomers becomes smaller with increasing C nos.
25. 25
    Deamer, D. W. Boundary Structures Are Formed by Organic Components of the Murchison Carbonaceous Chondrite. Nature 1985, 317 (6040), 792– 794, DOI: 10.1038/317792a0
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    Boundary structures are formed by organic components of the Murchison carbonaceous chondrite
    Deamer, David W.
    Nature (London, United Kingdom) (1985),317 (6040),792-4CODEN:NATUAS; ISSN:0028-0836.
    Formation of boundary structures by compds. extd. from the Murchison carbonaceous chondrite by org. solvents was investigated. A yellow, fluorescent ext. was obtained which appeared to contain carboxylate and phenolic groups and hydrocarbons. Two types of boundary structures were obsd. One class of microstructures was a viscous fluid droplet at alk. pH and a solid structure at neutral and acidic pH. The 2nd class of microstructure, formed only at alk. pH, was a thin boundary which functioned as a barrier to free diffusion, i.e., a membrane. The implications of these observations with respect to prebiotic evolution are discussed.
26. 26
    Lai, J. C.-Y.; Pearce, B. K. D.; Pudritz, R. E.; Lee, D. Meteoritic Abundances of Fatty Acids and Potential Reaction Pathways in Planetesimals. Icarus 2019, 319, 685– 700, DOI: 10.1016/j.icarus.2018.09.028
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    Meteoritic abundances of fatty acids and potential reaction pathways in planetesimals
    Lai, James C.-Y.; Pearce, Ben K. D.; Pudritz, Ralph E.; Lee, Drake
    Icarus (2019),319 (),685-700CODEN:ICRSA5; ISSN:0019-1035. (Elsevier Inc.)
    The origin of fatty acids on the prebiotic Earth is important as they likely formed the encapsulating membranes of the first protocells. Carbon-rich meteorites (i.e., carbonaceous chondrites) such as Murchison and Tagish Lake are well known to contain these mols., and their delivery to the early planet by intense early meteorite bombardments constitutes a key prebiotic source. We collect the fatty acid abundances measured in various carbonaceous chondrites from the literature and analyze them for patterns and correlations. Fatty acids in meteorites include straight-chain and branched-chain monocarboxylic and dicarboxylic acids up to 12 carbons in length-fatty acids with at least 8 carbons are required to form vesicles, and modern cell membranes employ lipids with ∼12-20 carbons. To understand the origin of meteoritic fatty acids, we search the literature for abiotic fatty acid reaction pathways and create a candidate list of 11 reactions that could potentially produce these fatty acids in meteorite parent bodies. Straight-chain monocarboxylic acids (SCMA) are the dominant fatty acids in meteorites, followed by branched-chain monocarboxylic acids (BCMA). SCMA are most abundant in CM2 and Tagish Lake (ungrouped) meteorites, ranging on av. from 102 ppb to 4 × 105 ppb, and 104 ppb to 5 × 106 ppb, resp. In CM, CV, and Tagish Lake meteorites, SCMA abundances generally decrease with increasing carbon chain length. Conversely, SCMA abundances in CR meteorites peak at 5 and 6 carbons in length, and decrease on either side of this peak. This unique CR fatty acid distribution may hint at terrestrial contamination, or that certain fatty acid reactions mechanisms are active in different meteorite parent bodies (planetesimals). We identify Fischer-Tropsch-type synthesis as the most promising pathway for further anal. in the prodn. of fatty acids in planetesimals.
27. 27
    Sephton, M. A. Organic Compounds in Carbonaceous Meteorites. Nat. Prod. Rep. 2002, 19 (3), 292– 311, DOI: 10.1039/b103775g
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    Organic compounds in carbonaceous meteorites
    Sephton, Mark A.
    Natural Product Reports (2002),19 (3),292-311CODEN:NPRRDF; ISSN:0265-0568. (Royal Society of Chemistry)
    A review with 193 refs. The carbonaceous chondrite meteorites are fragments of asteroids that have remained relatively unprocessed since the formation of the solar system 4.6 billion years ago. These carbon-rich objects contain a variety of extraterrestrial org. mols. that constitute a record of chem. evolution prior to the origin of life. Compd. classes include aliph. hydrocarbons, arom. hydrocarbons, amino acids, carboxylic acids, sulfonic acids, phosphonic acids, alcs., aldehydes, ketones, sugars, amines, amides, nitrogen heterocycles, sulfur heterocycles and a relatively abundant high mol. wt. macromol. material. Structural and stable isotopic characteristics suggest that a no. of environments may have contributed to the org. inventory, including interstellar space, the solar nebula and the asteroidal meteorite parent body. This review covers work published between 1950 and the present day and cites 193 refs.
28. 28
    Chyba, C. F.; Thomas, P. J.; Brookshaw, L.; Sagan, C. Cometary Delivery of Organic Molecules to the Early Earth. Science 1990, 249 (4967), 366– 373, DOI: 10.1126/science.11538074
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    Cometary delivery of organic molecules to the early Earth
    Chyba, Christopher F.; Thomas, Paul J.; Brookshaw, Leigh; Sagan, Carl
    Science (Washington, DC, United States) (1990),249 (4967),366-73CODEN:SCIEAS; ISSN:0036-8075.
    The possibility that prebiotic org. mols. were accreted to the early Earth's surface by impact of carbonaceous asteroids and comets is examd. through modeling of the atm. entry and ablation of halides and hydrodynamic simulation of their impact. The flux of cometary org. compds. delivered 4.5 to 3.5 Gyr ago is estd. taking into account the incident cometary mass, the cometary org. mass fraction, ablative loss during atm. passage, and the pyrolysis of org. compds. on impact. Org. mols. would not survive impacts at velocities >10 km/s and even bodies 100 m in radius carrot be aerobraked to below this velocity in an atm. with PCO2 = 10 bar, the flux of cometary org. compds. ∼106-107 kg/yr 4.S Gyr ago, decreasing with a half-life of ∼108 yr.
29. 29
    Pearce, B. K. D.; Pudritz, R. E.; Semenov, D. A.; Henning, T. K. Origin of the RNA World: The Fate of Nucleobases in Warm Little Ponds. Proc. Natl. Acad. Sci. USA 2017, 114 (43), 11327– 11332, DOI: 10.1073/pnas.1710339114
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    Origin of the RNA world: The fate of nucleobases in warm little ponds
    Pearce, Ben K. D.; Pudritz, Ralph E.; Semenov, Dmitry A.; Henning, Thomas K.
    Proceedings of the National Academy of Sciences of the United States of America (2017),114 (43),11327-11332CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Before the origin of simple cellular life, the building blocks of RNA (nucleotides) had to form and polymerize in favorable environments on early Earth. At this time, meteorites and interplanetary dust particles delivered orgs. such as nucleobases (the characteristic mols. of nucleotides) to warm little ponds whose wet-dry cycles promoted rapid polymn. We build a comprehensive numerical model for the evolution of nucleobases in warm little ponds leading to the emergence of the first nucleotides and RNA. We couple Earth's early evolution with complex prebiotic chem. in these environments. We find that RNA polymers must have emerged very quickly after the deposition of meteorites (less than a few years). Their constituent nucleobases were primarily meteoritic in origin and not from interplanetary dust particles. Ponds appeared as continents rose out of the early global ocean, but this increasing availability of "targets" for meteorites was offset by declining meteorite bombardment rates. Moreover, the rapid losses of nucleobases to pond seepage during wet periods, and to UV photodissocn. during dry periods, mean that the synthesis of nucleotides and their polymn. into RNA occurred in just one to a few wet-dry cycles. Under these conditions, RNA polymers likely appeared before 4.17 billion years ago.
30. 30
    Zamozdra, S. N.; Kocherov, A. V. Underwater Excavations and Welcoming the Big Meteorite. In Chelyabinsk Superbolide; Gorkavyi, N., Dudorov, A., Taskaev, S., Eds.; Springer Praxis Books; Springer International Publishing: Cham, 2019; pp 122– 147.
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    Popova, O. P.; Jenniskens, P.; Emel’yanenko, V.; Kartashova, A.; Biryukov, E.; Khaibrakhmanov, S.; Shuvalov, V.; Rybnov, Y.; Dudorov, A.; Grokhovsky, V. I.; Badyukov, D. D.; Yin, Q.-Z.; Gural, P. S.; Albers, J.; Granvik, M.; Evers, L. G.; Kuiper, J.; Kharlamov, V.; Solovyov, A.; Rusakov, Y. S.; Korotkiy, S.; Serdyuk, I.; Korochantsev, A. V.; Larionov, M. Yu.; Glazachev, D.; Mayer, A. E.; Gisler, G.; Gladkovsky, S. V.; Wimpenny, J.; Sanborn, M. E.; Yamakawa, A.; Verosub, K. L.; Rowland, D. J.; Roeske, S.; Botto, N. W.; Friedrich, J. M.; Zolensky, M. E.; Le, L.; Ross, D.; Ziegler, K.; Nakamura, T.; Ahn, I.; Lee, J. I.; Zhou, Q.; Li, X.-H.; Li, Q.-L.; Liu, Y.; Tang, G.-Q.; Hiroi, T.; Sears, D.; Weinstein, I. A.; Vokhmintsev, A. S.; Ishchenko, A. V.; Schmitt-Kopplin, P.; Hertkorn, N.; Nagao, K.; Haba, M. K.; Komatsu, M.; Mikouchi, T.; the Chelyabinsk Airburst Consortium Chelyabinsk Airburst, Damage Assessment, Meteorite Recovery, and Characterization. Science 2013, 342 (6162), 1069– 1073, DOI: 10.1126/science.1242642
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    Chelyabinsk Airburst, Damage Assessment, Meteorite Recovery, and Characterization
    Popova, Olga P.; Jenniskens, Peter; Emel'yanenko, Vacheslav; Kartashova, Anna; Biryukov, Eugeny; Khaibrakhmanov, Sergey; Shuvalov, Valery; Rybnov, Yurij; Dudorov, Alexandr; Grokhovsky, Victor I.; Badyukov, Dmitry D.; Yin, Qing-Zhu; Gural, Peter S.; Albers, Jim; Granvik, Mikael; Evers, Laeslo G.; Kuiper, Jacob; Kharlamov, Vladimir; Solovyov, Andrey; Rusakov, Yuri S.; Korotkiy, Stanislav; Serdyuk, Ilya; Korochantsev, Alexander V.; Larionov, Michail Yu.; Glazachev, Dmitry; Mayer, Alexander E.; Gisler, Galen; Gladkovsky, Sergei V.; Wimpenny, Josh; Sanborn, Matthew E.; Yamakawa, Akane; Verosub, Kenneth L.; Rowland, Douglas J.; Roeske, Sarah; Botto, Nicholas W.; Friedrich, Jon M.; Zolensky, Michael E.; Le, Loan; Ross, Daniel; Ziegler, Karen; Nakamura, Tomoki; Ahn, Insu; Lee, Jong Ik; Zhou, Qin; Li, Xian-Hua; Li, Qiu-Li; Liu, Yu; Tang, Guo-Qiang; Hiroi, Takahiro; Sears, Derek; Weinstein, Ilya A.; Vokhmintsev, Alexander S.; Ishchenko, Alexei V.; Schmitt-Kopplin, Phillipe; Hertkorn, Norbert; Nagao, Keisuke; Haba, Makiko K.; Komatsu, Mutsumi; Mikouchi, Takashi
    Science (Washington, DC, United States) (2013),342 (6162),1069-1073CODEN:SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The asteroid impact near the Russian city of Chelyabinsk on 15 Feb. 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and lab. techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorol., meteoritics, and cosmochem. and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.
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    Artemieva, N. A.; Shuvalov, V. V. From Tunguska to Chelyabinsk via Jupiter. Annual Review of Earth and Planetary Sciences 2016, 44 (1), 37– 56, DOI: 10.1146/annurev-earth-060115-012218
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    From Tunguska to Chelyabinsk via Jupiter
    Artemieva, Natalia A.; Shuvalov, Valery V.
    Annual Review of Earth and Planetary Sciences (2016),44 (),37-56CODEN:AREPCI; ISSN:0084-6597. (Annual Reviews)
    The Tunguska event remained enigmatic for almost 100 years until the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 helped to resolve this enigma and allowed us to adequately interpret the more recent Chelyabinsk event. Airbursts typically occur if a meteoroid entering Earth's atm. is 10-100 m in diam., i.e., its energy ranges from 0.5 (Chelyabinsk) to 20 (Tunguska) Mt TNT. All this energy is released in the atm. with strong shock waves generated during the entry reaching the surface and causing substantial damage. Atm. plumes are capable of dispersing extraterrestrial materials worldwide. Modern civilization is extremely vulnerable to those relatively small disturbances that recur on a decadal timescale and are still difficult to predict.
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    Mehta, C.; Perez, A.; Thompson, G.; Pasek, M. A. Caveats to Exogenous Organic Delivery from Ablation, Dilution, and Thermal Degradation. Life 2018, 8 (2), 13, DOI: 10.3390/life8020013
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    Caveats to exogenous organic delivery from ablation, dilution, and thermal degradation
    Mehta, Chris; Perez, Anthony; Thompson, Glenn; Pasek, Matthew A.
    Life (Basel, Switzerland) (2018),8 (2),13/1-13/15CODEN:LBSIB7; ISSN:2075-1729. (MDPI AG)
    A hypothesis in prebiotic chem. argues that orgs. were delivered to the early Earth in abundance by meteoritic sources. This study tests that hypothesis by measuring how the transfer of org. matter to the surface of Earth is affected by energy-dissipation processes such as ablation and airbursts. Exogenous delivery has been relied upon as a source of primordial material, but it must stand to reason that other avenues (i.e., hydrothermal vents, elec. discharge) played a bigger role in the formation of life as we know it on Earth if exogenous material was unable to deliver significant quantities of orgs. For this study, we look at various properties of meteors such as initial velocity and mass of the object, and atm. compn. to see how meteors with different initial velocities and masses ablate. We find that large meteors do not slow down fast enough and thus impact the surface, vaporizing their components; fast meteors with low masses are vaporized during entry; and meteors with low velocities and high initial masses reach the surface. For those objects that survive to reach the surface, about 60 to >99% of the mass is lost by ablation. Large meteors that fragment are also shown to spread out over increasingly larger areas with increasing mass, and small meteors (∼1 mm) are subjected to intense thermal heating, potentially degrading intrinsic orgs. These findings are generally true across most atm. compns. These findings provide several caveats to extraterrestrial delivery models that-while a viable point source of orgs.-likely did not supply as much prebiotic material as an effective endogenous prodn. route.
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    Clark, B. C.; Kolb, V. M. Comet Pond II: Synergistic Intersection of Concentrated Extraterrestrial Materials and Planetary Environments to Form Procreative Darwinian Ponds. Life 2018, 8 (2), 12, DOI: 10.3390/life8020012
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    Comet pond II: synergistic intersection of concentrated extraterrestrial materials and planetary environments to form procreative Darwinian ponds
    Clark, Benton C.; Kolb, Vera M.
    Life (Basel, Switzerland) (2018),8 (2),12/1-12/22CODEN:LBSIB7; ISSN:2075-1729. (MDPI AG)
    In the "comet pond" model, a rare combination of circumstances enables the entry and landing of pristine org. material onto a planetary surface with the creation of a pond by a soft impact and melting of entrained ices. Formation of the constituents of the comet in the cold interstellar medium and our circumstellar disk results in multiple constituents at disequil. which undergo rapid chem. reactions in the warmer, liq. environment. The planetary surface also provides minerals and atm. gases which chem. interact with the pond's organicand trace-element-rich constituents. Pond phys. morphol. and the heterogeneities imposed by gravitational forces (bottom sludge; surface scum) and weather result in a highly heterogeneous variety of macro- and microenvironments. Wet/dry, freeze/thaw, and natural chromatog. processes further promote certain reaction sequences. Evapn. concs. orgs. less volatile than water. Freezing concs. all sol. orgs. into a residual liq. phase, including CH3OH, HCN, etc. The pond's evolutionary processes culminate in the creation of a Macrobiont with the metabolically equiv. capabilities of energy transduction and replication of RNA (or its progenitor informational macromol.), from which smaller organisms can emerge. Planet-wide dispersal of microorganisms is achieved through wind transport, groundwater, and/or spillover from the pond into surface hydrol. networks.
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    Cape, J. L.; Monnard, P.-A.; Boncella, J. M. Prebiotically Relevant Mixed Fatty Acid Vesicles Support Anionic Solute Encapsulation and Photochemically Catalyzed Trans-Membrane Charge Transport. Chem. Sci. 2011, 2 (4), 661– 671, DOI: 10.1039/c0sc00575d
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    Prebiotically relevant mixed fatty acid vesicles support anionic solute encapsulation and photochemically catalyzed trans-membrane charge transport
    Cape, Jonathan L.; Monnard, Pierre-Alain; Boncella, James M.
    Chemical Science (2011),2 (4),661-671CODEN:CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The spontaneous assembly of amphiphile-based compartments in aq. soln. is widely viewed as a key step in models for the abiotic formation of primitive cell-like structures. Proposed org. components for such systems consist of mixed short chain fatty acids (FA) and polycyclic arom. hydrocarbon (PAH) species, the compn. of which have been modeled after org. exts. of carbonaceous meteorites. Self-assembly of amphiphiles from these exts. into aq. suspensions of bilayer structures was long ago demonstrated, although little has since been reported concerning the stability and potential functionality of these complex mixts. This work explores the thermodn. and kinetic stability of vesicles prepd. from complex mixts. of short chain FA species (CH3COOH-C9H19COOH) with membrane solubilized PAH species. Crit. vesicle concn. measurements and ultrafiltration analyses of decanoic acid in the presence of other shorter chain FA species indicate the formation of mixed component vesicle phases composed mainly of C10-C8 FA components. An electrostatic barrier to trans-membrane diffusion of neg. charges allows observation of stably encapsulated poly-anionic solutes inside these vesicles. As a model for primitive energy transduction, trans-membrane electron transfer between EDTA and encapsulated ferricyanide was demonstrated, driven catalytically via PAH photochem. without substantial decompn. of the chromophores or vesicles. These results indicate a plausible role for compartmentalization and catalysis by short chain fatty acids and PAH species in prebiotic vesicle-encapsulated systems.
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    Budin, I.; Prywes, N.; Zhang, N.; Szostak, J. W. Chain-Length Heterogeneity Allows for the Assembly of Fatty Acid Vesicles in Dilute Solutions. Biophys. J. 2014, 107 (7), 1582– 1590, DOI: 10.1016/j.bpj.2014.07.067
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    Chain-Length Heterogeneity Allows for the Assembly of Fatty Acid Vesicles in Dilute Solutions
    Budin, Itay; Prywes, Noam; Zhang, Na; Szostak, Jack W.
    Biophysical Journal (2014),107 (7),1582-1590CODEN:BIOJAU; ISSN:0006-3495. (Cell Press)
    A requirement for concd. and chem. homogeneous pools of mol. building blocks would severely restrict plausible scenarios for the origin of life. In the case of membrane self-assembly, models of prebiotic lipid synthesis yield primarily short, single-chain amphiphiles that can form bilayer vesicles only at very high concns. These high crit. aggregation concns. (cacs) pose significant obstacles for the self-assembly of single-chain lipid membranes. Here, we examine membrane self-assembly in mixts. of fatty acids with varying chain lengths, an expected feature of any abiotic lipid synthesis. We derive theor. predictions for the cac of mixts. by adapting thermodn. models developed for the analogous phenomenon of mixed micelle self-assembly. We then use several complementary methods to characterize aggregation exptl., and find cac values in close agreement with our theor. predictions. These measurements establish that the cac of fatty acid mixts. is dramatically lowered by minor fractions of long-chain species, thereby providing a plausible route for protocell membrane assembly. Using an NMR-based approach to monitor aggregation of isotopically labeled samples, we demonstrate the incorporation of individual components into mixed vesicles. These expts. suggest that vesicles assembled in dil., mixed solns. are depleted of the shorter-chain-length lipid species, a finding that carries implications for the compn. of primitive cell membranes.
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    Trigo-Rodríguez, J. M.; Rimola, A.; Tanbakouei, S.; Soto, V. C.; Lee, M. Accretion of Water in Carbonaceous Chondrites: Current Evidence and Implications for the Delivery of Water to Early Earth. Space Sci. Rev. 2019, 215 (1), 18, DOI: 10.1007/s11214-019-0583-0
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    Britt, D. T.; Consolmagno, G. J. S. J. Stony Meteorite Porosities and Densities: A Review of the Data through 2001. Meteoritics & Planetary Science 2003, 38 (8), 1161– 1180, DOI: 10.1111/j.1945-5100.2003.tb00305.x
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    Stony meteorite porosities and densities: A review of the data through 2001
    Britt, D. T.; Consolmagno S. J., G. J.
    Meteoritics & Planetary Science (2003),38 (8),1161-1180CODEN:MPSCFY; ISSN:1086-9379. (Meteoritical Society)
    In this review, we summarize the data published up to Dec. 2001 on the porosity and d. of stony meteorites. These data were taken from 925 samples of 454 different meteorites by a variety of techniques. Most meteorites have densities on the order of 3 to 4 g/cm3, with lower densities only for some volatile-rich carbonaceous meteorites and higher densities for stony irons. For the vast majority of stones, porosity data alone cannot distinguish between different meteorite compns. Av. porosities for most meteorite classes are around 10%, though individual samples can range as high as 30% porosity. Unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present. The measured d. of ordinary chondrites is strongly controlled by the amt. of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment. A theor. grain d. based on compn. can model "pre-weathered" porosities. The av. model porosity for H and LL chondrites is 10%, while L chondrite model porosities av. only 6%, a statistically significant difference.
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    de Klerk, A. Fischer–Tropsch Synthesis. In Fischer–Tropsch Refining; John Wiley & Sons, Ltd.: Hoboken, NJ; pp 73– 103.
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    Schulz, H.; Beck, K.; Erich, E. Mechanism of the Fischer–Tropsch Process. In Studies in Surface Science and Catalysis; Bibby, D. M., Chang, C. D., Howe, R. F., Yurchak, S., Eds.; Methane Conversion; Elsevier, 1988; Vol. 36, pp 457– 471.
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    Huber, C.; Wächtershäuser, G. Activated Acetic Acid by Carbon Fixation on (Fe,Ni)S Under Primordial Conditions. Science 1997, 276 (5310), 245– 247, DOI: 10.1126/science.276.5310.245
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    Activated acetic acid by carbon fixation on (Fe,Ni)S under primordial conditions
    Huber, Claudia; Waechterhaeuser, Guenter
    Science (Washington, D. C.) (1997),276 (5310),245-246, 247CODEN:SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    In expts. modeling the reactions of the reductive acetyl-CoA pathway at hydrothermal temps., it was found that an aq. slurry of copptd. NiS and FeS converted CO and CH3SH into the activated thioester CH3-CO-SCH3, which hydrolyzed to acetic acid. In the presence of aniline, acetanilide was formed. When NiS-FeS was modified with catalytic amts. of selenium, acetic acid and CH3SH were formed from CO and H2S alone. The reaction can be considered as the primordial initiation reaction for a chemoautotrophic origin of life.
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    Scheidler, C.; Sobotta, J.; Eisenreich, W.; Wächtershäuser, G.; Huber, C. Unsaturated C 3,5,7,9 -Monocarboxylic Acids by Aqueous, One-Pot Carbon Fixation: Possible Relevance for the Origin of Life. Sci. Rep. 2016, 6 (1), 1– 7, DOI: 10.1038/srep27595
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    Hudson, R.; de Graaf, R.; Strandoo Rodin, M.; Ohno, A.; Lane, N.; McGlynn, S. E.; Yamada, Y. M. A.; Nakamura, R.; Barge, L. M.; Braun, D.; Sojo, V. CO2 Reduction Driven by a PH Gradient. Proc. Natl. Acad. Sci. USA 2020, 117 (37), 22873– 22879, DOI: 10.1073/pnas.2002659117
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    CO2 reduction driven by a pH gradient
    Hudson, Reuben; de Graaf, Ruvan; Rodin, Mari Strandoo; Ohno, Aya; Lane, Nick; McGlynn, Shawn E.; Yamada, Yoichi M. A.; Nakamura, Ryuhei; Barge, Laura M.; Braun, Dieter; Sojo, Victor
    Proceedings of the National Academy of Sciences of the United States of America (2020),117 (37),22873-22879CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    All life on Earth is built of org. mols., so the primordial sources of reduced carbon remain a major open question in studies of the origin of life. A variant of the alk.-hydrothermal-vent theory for life's emergence suggests that orgs. could have been produced by the redn. of CO2 via H2 oxidn., facilitated by geol. sustained pH gradients. The process would be an abiotic analog-and proposed evolutionary predecessor-of the Wood-Ljungdahl acetyl-CoA pathway of modern archaea and bacteria. The first energetic bottleneck of the pathway involves the endergonic redn. of CO2 with H2 to formate (HCOO-), which has proven elusive in mild abiotic settings. Here we show the redn. of CO2 with H2 at room temp. under moderate pressures (1.5 bar), driven by microfluidic pH gradients across inorg. Fe(Ni)S ppts. Isotopic labeling with 13C confirmed formate prodn. Sep., deuterium (2H) labeling indicated that electron transfer to CO2 does not occur via direct hydrogenation with H2 but instead, freshly deposited Fe(Ni)S ppts. appear to facilitate electron transfer in an electrochem.-cell mechanism with two distinct half-reactions. Decreasing the pH gradient significantly, removing H2, or eliminating the ppt. yielded no detectable product. Our work demonstrates the feasibility of spatially sepd. yet elec. coupled geochem. reactions as drivers of otherwise endergonic processes. Beyond corroborating the ability of early-Earth alk. hydrothermal systems to couple carbon redn. to hydrogen oxidn. through biol. relevant mechanisms, these results may also be of significance for industrial and environmental applications, where other redox reactions could be facilitated using similarly mild approaches.
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    Preiner, M.; Igarashi, K.; Muchowska, K. B.; Yu, M.; Varma, S. J.; Kleinermanns, K.; Nobu, M. K.; Kamagata, Y.; Tüysüz, H.; Moran, J.; Martin, W. F. A Hydrogen-Dependent Geochemical Analogue of Primordial Carbon and Energy Metabolism. Nat. Ecol Evol 2020, 4 (4), 534– 542, DOI: 10.1038/s41559-020-1125-6
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    A hydrogen-dependent geochemical analogue of primordial carbon and energy metabolism
    Preiner Martina; Martin William F; Igarashi Kensuke; Muchowska Kamila B; Moran Joseph; Yu Mingquan; Tuysuz Harun; Varma Sreejith J; Kleinermanns Karl; Nobu Masaru K; Kamagata Yoichi
    Nature ecology & evolution (2020),4 (4),534-542 ISSN:.
    Hydrogen gas, H2, is generated by alkaline hydrothermal vents through an ancient geochemical process called serpentinization, in which water reacts with iron-containing minerals deep within the Earth's crust. H2 is the electron donor for the most ancient and the only energy-releasing route of biological CO2 fixation, the acetyl-CoA pathway. At the origin of metabolism, CO2 fixation by hydrothermal H2 within serpentinizing systems could have preceded and patterned biotic pathways. Here we show that three hydrothermal minerals-greigite (Fe3S4), magnetite (Fe3O4) and awaruite (Ni3Fe)-catalyse the fixation of CO2 with H2 at 100 °C under alkaline aqueous conditions. The product spectrum includes formate (up to 200 mM), acetate (up to 100 μM), pyruvate (up to 10 μM), methanol (up to 100 μM) and methane. The results shed light on both the geochemical origin of microbial metabolism and the nature of abiotic formate and methane synthesis in modern hydrothermal vents.
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    McCollom, T. M.; Seewald, J. S. A Reassessment of the Potential for Reduction of Dissolved CO2 to Hydrocarbons during Serpentinization of Olivine. Geochim. Cosmochim. Acta 2001, 65 (21), 3769– 3778, DOI: 10.1016/S0016-7037(01)00655-X
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    A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine
    McCollom, Thomas M.; Seewald, Jeffrey S.
    Geochimica et Cosmochimica Acta (2001),65 (21),3769-3778CODEN:GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    The concept that aq. CO2 can be reduced to hydrocarbons abiotically during serpentinization of olivine has become widespread in the earth and planetary sciences. This concept was reevaluated through an exptl. study of the reaction of dissolved CO2 in the presence of olivine under hydrothermal conditions (300°, 350 bar). Redn. of CO2 to formate (HCOO-) proceeded rapidly, with H2 generated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decompn. of formic acid to CO2 and H2, also proceeded rapidly. Although dissolved hydrocarbon concns. increased throughout the expts., isotopic labeling of dissolved CO2 with 13C indicated that these compds. were primarily generated from reduced carbon compds. already present in olivine at the beginning of the expt. rather than by redn. of CO2. The only hydrocarbon product from redn. of CO2 obsd. in the expts. was a small amt. of methane (<0.04% conversion of dissolved CO2 after >2500 h heating). Comparison of the reaction products with thermodn. data indicated that reactions between dissolved CO2 and formate rapidly achieved metastable equil. at the exptl. conditions, suggesting that similar reactions could control the concn. of formate in geol. fluids. Thus, the potential for abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed, and other mineral catalysts or vapor phase reactions (e.g., Fischer-Tropsch-like) may be required to explain many occurrences of abiotic hydrocarbons in serpentinites and igneous rocks.
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    McCollom, T. M.; Seewald, J. S. Experimental Constraints on the Hydrothermal Reactivity of Organic Acids and Acid Anions: I. Formic Acid and Formate. Geochim. Cosmochim. Acta 2003, 67 (19), 3625– 3644, DOI: 10.1016/S0016-7037(03)00136-4
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    Experimental constraints on the hydrothermal reactivity of organic acids and acid anions: I. Formic acid and formate
    McCollom, Thomas M.; Seewald, Jeffrey S.
    Geochimica et Cosmochimica Acta (2003),67 (19),3625-3644CODEN:GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    A series of hydrothermal expts. covering a range of temps. from 175 to 260°C examd. the decompn. of formic acid and formate and also investigated the prodn. of formate from redn. of CO2. Decompn. rates measured in this study, which were conducted in gold-TiO2 reactors, were several orders of magnitude slower than those reported in previous studies conducted in steel and Ti-metal reactors, indicating the previous studies substantially overestimated the rate of the reaction owing to reactor catalysis. Although expts. were conducted with several different minerals present (hematite, magnetite, serpentinized olivine, NiFe-alloy), the decompn. rates were similar in each expt. once the effects of fluid pH were accounted for, suggesting that the minerals had no effect on the stability of formic acid or formate. At higher temps. (> 225°C), the rates of both the decompn. of formate and the redn. of CO2 to formate were sufficiently rapid that reactions between dissolved CO2 and formate rapidly attained a state of metastable thermodn. equil. The results suggest that the amt. of formate in many subsurface and hydrothermal fluids is likely to be controlled by equil. with dissolved CO2 at the prevailing oxidn. state and pH of the fluid. This may account for the high concns. of formate obsd. in strongly reducing environments such as serpentinites, as well as the low concns. relative to other org. acid anions in mildly reducing environments such as oil-filled brines and formation waters in sedimentary basins. Although formate has been suggested to be a reaction intermediate in the formation of abiotic hydrocarbons from redn. of aq. CO2, prodn. of hydrocarbons was not obsd. in any of the expts., except for trace amts. of methane, despite high concns. of formate and strongly reducing conditions.
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    McCollom, T. M.; Seewald, J. S. Experimental Study of the Hydrothermal Reactivity of Organic Acids and Acid Anions: II. Acetic Acid, Acetate, and Valeric Acid. Geochim. Cosmochim. Acta 2003, 67 (19), 3645– 3664, DOI: 10.1016/S0016-7037(03)00135-2
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    Experimental study of the hydrothermal reactivity of organic acids and acid anions: II. Acetic acid, acetate, and valeric acid
    McCollom, Thomas M.; Seewald, Jeffrey S.
    Geochimica et Cosmochimica Acta (2003),67 (19),3645-3664CODEN:GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    Org. acids and acid anions occur in substantial concns. in many aq. geol. fluids and are thought to take part in a variety of geochem. processes ranging from the transport of metals in ore-forming fluids to the formation of natural gas to serving as a metabolic energy source for microbes in subsurface habitats. The widespread occurrence of org. acids and their potential role in diverse geol. processes has led to numerous exptl. studies of their thermal stability, yet there remain substantial gaps in our knowledge of the factors that control the rates and reaction pathways for the decompn. of these compds. under geol. conditions. In order to address some of these uncertainties, a series of lab. expts. were conducted to examine the behavior of org. acids and acid anions under hydrothermal conditions in the presence of minerals. Results are reported of expts. in which aq. solns. of acetic acid, sodium acetate, or valeric acid (n-pentanoic acid) were heated at 325°C, 350 bars in the presence of the mineral assemblages hematite + magnetite + pyrite, pyrite + pyrrhotite + magnetite, and hematite + magnetite. The results indicate that aq. acetic acid and acetate decomp. by a combination of two reaction pathways: decarboxylation and oxidn. Both reactions are promoted by minerals, with hematite catalyzing the oxidn. reaction while magnetite catalyzes decarboxylation. The oxidn. reaction is much faster, so that oxidn. dominates the decompn. of acetic acid and acetate when hematite is present. In contrast to previous reports that acetate decompd. more slowly than acetic acid, acetate decompd. at slightly faster rates than the acid in the presence of minerals. Although longer-chain monocarboxylic acids are generally thought to decomp. by decarboxylation, valeric acid appeared to decomp. primarily by "deformylation" to 1-butene plus formic acid. Subsequent decompn. of 1-butene and formic acid generated a variety of short-chain (≤C4) hydrocarbons and moncarboxylic acids as well as CO2. Valeric acid decompn. proceeded more rapidly (by a factor of 2) in the presence of hematite-magnetite-pyrite than with the other mineral assemblages, with the greater reaction rate apparently attributable to the effects of fluid chem. Valeric acid was obsd. to decomp. at a substantially faster rate than acetic acid under similar conditions. The results suggest that decompn. of aq. monocarboxylic acids may make a significant contribution to the conversion of petroleum to light hydrocarbons in natural gas and thermal fluids.
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    McCollom, T. M.; Seewald, J. S. Carbon Isotope Composition of Organic Compounds Produced by Abiotic Synthesis under Hydrothermal Conditions. Earth and Planetary Science Letters 2006, 243 (1), 74– 84, DOI: 10.1016/j.epsl.2006.01.027
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    Carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions
    McCollom, Thomas M.; Seewald, Jeffrey S.
    Earth and Planetary Science Letters (2006),243 (1-2),74-84CODEN:EPSLA2; ISSN:0012-821X. (Elsevier B.V.)
    Although it is widely believed that prodn. of org. compds. by Fischer-Tropsch synthesis and related processes occurs in many geol. environments, unambiguous identification of compds. with an abiotic origin in natural samples has been hampered by a lack of means to discriminate between abiotic compds. and org. matter from biol. sources. While isotopic compns. might provide a means to discriminate between biol. and non-biol. sources of org. matter, there are few data presently available to constrain the isotopic compn. of compds. produced by abiotic processes in geol. systems. Here, we report results of lab. expts. conducted to evaluate the isotopic compn. of org. compds. synthesized abiotically under hydrothermal conditions. We find the org. products are depleted in 13C to a degree typically ascribed to biol. processes, indicating that carbon isotopic compn. may not be a particularly effective diagnostic means to differentiate between biol. and non-biol. sources. Furthermore, our results suggest that the isotopic compns. of reduced carbon compds. found in many ancient rocks that have heretofore been attributed to biol. sources could be consistent with an abiotic origin in a hydrothermal setting.
49. 49
    McCollom, T. M.; Ritter, G.; Simoneit, B. R. T. Lipid Synthesis Under Hydrothermal Conditions by Fischer- Tropsch-Type Reactions. Orig Life Evol Biosph 1999, 29 (2), 153– 166, DOI: 10.1023/A:1006592502746
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    Lipid synthesis under hydrothermal conditions by Fischer-Tropsch-type reactions
    Mccollom, Thomas M.; Ritter, Gilles; Simoneit, Bernd R. T.
    Origins of Life and Evolution of the Biosphere (1999),29 (2),153-166CODEN:OLEBEM; ISSN:0169-6149. (Kluwer Academic Publishers)
    Ever since their discovery in the late 1970's, mid-ocean-ridge hydrothermal systems have received a great deal of attention as a possible site for the origin of life on Earth (and environments analogous to mid-ocean-ridge hydrothermal systems are postulated to have been sites where life could have originated on Mars and elsewhere as well). Because no modern-day terrestrial hydrothermal systems are free from the influence of org. compds. derived from biol. processes, lab. expts. provide the best opportunity for confirmation of the potential for org. synthesis in hydrothermal systems. Here we report on the formation of lipid compds. during Fischer-Tropsch-type synthesis from aq. solns. of formic acid or oxalic acid. Optimum synthesis occurs in stainless steel vessels by heating at 175° for 2-3 days and produces lipid compds. ranging from C2 to >C35 which consist of n-alkanols, n-alkanoic acids, n-alkenes, n-alkanes and alkanones. The precursor carbon sources used are either formic acid or oxalic acid, which disproportionate to H2, CO2 and probably CO. Both carbon sources yield the same lipid classes with essentially the same ranges of compds. The synthesis reactions were confirmed by using 13C labeled precursor acids.
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    Nooner, D. W.; Gibert, J. M.; Gelpi, E.; Oró, J. Closed System Fischer–Tropsch Synthesis over Meteoritic Iron, Iron Ore and Nickel-Iron Alloy. Geochim. Cosmochim. Acta 1976, 40 (8), 915– 924, DOI: 10.1016/0016-7037(76)90140-X
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    Closed system Fischer-Tropsch synthesis over meteoritic iron, iron ore and nickel-iron alloy
    Nooner, D. W.; Gibert, J. M.; Gelpi, E.; Oro, J.
    Geochimica et Cosmochimica Acta (1976),40 (8),915-24CODEN:GCACAK; ISSN:0016-7037.
    Meteoritic Fe, Fe ore, and Ni-Fe alloy, either alone or mixed with alumina, carbonaceous chondrite, K2CO3, or Na2CO3, were used to catalyze the reaction of D and CO in a closed reaction vessel, as a possible model for the synthesis of org. matter in the solar nebula and in the parent bodies of carbonaceous chondrites. The mole ratios of D to CO were 0.5:1-10:1, the reaction temps. 195-370°, and the reaction times 6-480 hr. Anal. of the reaction products showed that normal C11-C25 alkanes and alkenes, their mono-Me substituted isomers, and arom. hydrocarbons, e.g. naphthalene, acenaphthene, fluorene, phenanthrene, and their Me derivs., were formed. One reaction product contained perdeutero normal fatty acids (10:0-16:0).
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    Rushdi, A. I.; Simoneit, B. R. T. Lipid Formation by Aqueous Fischer–Tropsch-Type Synthesis over a Temperature Range of 100 to 400 °C. Orig Life Evol Biosph 2001, 31 (1), 103– 118, DOI: 10.1023/A:1006702503954
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    Lipid formation by aqueous Fischer-Tropsch-type synthesis over a temperature range of 100 to 400°C
    Rushdi, Ahmed I.; Simoneit, Bernd R. T.
    Origins of Life and Evolution of the Biosphere (2001),31 (1-2),103-118CODEN:OLEBEM; ISSN:0169-6149. (Kluwer Academic Publishers)
    The formation of lipid compds. during an aq. Fischer-Tropsch-type reaction was studied with solns. of oxalic acid as the carbon and hydrogen source. The reactions were conducted in stainless steel vessels by heating the oxalic acid soln. at discrete temps. from 100 to 400°C, at intervals of 50°C for two days each. The max. lipid yield, esp. for oxygenated compds., is in the window of 150-250°C. At a temp. of 100°C only a trace amt. of lipids was detected. At temps. above 150°C the lipid components ranged from C12 to > C33 and included n-alkanols, n-alkanoic acids, n-alkyl formates, n-alkanals, n-alkanones, n-alkanes, and n-alkenes, all with essentially no carbon no. preference. The n-alkanes increased in concn. over the oxygenated compds. at temps. of 200°C and above, with a slight redn. in their carbon no. ranges due to cracking. The n-alkanoic acids increased while n-alkanols decreased with increasing temp. above 200°C. At temps. above 300°C synthesis competes with cracking and reforming reactions. At 400°C significant cracking was obsd. and polynuclear arom. hydrocarbons and their alkylated homologs were detected. The formation of lipid compds. by aq. FTT reactions proceeds by insertion of a CO group at the terminal end of a carboxylic acid functionality to form n-oxoalkanoic acids, followed by redn. to n-alkanoic acids, to n-alkanals, then to n-alkanols. The n-alkenes are intermediate homologs for n-alkan-2-ones and n-alkanes. This proposed mechanism for aq. FTT synthesis differs from the surface-catalyzed stepwise FT process (i.e., gaseous) of polymn. of methylene reported in the literature.
52. 52
    Rushdi, A. I.; Simoneit, B. R. T. Condensation Reactions and Formation of Amides, Esters, and Nitriles Under Hydrothermal Conditions. Astrobiology 2004, 4 (2), 211– 224, DOI: 10.1089/153110704323175151
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    Condensation reactions and formation of amides, esters, and nitriles under hydrothermal conditions
    Rushdi, Ahmed I.; Simoneit, Bernd R. T.
    Astrobiology (2004),4 (2),211-224CODEN:ASTRC4; ISSN:1531-1074. (Mary Ann Liebert, Inc.)
    Hydrothermal pyrolysis expts. were performed to assess condensation (dehydration) reactions to amide, ester, and nitrile functionalities from lipid precursors. Beside product formation, org. compd. alteration and stability were also evaluated. Mixts. of nonadecanoic acid, hexadecanedioic acid, or hexadecanamide with water, ammonium bicarbonate, and oxalic acid were heated at 300°C for 72 h. In addn., mixts. of ammonium bicarbonate and oxalic acid solns. were used to test the abiotic formation of org. nitrogen compds. at the same temp. The resulting products were condensation compds. such as amides, nitriles, and minor quantities of N-methylalkyl amides, alkanols, and esters. Mixts. of alkyl amide in water or oxalic acid yielded mainly hydrolysis and dehydration products, and with ammonium bicarbonate and oxalic acid the yield of condensation products was enhanced. The synthesis expts. with oxalic acid and ammonium bicarbonate solns. yielded homologous series of alkyl amides, alkyl amines, alkanes, and alkanoic acids, all with no carbon no. predominances. These org. nitrogen compds. are stable and survive under the elevated temps. of hydrothermal fluids.
53. 53
    Mißbach, H.; Schmidt, B. C.; Duda, J.-P.; Lünsdorf, N. K.; Goetz, W.; Thiel, V. Assessing the Diversity of Lipids Formed via Fischer–Tropsch-Type Reactions. Org. Geochem. 2018, 119, 110– 121, DOI: 10.1016/j.orggeochem.2018.02.012
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    Assessing the diversity of lipids formed via Fischer-Tropsch-type reactions
    Missbach, Helge; Schmidt, Burkhard C.; Duda, Jan-Peter; Luensdorf, Nils Keno; Goetz, Walter; Thiel, Volker
    Organic Geochemistry (2018),119 (),110-121CODEN:ORGEDE; ISSN:0146-6380. (Elsevier Ltd.)
    Lipid biomarkers are commonly used for tracking life through Earth's history and are also gaining in importance in the search for extraterrestrial life. However, some lipids may also be formed in-situ via abiotic Fischer-Tropsch-type (FTT) reactions. These processes have been considered as a source of prebiotic org. matter. Here we report on a FTT synthesis expt. performed under hydrothermal conditions, focusing on more complex, previously undescribed FTT reaction products that may potentially mimic biol. signals. The expt. was carried out in stainless steel reactors by heating aq. solns. of oxalic acid mixed with montmorillonite to 175 °C for 3 days. Org. exts. of the products and extn. residues were analyzed by gas chromatog.-mass spectrometry (GC-MS) and combustion-IR detection, attenuated total reflectance Fourier transform IR spectroscopy and pyrolysis GC-MS. FTT reactions led to a no. of biomarker-like lipids such as linear and methyl-branched alkanes and alkanols as well as n-alkanoic acids. However, FTT reactions a priori produce unimodal chain length distributions and isomeric mixts. of methyl-branched compds., as opposed to biolipids which typically show preferences of individual homologs and/or isomers. Exptl. maturation of an immature kerogen demonstrated how these biol. signatures may pass into unimodal distributions similar to FTT reaction products by thermal overprint. Therefore, discrimination of biol. derived compds. from FTT orgs. may become increasingly problematic with ongoing thermal maturation. Extn. residues of FTT reaction products contained org. material, possibly in form of an org. polymer. However, it remains to be tested whether these residual orgs. can evolve into kerogen-like material.
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    Bonfio, C.; Caumes, C.; Duffy, C. D.; Patel, B. H.; Percivalle, C.; Tsanakopoulou, M.; Sutherland, J. D. Length-Selective Synthesis of Acylglycerol-Phosphates through Energy-Dissipative Cycling. J. Am. Chem. Soc. 2019, 141 (9), 3934– 3939, DOI: 10.1021/jacs.8b12331
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    Length-Selective Synthesis of Acylglycerol-Phosphates through Energy-Dissipative Cycling
    Bonfio, Claudia; Caumes, Cecile; Duffy, Colm D.; Patel, Bhavesh H.; Percivalle, Claudia; Tsanakopoulou, Maria; Sutherland, John D.
    Journal of the American Chemical Society (2019),141 (9),3934-3939CODEN:JACSAT; ISSN:0002-7863. (American Chemical Society)
    The main aim of origins of life research is to find a plausible sequence of transitions from prebiotic chem. to nascent biol. In this context, understanding how and when phospholipid membranes appeared on early Earth is crit. to elucidating the prebiotic pathways that led to the emergence of primitive cells. Here we show that exposing glycerol-2-phosphate to acylating agents leads to the formation of a library of acylglycerol-phosphates. Medium-chain acylglycerol-phosphates were found to self-assemble into vesicles stable across a wide range of conditions and capable of retaining mono- and oligonucleotides. Starting with a mixt. of activated carboxylic acids of different lengths, iterative cycling of acylation and hydrolysis steps allowed for the selection of longer-chain acylglycerol-phosphates. Our results suggest that a selection pathway based on energy-dissipative cycling could have driven the selective synthesis of phospholipids on early Earth.
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    Zahnle, K. J.; Lupu, R.; Catling, D. C.; Wogan, N. Creation and Evolution of Impact-Generated Reduced Atmospheres of Early Earth. Planet. Sci. J. 2020, 1 (1), 11, DOI: 10.3847/PSJ/ab7e2c
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    Citron, R. I.; Stewart, S. T. Large Impacts onto the Early Earth: Planetary Sterilization and Iron Delivery. arXiv 2022, arXiv:2201.09349; https://arxiv.org/abs/2201.09349.
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    Benner, S. A.; Bell, E. A.; Biondi, E.; Brasser, R.; Carell, T.; Kim, H.-J.; Mojzsis, S. J.; Omran, A.; Pasek, M. A.; Trail, D. When Did Life Likely Emerge on Earth in an RNA-First Process?. ChemSystemsChem. 2020, 2 (2), e1900035, DOI: 10.1002/syst.202000009
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    When Did Life Likely Emerge on Earth in an RNA-First Process?
    Benner, Steven A.; Bell, Elizabeth A.; Biondi, Elisa; Brasser, Ramon; Carell, Thomas; Kim, Hyo-Joong; Mojzsis, Stephen J.; Omran, Arthur; Pasek, Matthew A.; Trail, Dustin
    ChemSystemsChem (2020),2 (2),e1900035CODEN:CSCHCD; ISSN:2570-4206. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The widespread presence of RNA (RNA) catalysts and cofactors in the Earth's biosphere today suggests that RNA was the first biopolymer to support Darwinian evolution. However, most "path-hypotheses" to generate building blocks for RNA require reduced nitrogen-contg. compds. not made in useful amts. in the CO2-N2-H2O atmospheres of the Hadean. We review models for Earth's impact history that invoke a single ∼1023 kg impactor (Moneta) to account for measured amts. of platinum, gold, and other siderophilic ("iron-loving") elements on the Earth and Moon. If it were the last sterilizing impactor, by reducing the atm. but not the mantle Moneta, would have opened a "window of opportunity" for RNA synthesis, a period when RNA precursors rained from the atm. onto land holding oxidized minerals that stabilize advanced RNA precursors and RNA. Surprisingly, this combination of physics, geol., and chem. suggests a time when RNA formation was most probable, ∼120±100 million years after Moneta's impact, or ∼4.36±0.1 billion years ago. Uncertainties in this time are driven by uncertainties in rates of productive atm. loss and amts. of sub-aerial land.
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    Zhou, L.-P.; Hao, X.; Gao, J.-H.; Yang, Y.; Wu, B.-S.; Xu, J.; Xu, Y.-Y.; Li, Y.-W. Studies and Discriminations of the Kinetic Models for the Iron-Based Fischer–Tropsch Catalytic Reaction in a Recycle Slurry Reactor. Energy Fuels 2011, 25 (1), 52– 59, DOI: 10.1021/ef101270u
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    Studies and Discriminations of the Kinetic Models for the Iron-Based Fischer-Tropsch Catalytic Reaction in a Recycle Slurry Reactor
    Zhou, Li-Ping; Hao, Xu; Gao, Jun-Hu; Yang, Yong; Wu, Bao-Shan; Xu, Jian; Xu, Yuan-Yuan; Li, Yong-Wang
    Energy & Fuels (2011),25 (1),52-59CODEN:ENFUEM; ISSN:0887-0624. (American Chemical Society)
    The Fischer-Tropsch synthesis kinetic rates were studied by discussing the hydrogenation steps of the favored hydrogen-assisted CO dissocn. mechanism and the surface site balance over an industrial iron-based catalyst. Their math. expressions are the two proposed models -rFT = (kFTPCOPH2α)/(kCOPCO + .sum.i=1ixi) and -rFT = (kFTPCOPH2α)/((kCOPCO + .sum.i=1ixi)2) in the literature. Unresolved relationships and differences between the two models are addressed here and analyzed specifically. When the significance of each of the terms in the two rival models is critically evaluated, both surface site balances of the two could be simplified to two involved terms: an adsorbed CO term and a vacant site term. Although the two models could hardly be distinguished from each other at the typical range of exptl. conditions, further theor. analyses and extrapolated exptl. data favor the latter [its simplified form is -rFT = (kFTPCOPH20.5)/((1 + kCOPCO)2)]. These studies are helpful to understand the CO activation processes, and some contradictory conclusions and exptl. phenomena in the literature can be explained consistently and reasonably.
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    Shock, E.; Canovas, P. The Potential for Abiotic Organic Synthesis and Biosynthesis at Seafloor Hydrothermal Systems. Geofluids 2011, 10 (1–2), 161– 192, DOI: 10.1002/9781444394900.ch12
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    Konn, C.; Charlou, J. L.; Donval, J. P.; Holm, N. G.; Dehairs, F.; Bouillon, S. Hydrocarbons and Oxidized Organic Compounds in Hydrothermal Fluids from Rainbow and Lost City Ultramafic-Hosted Vents. Chem. Geol. 2009, 258 (3), 299– 314, DOI: 10.1016/j.chemgeo.2008.10.034
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    Hydrocarbons and oxidized organic compounds in hydrothermal fluids from Rainbow and Lost City ultramafic-hosted vents
    Konn, C.; Charlou, J. L.; Donval, J. P.; Holm, N. G.; Dehairs, F.; Bouillon, S.
    Chemical Geology (2009),258 (3-4),299-314CODEN:CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    The first building blocks of life could be produced in ultramafic-hosted hydrothermal systems considering the large amts. of hydrogen and methane generated by serpentinization and Fischer-Tropsch-type synthesis, resp., in those systems. The purpose of this study was to detect and characterize org. mols. in hydrothermal fluids from ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge (MAR) region. During the EXOMAR cruise 2005, fluids from the Rainbow (36°14'N) and the Lost City (30°N) hydrothermal fields were collected and treated by Stir Bar Sorptive Extn. (SBSE) and Solid Phase Extn. (SPE). The exts. were analyzed by Thermal Desorption-Gas Chromatog.-Mass Spectrometry (TD-GC-MS) and GC-MS, resp. Compared to nearby deep seawater, hydrothermal fluids were clearly enriched in org. compds., with a more diverse spectrum of mols. We obsd. a very similar range of org. compds. in fluids from both sites, with a dominance of aliph. hydrocarbons (C9-C14), arom. compds. (C6-C16) and carboxylic acids (C8-C18). The occurrence of these compds. is supported by other field studies on serpentinites and sulfide deposits. Literature on thermodn. data and exptl. work has suggested the possible abiogenic origin of hydrocarbons and org. acids. In addn., it has been shown elsewhere that catalytic reactions producing hydrocarbons likely occur at both Lost City and Rainbow hydrothermal fields as suggested by the evolution of δ 13C with increasing C no. for methane, ethane, propane and butane. In order to investigate the origin of the org. mols. in the fluids, compd.-specific carbon isotope ratio measurements were performed on n-alkanes and carboxylic acids, for which the δ 13C values were in the range of - 46 to - 20‰ (vs. V-PDB). These preliminary data did not allow conclusive support or rejection of an abiogenic origin of the compds. Indeed, predicting δ13C signatures in hydrothermal systems is likely to be complicated, due to differences in source δ13C signatures (i.e., of the C building blocks), and a variety of, mostly unknown, fractionation steps which may occur along the synthesis pathways. In addn., even though a fraction of the compds. detected in the fluids is likely abiotically produced, a dominance of biogenic sources and/or processes might hide their characteristic signature.
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    Bach, W.; Banerjee, N. R.; Dick, H. J. B.; Baker, E. T. Discovery of Ancient and Active Hydrothermal Systems along the Ultra-Slow Spreading Southwest Indian Ridge 10°–16°E. Geochemistry, Geophysics, Geosystems 2002, 3 (7), 1– 14, DOI: 10.1029/2001GC000279
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    McCollom, T. M.; Seewald, J. S. Abiotic Synthesis of Organic Compounds in Deep-Sea Hydrothermal Environments. Chem. Rev. 2007, 107 (2), 382– 401, DOI: 10.1021/cr0503660
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    Abiotic Synthesis of Organic Compounds in Deep-Sea Hydrothermal Environments
    McCollom, Thomas M.; Seewald, Jeffrey S.
    Chemical Reviews (Washington, DC, United States) (2007),107 (2),382-401CODEN:CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review concerning abiotic synthesis of org. compds., particularly hydrocarbons, and a summary of observations of natural system org. compds. which place constraints on abiotic input in deep sea hydrothermal environments is given. Topics discussed include: geochem. and thermodn. constraints on abiotic synthesis; abiotic org. synthesis pathways (overview of Fischer-Tropsch synthesis, exptl. studies of Fischer-Tropsch-type [FTT] synthesis under hydrothermal conditions, isotope fractionation during FTT synthesis, alternative pathways to org. synthesis in hydrothermal systems); geothermal evidence for abiotic org. compds. in hydrothermal fluids; and concluding remarks.
63. 63
    Wilcox, E. M.; Roberts, G. W.; Spivey, J. J. Direct Catalytic Formation of Acetic Acid from CO2 and Methane. Catal. Today 2003, 88 (1), 83– 90, DOI: 10.1016/j.cattod.2003.08.007
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    Direct catalytic formation of acetic acid from CO2 and methane
    Wilcox, Esther M.; Roberts, George W.; Spivey, James J.
    Catalysis Today (2003),88 (1-2),83-90CODEN:CATTEA; ISSN:0920-5861. (Elsevier Science B.V.)
    The direct synthesis of acetic acid from methane and carbon dioxide was investigated. Diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) expts. showed the formation of an adsorbed acetate on both a 5% Pd/carbon and a 5% Pt/alumina catalyst when the catalyst was exposed to a mixt. of methane and carbon dioxide at a temp. of about 400 °C. Temp. programmed reaction (TPR) expts. showed the formation of gas phase acetic acid from carbon dioxide and methane at about 400 °C over a 5% Pt/alumina catalyst.
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    Cody, G. D.; Boctor, N. Z.; Brandes, J. A.; Filley, T. R.; Hazen, R. M.; Yoder, H. S. Assaying the Catalytic Potential of Transition Metal Sulfides for Abiotic Carbon Fixation 1 1Associate Editor: P. A. O’Day. Geochim. Cosmochim. Acta 2004, 68 (10), 2185– 2196, DOI: 10.1016/j.gca.2003.11.020
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    Assaying the catalytic potential of transition metal sulfides for abiotic carbon fixation
    Cody, G. D.; Boctor, N. Z.; Brandes, J. A.; Filley, T. R.; Hazen, R. M.; Yoder, H. S.
    Geochimica et Cosmochimica Acta (2004),68 (10),2185-2196CODEN:GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    A suite of nickel, cobalt, iron, copper, and zinc contg. sulfides are assayed for the promotion of a model carbon fixation reaction with relevance to local reducing environments of the early Earth. The assay tests the promotion of hydrocarboxylation (the Koch reaction) wherein a carboxylic acid is synthesized via carbonyl insertion at a metal-sulfide-bound alkyl group. The exptl. conditions are chosen for optimal assay, i.e., high reactant concns. and pressures (200 MPa) to enhance chemisorption, and high temp. (250°C) to enhance reaction kinetics. All of the metal sulfides studied, with the exception CuS, promote hydrocarboxylation. Two other significant reactions involve the catalytic redn. of CO to form a surface-bound Me group, detected after nucleophilic attack by nonane thiol to form Me nonyl sulfide, and the formation of dinonyl sulfide via a similar reaction. Estn. of the catalytic turnover frequencies for each of the metal sulfides with respect to each of the primary reactions reveals that NiS, Ni3S2, and CoS perform comparably to commonly employed industrial catalysts. A pos. correlation between the yield of primary product to NiS and Ni3S2 surface areas provides strong evidence that the reactions are surface catalytic in these cases. The sulfides FeS and Fe(1-x)S are unique in that they exhibit evidence of extensive dissoln., thus, complicating interpretation regarding heterogeneous vs. homogeneous catalysis. With the exception of CuS, each of the metal sulfides promotes reactions that mimic key intermediate steps manifest in the mechanistic details of an important autotrophic enzyme, acetyl-CoA synthase. The relatively high temps. chosen for assaying purposes, however, are incompatible with the accumulation of thioesters. The results of this study support the hypothesis that transition metal sulfides may have provided useful catalytic functionality for geochem. carbon fixation in a prebiotic world (at least intially) devoid of peptide-based enzymes.
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    He, C.; Tian, G.; Liu, Z.; Feng, S. A Mild Hydrothermal Route to Fix Carbon Dioxide to Simple Carboxylic Acids. Org. Lett. 2010, 12 (4), 649– 651, DOI: 10.1021/ol9025414
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    A Mild Hydrothermal Route to Fix Carbon Dioxide to Simple Carboxylic Acids
    He, Chao; Tian, Ge; Liu, Ziwei; Feng, Shouhua
    Organic Letters (2010),12 (4),649-651CODEN:ORLEF7; ISSN:1523-7060. (American Chemical Society)
    Redn. of carbon dioxide (CO2) under mild hydrothermal conditions was carried out in the presence of iron nanoparticles. In this reaction system, the iron nanoparticles not only act as the reducing agent but also catalyze the redn. of CO2 to form formic acid and acetic acid.
66. 66
    Rotelli, L.; Trigo-Rodríguez, J. M.; Moyano-Cambero, C. E.; Carota, E.; Botta, L.; Di Mauro, E.; Saladino, R. The Key Role of Meteorites in the Formation of Relevant Prebiotic Molecules in a Formamide/Water Environment. Sci. Rep 2016, 6 (1), 38888, DOI: 10.1038/srep38888
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    The key role of meteorites in the formation of relevant prebiotic molecules in a formamide/water environment
    Rotelli, Luca; Trigo-Rodriguez, Josep M.; Moyano-Cambero, Carles E.; Carota, Eleonora; Botta, Lorenzo; Di Mauro, Ernesto; Saladino, Raffaele
    Scientific Reports (2016),6 (),38888CODEN:SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    We show that carbonaceous chondrite meteorites actively and selectively catalyze the formation of relevant prebiotic mols. from formamide in aq. media. Specific catalytic behaviors are obsd., depending on the origin and compn. of the chondrites and on the type of water present in the system (activity: thermal > seawater > pure). We report the one-pot synthesis of all the natural nucleobases, of aminoacids and of eight carboxylic acids (forming, from pyruvic acid to citric acid, a continuous series encompassing a large part of the extant Krebs cycle). These data shape a general prebiotic scenario consisting of carbonaceous meteorites acting as catalysts and of a volcanic-like environment providing heat, thermal waters and formamide. This scenario also applies to the other solar system locations that experienced rich delivery of carbonaceous materials, and whose phys.-chem. conditions could have allowed chem. evolution.
67. 67
    Varma, S. J.; Muchowska, K. B.; Chatelain, P.; Moran, J. Native Iron Reduces CO2 to Intermediates and End-Products of the Acetyl-CoA Pathway. Nat. Ecol Evol 2018, 2 (6), 1019– 1024, DOI: 10.1038/s41559-018-0542-2
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    Native iron reduces CO2 to intermediates and end-products of the acetyl-CoA pathway
    Varma Sreejith J; Muchowska Kamila B; Chatelain Paul; Moran Joseph
    Nature ecology & evolution (2018),2 (6),1019-1024 ISSN:.
    Autotrophic theories for the origin of life propose that CO2 was the carbon source for primordial biosynthesis. Among the six known CO2 fixation pathways in nature, the acetyl-CoA (AcCoA; or Wood-Ljungdahl) pathway is the most ancient, and relies on transition metals for catalysis. Modern microbes that use the AcCoA pathway typically fix CO2 with electrons from H2, which requires complex flavin-based electron bifurcation. This presents a paradox: how could primitive metabolic systems have fixed CO2 before the origin of proteins? Here, we show that native transition metals (Fe(0), Ni(0) and Co(0)) selectively reduce CO2 to acetate and pyruvate-the intermediates and end-products of the AcCoA pathway-in near millimolar concentrations in water over hours to days using 1-40 bar CO2 and at temperatures from 30 to 100 °C. Geochemical CO2 fixation from native metals could have supplied critical C2 and C3 metabolites before the emergence of enzymes.
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    Miller, S. L. A Production of Amino Acids Under Possible Primitive Earth Conditions. Science 1953, 117 (3046), 528– 529, DOI: 10.1126/science.117.3046.528
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    Production of amino acids under possible primitive earth conditions
    Miller, Stanley L.
    Science (Washington, DC, United States) (1953),117 (),528-9CODEN:SCIEAS; ISSN:0036-8075.
    The hypothesis that org. compds. that served as the basis of life were formed when the earth had an atm. of CH4, NH3, H2O, and H2 was tested by circulating these gases past an elec. discharge for a week. A description of the app. is given. The resulting mixt. was assayed for amino acids by paper chromatography, α-Alanine and β-alanine were positively identified in significant amts. A tentative identification of aspartic acid, α-aminobutyric acid, and possibly β- and γ-amino acids was also made.
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    Johnson, A. P.; Cleaves, H. J.; Dworkin, J. P.; Glavin, D. P.; Lazcano, A.; Bada, J. L. The Miller Volcanic Spark Discharge Experiment. Science 2008, 322 (5900), 404– 404, DOI: 10.1126/science.1161527
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    The Miller Volcanic Spark Discharge Experiment
    Johnson, Adam P.; Cleaves, H. James; Dworkin, Jason P.; Glavin, Daniel P.; Lazcano, Antonio; Bada, Jeffrey L.
    Science (Washington, DC, United States) (2008),322 (5900),404CODEN:SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Miller's 1950s expts. used, besides the app. known in textbooks, one that generated a hot water mist in the spark flask, simulating a water vapor-rich volcanic eruption. We found the original exts. of this expt. in Miller's material and reanalyzed them. The volcanic app. produced a wider variety of amino acids than the classic one. Release of reduced gases in volcanic eruptions accompanied by lightning could have been common on the early Earth. Prebiotic compds. synthesized in these environments could have locally accumulated, where they could have undergone further processing.
70. 70
    Ferus, M.; Pietrucci, F.; Saitta, A. M.; Knížek, A.; Kubelík, P.; Ivanek, O.; Shestivska, V.; Civiš, S. Formation of Nucleobases in a Miller–Urey Reducing Atmosphere. Proc. Natl. Acad. Sci. USA 2017, 114 (17), 4306– 4311, DOI: 10.1073/pnas.1700010114
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    Formation of nucleobases in a Miller-Urey reducing atmosphere
    Ferus, Martin; Pietrucci, Fabio; Saitta, Antonino Marco; Knizek, Antonin; Kubelik, Petr; Ivanek, Ondrej; Shestivska, Violetta; Civis, Svatopluk
    Proceedings of the National Academy of Sciences of the United States of America (2017),114 (17),4306-4311CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The Miller-Urey expts. pioneered modern research on the mol. origins of life, but their actual relevance in this field was later questioned because the gas mixt. used in their research was considered too reducing with respect to the most accepted hypotheses for the conditions on primordial Earth. In particular, the prodn. of only amino acids has been taken as evidence of the limited relevance of the results. Here, we report exptl. work, combined with state-of-the-art computational methods, in which both elec. discharge and laser-driven plasma impact simulations were carried out in a reducing atm. contg. NH3 + CO. We show that RNA nucleobases are synthesized in these expts., strongly supporting the possibility of the emergence of biol. relevant mols. in a reducing atm. The reconstructed synthetic pathways indicate that small radicals and formamide play a crucial role, in agreement with a no. of recent exptl. and theor. results.
71. 71
    Criado-Reyes, J.; Bizzarri, B. M.; García-Ruiz, J. M.; Saladino, R.; Di Mauro, E. The Role of Borosilicate Glass in Miller–Urey Experiment. Sci. Rep 2021, 11 (1), 21009, DOI: 10.1038/s41598-021-00235-4
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    The role of borosilicate glass in Miller-Urey experiment
    Criado-Reyes, Joaquin; Bizzarri, Bruno M.; Garcia-Ruiz, Juan Manuel; Saladino, Raffaele; Di Mauro, Ernesto
    Scientific Reports (2021),11 (1),21009CODEN:SRCEC3; ISSN:2045-2322. (Nature Research)
    We have designed a set of expts. to test the role of borosilicate reactor on the yielding of the Miller-Urey type of expt. Two expts. were performed in borosilicate flasks, two in a Teflon flask and the third couple in a Teflon flask with pieces of borosilicate submerged in the water. The expts. were performed in CH4, N2, and NH3 atmosphere either buffered at pH 8.7 with NH4Cl or unbuffered solns. at pH ca. 11, at room temp. The Gas Chromatog.-Mass Spectroscopy results show important differences in the yields, the no. of products, and mol. wt. In particular, a dipeptide, multi-carbon dicarboxylic acids, PAHs, and a complete panel of biol. nucleobases form more efficiently or exclusively in the borosilicate vessel. Our results offer a better explanation of the famous Miller's expt. showing the efficiency of borosilicate in a triphasic system including water and the reduced Miller-Urey atm.
72. 72
    Allen, W. V.; Ponnamperuma, C. A Possible Prebiotic Synthesis of Monocarboxylic Acids. Biosystems 1967, 1 (1), 24– 28, DOI: 10.1016/0303-2647(67)90017-2
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    Dicarboxylic acids from electric discharge
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    Chloro and Me substituted, satd., and unsatd. dicarboxylic acids were detected by gas chromatog.-mass spectrometry during elec. discharge through a simulated primitive earth atm.
74. 74
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75. 75
    Orville, R. E. A High-Speed Time-Resolved Spectroscopic Study of the Lightning Return Stroke: Part II. A Quantitative Analysis. Journal of the Atmospheric Sciences 1968, 25 (5), 839– 851, DOI: 10.1175/1520-0469(1968)025<0839:AHSTRS>2.0.CO;2
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    Chyba, C.; Sagan, C. Electrical Energy Sources for Organic Synthesis on the Early Earth. Origins Life Evol Biosphere 1991, 21 (1), 3– 17, DOI: 10.1007/BF01809509
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    Electrical energy sources for organic synthesis on the early Earth
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    Much contemporary research on the origins of life continues to employ the original ests. for terrestrial energy dissipation by lightning and coronal discharges, 2 × 1019 J/yr and 6 × 1019 J/yr, resp., as given by S. L. Miller and H. C. Urey (1959). However, more recent work in terrestrial lightning and point discharge research suggests that these values are overests. by factors of about 20 and 120, resp. Calcd. concs. of amino acids (or other prebiotic org. products) in the early terrestrial oceans due to elec. discharge sources may therefore have been equally overestimated. A review of efficiencies for those expts. that provide good analogs to naturally-occurring lightning and coronal discharges suggests that lightning energy yields for org. synthesis (nmole J-1) are about one order of magnitude higher than those for coronal discharge. Therefore org. prodn. by lightning may be expected to have dominated that due to coronae on early Earth. Limited data available for prodn. of nitric oxide in clouds suggests that coronal emission within clouds, a source of energy heretofore too uncertain to be included in the total coronal energy inventory, is insufficient to change this conclusion. New recommended values for lightning and coronal discharge dissipation rates on the early Earth are, resp., 1 × 1018 J/yr-1 and 5 × 1017 J/yr-1.
77. 77
    Schlesinger, G.; Miller, S. L. Prebiotic Synthesis in Atmospheres Containing CH4, CO, and CO2. J. Mol. Evol 1983, 19 (5), 376– 382, DOI: 10.1007/BF02101642
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    Prebiotic synthesis in atmospheres containing methane, carbon monoxide and carbon dioxide. I. Amino acids
    Schlesinger, Gordon; Miller, Stanley L.
    Journal of Molecular Evolution (1983),19 (5),376-82CODEN:JMEVAU; ISSN:0022-2844.
    The prebiotic synthesis of org. compds. using a spark discharge on various simulated primitive earth atms. at 25° was studied. CH4 mixts. contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4 molar ratios of 0-4 and pNH3 = 0.1 torr. A similar set of expts. without added NH3 was performed. The yields of amino acids (1.2-4.7% based on C) are approx. independent of the H2/CH4 ratio and the presence of NH3. A wide variety of amino acids is obtained. Mixts. of H2 + CO + H2O + N2 and H2 + CO2 + H2O + N2, with and without added NH3, all gave ∼2% yields of amino acids at H2/CO and H2/CO2 ratios of 2-4. For a H2/CO2 ratio of 0, the yield of amino acids is extremely low (10-3%). Glycine is almost the only amino acid produced from CO and CO2 model atms. Thus, the max. yield is about the same for the 3 C sources at high H2/C ratios, but CH4 is superior at low H2/C ratios. In addn., CH4 gives a much greater variety of amino acids than either CO or CO2. If an abundance of amino acids more complex than glycine was required for the origin of life, CH4 probably was required in the primitive atm.
78. 78
    Roldan, A.; Hollingsworth, N.; Roffey, A.; Islam, H.-U.; Goodall, J. B. M.; Catlow, C. R. A.; Darr, J. A.; Bras, W.; Sankar, G.; Holt, K. B.; Hogarth, G.; de Leeuw, N. H. Bio-Inspired CO2 Conversion by Iron Sulfide Catalysts under Sustainable Conditions. Chem. Commun. 2015, 51 (35), 7501– 7504, DOI: 10.1039/C5CC02078F
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    Chemical Communications (Cambridge, United Kingdom) (2015),51 (35),7501-7504CODEN:CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The mineral greigite presents similar surface structures to the active sites found in many modern-day enzymes. We show that particles of greigite can reduce CO2 under ambient conditions into chems. such as methanol, formic, acetic and pyruvic acid. Our results also lend support to the Origin of Life theory on alk. hydrothermal vents.
79. 79
    Groth, W. E.; Weyssenhoff, H. v. Photochemical Formation of Organic Compounds from Mixtures of Simple Gases. Planetary and Space Science 1960, 2 (2), 79– 85, DOI: 10.1016/0032-0633(60)90001-5
80. 80
    Kaiser, R. I.; Gabrysch, A.; Roessler, K. Cosmic Ray Simulator: A Versatile Apparatus for Quantitative Studies on the Interaction of Cosmic Rays with Frozen Solids by on Line and in Situ Quadrupole Mass Spectrometry and Fourier Transform Infrared Spectroscopy. Rev. Sci. Instrum. 1995, 66 (4), 3058– 3066, DOI: 10.1063/1.1145529
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    Cosmic ray simulator: a versatile apparatus for quantitative studies on the interaction of cosmic rays with frozen solids by on line and in situ quadrupole mass spectrometry and Fourier transform infrared spectroscopy
    Kaiser, R. I.; Gabrysch, A.; Roessler, K.
    Review of Scientific Instruments (1995),66 (4),3058-66CODEN:RSINAK; ISSN:0034-6748. (American Institute of Physics)
    The cosmic ray simulator consists of a 50 L cylindrical stainless steel chamber. A rotable cold finger milled of a Ag (111) monocrystal optimizes heat cond. and is connected to a programmable, closed cycle He refrigerator allowing temp. control of an attached Ag wafer between 10 and 340 K (±0.5 K). Oil-free ultrahigh vacuum (UHV) conditions of ≈10-10 mbar are provided by a membrane, drag, and cryo-pump, hence guaranteeing a vacuum system free of any contamination. Ice layers of defined crystal structures and reproducible thickness of (5 ± 1) μm are achieved by depositing gases, e.g., CH4, CD4, CD4/O2, and CH4/O2, with a computer-assisted thermo-valve on the cooled wafer. These frosts are irradiated at 10 and 50 K with 7.3 MeV protons and 9 MeV α particles of the compact cyclotron CV28 in Forschungszentrum Juelich up to doses of 150 eV per mol., i.e., simulating the distribution max. of galactic cosmic ray particles interacting with primordial matter in space during 0.7 × 109 yr. During the expts., gas phase and solid state were monitored for the 1st time quant. on line and in situ by a quadrupole mass spectrometer (QMS) via matrix interval arithmetic and a FTIR spectrometer (FTIR) in an absorption-reflection mode at 62.5°. For the 1st time, a cosmic ray simulator allows detailed and reproducible mechanistic studies on the interaction of cosmic ray particles with frozen gases in space based on pressure conditions (hydrocarbon free UHV conditions, the limitation of condensations of residual gases during an expt. to less than one monolayer), temp. regime (the use of Ag monocrystals, FTIR in reflection, optimized ion currents, and target thicknesses <5 μm restrict temp. increasing to 14 K), and defined target systems. In combination with two on line and in situ analyses techniques, i.e., FTIR and QMS, the machine yields unprecedented options such as computing the heating of the ice surfaces directly exposed to the ion beam by a calibrated QMS and a complete quantification of product distribution. Preliminary results indicate a strong temp.-dependent component of the reaction mechanisms in the frosts: surface layers are heated by impinging ions to (14 ± 1) K and yield (70%-100%) of higher mol. wt. species, such as C11D24, whereas 10 K regions produce majority of simpler hydrocarbons, e.g., C3D8. Second, O2 contaminations influence the expts. dramatically by trapping of diffusive H atoms as O2 H and, thus, yield O-contg. yellow to brown residues after heating to 293 K. Irradn. of pure methane targets, however, produce no residues. But an increasing concn. of H atoms exceeding (6% ± 3%) leads to ejection of up to 90% of the frosts into vacuum.
81. 81
    Kim, Y. S.; Kaiser, R. I. Abiotic Formation of Carboxylic Acids (RCOOH) in Interstellar and Solar System Model Ices. ApJ. 2010, 725 (1), 1002– 1010, DOI: 10.1088/0004-637X/725/1/1002
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    Abiotic formation of carboxylic acids (RCOOH) in interstellar and solar system model ices
    Kim, Y. S.; Kaiser, R. I.
    Astrophysical Journal (2010),725 (1, Pt. 1),1002-1010CODEN:ASJOAB; ISSN:0004-637X. (Institute of Physics Publishing)
    The present lab. study simulated the abiotic formation of carboxylic acids (RCOOH) in interstellar and solar system model ices of carbon dioxide (CO2)-hydrocarbon mix CnH2n + 2 (n = 1-6). The pristine model ices were irradiated at 10 K under contamination-free, ultrahigh vacuum conditions with energetic electrons generated in the track of galactic cosmic-ray particles. The chem. processing of the ices was monitored by a Fourier transform IR spectrometer and a quadrupole mass spectrometer during the irradn. phase and subsequent warm-up phases on line and in situ in order to ext. qual. (carriers) and quant. (rate consts. and yields) information on the newly synthesized species. Carboxylic acids were identified to be the main carrier, together with carbon monoxide (CO) and a trace of formyl (HCO) and hydroxycarbonyl (HOCO) radicals at 10 K. The upper limit of acid column d. at 10 K was estd. as much as (1.2 ± 0.1) × 1017 mols. cm-2 at doses of 17 ± 2 eV mol.-1, or the yield of 39% ± 4% from the initial column d. of carbon dioxide. The temporal column d. profiles of the products were then numerically fit using two independent kinetic schemes of reaction mechanisms. Finally, we transfer this lab. simulation to star-forming regions of the interstellar medium, wherein cosmic-ray-induced processing of icy grains at temps. as low as 10 K could contribute to the current level of chem. complexity as evidenced in astronomical observations and in exts. of carbonaceous meteorites.
82. 82
    Blagojevic, V.; Petrie, S.; Bohme, D. K. Gas-Phase Syntheses for Interstellar Carboxylic and Amino Acids. Mon. Not. R. Astron. Soc. 2003, 339 (1), L7– L11, DOI: 10.1046/j.1365-8711.2003.06351.x
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    Gas-phase syntheses for interstellar carboxylic and amino acids
    Blagojevic, Voislav; Petrie, Simon; Bohme, Diethard K.
    Monthly Notices of the Royal Astronomical Society (2003),339 (1),L7-L11CODEN:MNRAA4; ISSN:0035-8711. (Blackwell Publishing Ltd.)
    We report exptl. results that demonstrate gas-phase, ionic syntheses of glycine and β-alanine, as well as acetic and propanoic acid, from smaller mols. found in space; in doing so, we infer the formation of these acids in the interstellar environment. We show that ionized glycine and β-alanine are produced in the reactions of hydroxylamine ions, NH2OH+, with acetic and propanoic acid resp. Even more promising in the context of interstellar synthesis are our results that demonstrate the corresponding prodn. of the protonated amino acids from analogous reactions with protonated hydroxylamine. The striking specificity of these syntheses for the β-isomer of alanine suggests that the amino acids of CI (Carbonaceous Ivona) chondrite meteorites are products of interstellar chem. and supports the hypothesis that these meteorites are of cometary origin.
83. 83
    Lowe, C. U.; Rees, M. W.; Markham, R. Synthesis of Complex Organic Compounds from Simple Precursors: Formation of Amino-Acids, Amino-Acid Polymers, Fatty Acids and Purines from Ammonium Cyanide. Nature 1963, 199 (4890), 219– 222, DOI: 10.1038/199219a0
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    Synthesis of complex organic compounds from simple precursors: formation of amino acids, amino acid polymers, fatty acids, and purines from ammonium cyanide
    Lowe, C. U.; Rees, M. W.; Markham, R.
    Nature (London, United Kingdom) (1963),199 (),219-22CODEN:NATUAS; ISSN:0028-0836.
    On the assumption that HCN and NH3 were present on the earth under the environmental conditions at the beginning of time, a series of combined org. compds. HCN were prepd. from these compds. (1.5 moles) was added to 1l. 1.5M aq. NH4OH, and the mixt. was heated to 90°. After 18 hrs., 30-40 g. of solid, amorphous, black ppt./l. was formed, and centrifuged. The supernatant liquid and the ppt. were sepd. directly or after acid hydrolysis. Of the total glycines, 7-14% was in the free state and the rest occurred in combination. Also found were arginine, α-alanine, β-alanine, α,β-diaminopropionic acid, serine, aspartic acid, glutamic acid, leucine, isoleucine, α-aminobutyric acid, threonine, urea, and guanidine. In the ppt., 3 polymers sepd. electrophoretically showed different amino acid compn. When H2S was added to the reaction mixt. of HCN and NH4OH, the proportions of the products formed were much changed. In addn. a total of 1 μmole of adenine and hypoxanthine/l. was isolated from the whole hydrolyzate, as well as a little formic acid. Guanine was not found.
84. 84
    Novotný, O.; Cejpek, K.; Velíšek, J. Formation of Carboxylic Acids during Degradation of Monosaccharides.. Czech J. Food Sci. 2008, 26 (2), 113– 131, DOI: 10.17221/2465-CJFS
85. 85
    Sato, K.; Hyodo, M.; Takagi, J.; Aoki, M.; Noyori, R. Hydrogen Peroxide Oxidation of Aldehydes to Carboxylic Acids: An Organic Solvent-, Halide- and Metal-Free Procedure. Tetrahedron Lett. 2000, 41 (9), 1439– 1442, DOI: 10.1016/S0040-4039(99)02310-2
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    Hydrogen peroxide oxidation of aldehydes to carboxylic acids: an organic solvent-, halide- and metal-free procedure
    Sato, Kazuhiko; Hyodo, Mamoru; Takagi, Junko; Aoki, Masao; Noyori, Ryoji
    Tetrahedron Letters (2000),41 (9),1439-1442CODEN:TELEAY; ISSN:0040-4039. (Elsevier Science Ltd.)
    Aq. hydrogen peroxide oxidizes aldehydes to carboxylic acids without affecting olefinic or alc. functions under aq./org. biphasic conditions in the presence of a quaternary ammonium salt.
86. 86
    Ranjan, S.; Sasselov, D. D. Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry. Astrobiology 2017, 17 (3), 169– 204, DOI: 10.1089/ast.2016.1519
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    Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry
    Ranjan, Sukrit; Sasselov, Dimitar D.
    Astrobiology (2017),17 (3),169-204CODEN:ASTRC4; ISSN:1557-8070. (Mary Ann Liebert, Inc.)
    The UV environment is a key boundary condition to abiogenesis. However, considerable uncertainty exists as to planetary conditions and hence surface UV at abiogenesis. Here, we present two-stream multilayer clear-sky calcns. of the UV surface radiance on Earth at 3.9 Ga to constrain the UV surface fluence as a function of albedo, solar zenith angle (SZA), and atm. compn. Variation in albedo and latitude (through SZA) can affect max. photoreaction rates by a factor of >10.4; for the same atm., photoreactions can proceed an order of magnitude faster at the equator of a snowball Earth than at the poles of a warmer world. Hence, surface conditions are important considerations when computing prebiotic UV fluences. For climatically reasonable levels of CO2, fluence shortward of 189 nm is screened out, meaning that prebiotic chem. is robustly shielded from variations in UV fluence due to solar flares or variability. Strong shielding from CO2 also means that the UV surface fluence is insensitive to plausible levels of CH4, O2, and O3. At scattering wavelengths, UV fluence drops off comparatively slowly with increasing CO2 levels. However, if SO2 and/or H2S can build up to the ≥1-100 ppm level as hypothesized by some workers, then they can dramatically suppress surface fluence and hence prebiotic photoprocesses. H2O is a robust UV shield for λ < 198 nm. This means that regardless of the levels of other atm. gases, fluence .ltorsim.198 nm is only available for cold, dry atmospheres, meaning sources with emission .ltorsim.198 (e.g., ArF excimer lasers) can only be used in simulations of cold environments with low abundance of volcanogenic gases. On the other hand, fluence at 254 nm is unshielded by H2O and is available across a broad range of , meaning that mercury lamps are suitable for initial studies regardless of the uncertainty in primordial H2O and CO2 levels.
87. 87
    Dworkin, J. P.; Deamer, D. W.; Sandford, S. A.; Allamandola, L. J. Self-Assembling Amphiphilic Molecules: Synthesis in Simulated Interstellar/Precometary Ices. Proc. Natl. Acad. Sci. USA 2001, 98 (3), 815– 819, DOI: 10.1073/pnas.98.3.815
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    Self-assembling amphiphilic molecules: synthesis in simulated interstellar/precometary ices
    Dworkin, Jason P.; Deamer, David W.; Sandford, Scott A.; Allamandola, Louis J.
    Proceedings of the National Academy of Sciences of the United States of America (2001),98 (3),815-819CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Interstellar gas and dust constitute the primary material from which the solar system formed. Near the end of the hot early phase of star and planet formation, volatile, less refractory materials were transported into the inner solar system as comets and interplanetary dust particles. Once the inner planets had sufficiently cooled, late accretionary infall seeded them with complex org. compds. Delivery of such extraterrestrial compds. may have contributed to the org. inventory necessary for the origin of life. Interstellar ices, the building blocks of comets, tie up a large fraction of the biogenic elements available in mol. clouds. In the efforts to understand their synthesis, chem. compn., and phys. properties, the authors report here that a complex mixt. of mols. is produced by UV photolysis of realistic, interstellar ice analogs, and that some of the components have properties relevant to the origin of life, including the ability to self-assemble into vesicular structures.
88. 88
    Ehrenfreund, P.; d’Hendecourt, L.; Charnley, S.; Ruiterkamp, R. Energetic and Thermal Processing of Interstellar Ices. Journal of Geophysical Research: Planets 2001, 106 (E12), 33291– 33301, DOI: 10.1029/2000JE001349
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    Energetic and thermal processing of interstellar ices
    Ehrenfreund, P.; d'Hendecourt, L.; Charnley, S.; Ruiterkamp, R.
    Journal of Geophysical Research, [Planets] (2001),106 (E12),33291-33301CODEN:JGPLEH; ISSN:1934-8592. (American Geophysical Union)
    A review. Our current knowledge on the chem. compn. of interstellar ices is summarized with respect to the possible contribution of energetic photons and particles to their obsd. state. We describe the inventory of astronomical ices as detd. by IR observations from the IR Space Observatory (ISO) and lab. spectroscopy. Sources of radiolysis, UV photolysis, and ice heating are then discussed in the context of the chem. state of material in dense mol. clouds and in protostellar cores. Through specific examples we show how energetic processing can explain the obsd. solid-state characteristics of several key mols.: CH3OH, CO2, and OCN-. We also discuss the gaseous and solid-state photochem. of the first org. acid detected in interstellar ices, HCOOH.
89. 89
    Pecoits, E.; Smith, M. L.; Catling, D. C.; Philippot, P.; Kappler, A.; Konhauser, K. O. Atmospheric Hydrogen Peroxide and Eoarchean Iron Formations. Geobiology 2015, 13 (1), 1– 14, DOI: 10.1111/gbi.12116
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    Atmospheric hydrogen peroxide and Eoarchean iron formations
    Pecoits, E.; Smith, M. L.; Catling, D. C.; Philippot, P.; Kappler, A.; Konhauser, K. O.
    Geobiology (2015),13 (1),1-14CODEN:GEOBCZ; ISSN:1472-4669. (Wiley-Blackwell)
    It is widely accepted that photosynthetic bacteria played a crucial role in Fe(II) oxidn. and the pptn. of iron formations (IF) during the Late Archean-Early Paleoproterozoic (2.7-2.4 Ga). It is less clear whether microbes similarly caused the deposition of the oldest IF at ca. 3.8 Ga, which would imply photosynthesis having already evolved by that time. Abiol. alternatives, such as the direct oxidn. of dissolved Fe(II) by UV radiation may have occurred, but its importance has been discounted in environments where the injection of high concns. of dissolved iron directly into the photic zone led to chem. pptn. reactions that overwhelmed photooxidn. rates. However, an outstanding possibility remains with respect to photochem. reactions occurring in the atm. that might generate hydrogen peroxide (H2O2), a recognized strong oxidant for ferrous iron. Here, we modeled the amt. of H2O2 that could be produced in an Eoarchean atm. using updated solar fluxes and plausible CO2, O2, and CH4 mixing ratios. Irresp. of the atm. simulations, the upper limit of H2O2 rainout was calcd. to be <106 mols. cm-2 s-1. Using conservative Fe(III) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H2O2 was insufficient by several orders of magnitude to account for IF deposition (requiring ∼1011 H2O2 mols. cm-2 s-1). This finding further constrains the plausible Fe(II) oxidn. mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe(II)-oxidizing micro-organisms the most likely mechanism responsible for Earth's oldest IF.
90. 90
    Omran, A.; Menor-Salvan, C.; Springsteen, G.; Pasek, M. The Messy Alkaline Formose Reaction and Its Link to Metabolism. Life 2020, 10 (8), 125, DOI: 10.3390/life10080125
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    The messy alkaline formose reaction and its link to metabolism
    Omran, Arthur; Menor-Salvan, Cesar; Springsteen, Greg; Pasek, Matthew
    Life (Basel, Switzerland) (2020),10 (8),125CODEN:LBSIB7; ISSN:2075-1729. (MDPI AG)
    Sugars are essential for the formation of genetic elements such as RNA and as an energy/food source. Thus, the formose reaction, which autocatalytically generates a multitude of sugars from formaldehyde, has been viewed as a potentially important prebiotic source of biomols. at the origins of life. When analyzing our formose solns. we fiend that many of the chem. species are simple carboxylic acids, including a-hydroxy acids, assocd. with metab. In this work we posit that the study of the formose reaction, under alk. conditions and moderate hydrothermal temps., should not be solely focused on sugars for genetic materials, but should focus on the origins of metab. (via metabolic mols.) as well.
91. 91
    Toner, J. D.; Catling, D. C. Alkaline Lake Settings for Concentrated Prebiotic Cyanide and the Origin of Life. Geochim. Cosmochim. Acta 2019, 260, 124– 132, DOI: 10.1016/j.gca.2019.06.031
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    Alkaline lake settings for concentrated prebiotic cyanide and the origin of life
    Toner, J. D.; Catling, D. C.
    Geochimica et Cosmochimica Acta (2019),260 (),124-132CODEN:GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Cyanide plays a crit. role in origin of life hypotheses that have received strong exptl. support from cyanide-driven synthesis of amino acids, nucleotides, and lipid precursors. However, relatively high cyanide concns. are needed. Such cyanide could have been supplied by reaction networks in which hydrogen cyanide in early Earth's atm. reacted with iron to form ferrocyanide salts, followed by thermal decompn. of ferrocyanide salts to cyanide. Using an aq. model supported by new exptl. data, we show that sodium ferrocyanide salts ppt. in closed-basin, alk. lakes over a wide range of plausible early Earth conditions. Such lakes were likely common on the early Earth because of chem. weathering of mafic or ultramafic rocks and evaporative concn. Subsequent thermal decompn. of sedimentary sodium ferrocyanide yields sodium cyanide (NaCN), which dissolves in water to form NaCN-rich solns. Thus, geochem. considerations newly identify a particular geol. setting and NaCN feedstock nucleophile for prebiotic chem.
92. 92
    Toner, J. D.; Catling, D. C. A Carbonate-Rich Lake Solution to the Phosphate Problem of the Origin of Life. Proc. Natl. Acad. Sci. USA 2020, 117 (2), 883– 888, DOI: 10.1073/pnas.1916109117
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    A carbonate-rich lake solution to the phosphate problem of the origin of life
    Toner, Jonathan D.; Catling, David C.
    Proceedings of the National Academy of Sciences of the United States of America (2020),117 (2),883-888CODEN:PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Phosphate is central to the origin of life because it is a key component of nucleotides in genetic mols., phospholipid cell membranes, and energy transfer mols. such as ATP. To incorporate phosphate into biomols., prebiotic expts. commonly use molar phosphate concns. to overcome phosphate's poor reactivity with orgs. in water. However, phosphate is generally limited to micromolar levels in the environment because it ppts. with calcium as low-soly. apatite minerals. This disparity between lab. conditions and environmental constraints is an enigma known as "the phosphate problem.". Here we show that carbonate-rich lakes are a marked exception to phosphate-poor natural waters. In principle, modern carbonate-rich lakes could accumulate up to ∼ 1 m phosphate under steady-state conditions of evapn. and stream inflow because calcium is sequestered into carbonate minerals. This prevents the loss of dissolved phosphate to apatite pptn. Even higher phosphate concns. (> 1 m) can form during evapn. in the absence of inflows. On the prebiotic Earth, carbonate-rich lakes were likely abundant and phosphate-rich relative to the present day because of the lack of microbial phosphate sinks and enhanced chem. weathering of phosphate minerals under relatively CO2-rich atmospheres. Furthermore, the prevailing CO2 conditions would have buffered phosphate-rich brines to moderate pH (pH 6.5 to 9). The accumulation of phosphate and other prebiotic reagents at concn. and pH levels relevant to exptl. prebiotic syntheses of key biomols. is a compelling reason to consider carbonate-rich lakes as plausible settings for the origin of life.
93. 93
    Sasselov, D. D.; Grotzinger, J. P.; Sutherland, J. D. The Origin of Life as a Planetary Phenomenon. Sci. Adv. 2020, 6 (6), eaax3419, DOI: 10.1126/sciadv.aax3419
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    Yoneda, N.; Kusano, S.; Yasui, M.; Pujado, P.; Wilcher, S. Recent Advances in Processes and Catalysts for the Production of Acetic Acid. Appl. Catal. A: General 2001, 221 (1), 253– 265, DOI: 10.1016/S0926-860X(01)00800-6
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    Recent advances in processes and catalysts for the production of acetic acid
    Yoneda, Noriyuki; Kusano, Satoru; Yasui, Makoto; Pujado, Peter; Wilcher, Steve
    Applied Catalysis, A: General (2001),221 (1-2),253-265CODEN:ACAGE4; ISSN:0926-860X. (Elsevier Science B.V.)
    A review on AcOH prodn. processes and catalyst development. The processes include: MeOH carbonylation, Me formate isomerization, synthesis gas conversion and vapor phase oxidn. of ethylene.
95. 95
    Berg, J. M.; Tymoczko, J. L.; Stryer, L. Biochemistry, 5th ed.; W. H. Freeman: New York, NY, 2002.
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    Origin of fatty acid synthesis: thermodynamics and kinetics of reaction pathways
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    Journal of Molecular Evolution (1991),32 (2),93-100CODEN:JMEVAU; ISSN:0022-2844.
    The primitiveness of contemporary fatty acid biosynthesis was evaluated by using the thermodn. and kinetics of its component reactions to est. the extent of its dependence on powerful and selective catalysis by enzymes. Since this anal. indicated that the modern pathway is not primitive because it requires sophisticated enzymic catalysis, an alternative pathway of primitive fatty acid synthesis that uses glycolaldehyde as a substrate is proposed. In contrast to the modern pathway, this primitive pathway is not dependent on an exogenous source of phosphoanhydride energy (ATP). Furthermore, the chem. spontaneity of its reactions suggests that it could have been readily catalyzed by the rudimentary biocatalysts available at an early stage in the origin of life.
97. 97
    Shock, E. L.; Schulte, M. D. Amino-Acid Synthesis in Carbonaceous Meteorites by Aqueous Alteration of Polycyclic Aromatic Hydrocarbons. Nature 1990, 343 (6260), 728– 731, DOI: 10.1038/343728a0
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    Amino-acid synthesis in carbonaceous meteorites by aqueous alteration of polycyclic aromatic hydrocarbons
    Shock, Everett L.; Schulte, Mitchell D.
    Nature (London, United Kingdom) (1990),343 (6260),728-31CODEN:NATUAS; ISSN:0028-0836.
    Calcns. were made of the distribution of aq. org. compds in metastable equil. with pyrene (I) and fluoranthene (II), as functions of the fugacities (f) of O2, CO2, and NH3, under conditions simulating those within meteorite parent bodies during alteration: 100°, 100 bar, and f of NH3 = 1 X 10-3, with initial activities of solid I and II of 10-2. Equil. consts. for reactions involving 76 aq. alkanes, alkenes, alkynes, alkylbenzenes, ketones, alcs., carboxylic acids, amines, and amino acids were calcd. with equations and data given by E.L. Shock and H.C. Helgeson (in press) and thermodn. data for I and II from D.R. Stull et al. (1969) and W.K. Wong and E.Rf. Westrum, Jr. (1971). With decreasing fCO2, I is reduced to an aq. assemblage dominated by simple alkanes; at high fCO2, org. acids are the favored reaction products. Activities of carboxylic acids were ∼1 order of magnitude higher in the II assemblage than that of I; however, amino acids have higher activities in the I assemblage. The results of calcn. demonstrate that reactions involving aq. solns. and polycyclic arom. hydrocarbons could have formed assemblages of aq. org. species, including amino acids, during the alteration events documented in the mineralogy of carbonaceous chondrites. If higher fNH3 values were used, the activities of the amino acids would dramatically increase.
98. 98
    Sleep, N. H.; Zahnle, K. J.; Kasting, J. F.; Morowitz, H. J. Annihilation of Ecosystems by Large Asteroid Impacts on the Early Earth. Nature 1989, 342 (6246), 139– 142, DOI: 10.1038/342139a0
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    Annihilation of ecosystems by large asteroid impacts on the early Earth
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    Nature (1989),342 (6246),139-42 ISSN:0028-0836.
    Large asteroid impacts produced globally lethal conditions by evaporating large volumes of ocean water on the early Earth. The Earth may have been continuously habitable by ecosystems that did not depend on photosynthesis as early as 4.44 Gyr BP (before present). Only a brief interval after 3.8 Gyr exists between the time when obligate photosynthetic organisms could continuously evolve and the time when the palaeontological record indicates highly evolved photosynthetic ecosystems.
99. 99
    Marchi, S.; Bottke, W. F.; Elkins-Tanton, L. T.; Bierhaus, M.; Wuennemann, K.; Morbidelli, A.; Kring, D. A. Widespread Mixing and Burial of Earth’s Hadean Crust by Asteroid Impacts. Nature 2014, 511 (7511), 578– 582, DOI: 10.1038/nature13539
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    Widespread mixing and burial of Earth's Hadean crust by asteroid impacts
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    Nature (London, United Kingdom) (2014),511 (7511),578-582CODEN:NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The history of the Hadean Earth (∼4.0-4.5 billion years ago) is poorly understood because few known rocks are older than ∼3.8 billion years old. The main constraints from this era come from ancient submillimeter zircon grains. Some of these zircons date back to ∼4.4 billion years ago when the Moon, and presumably the Earth, was being pummeled by an enormous flux of extraterrestrial bodies. The magnitude and exact timing of these early terrestrial impacts, and their effects on crustal growth and evolution, are unknown. Here we provide a new bombardment model of the Hadean Earth that has been calibrated using existing lunar and terrestrial data. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the age distribution of Hadean zircons and the absence of early terrestrial rocks. Existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.
100. 100
     Chyba, C.; Sagan, C. Endogenous Production, Exogenous Delivery and Impact-Shock Synthesis of Organic Molecules: An Inventory for the Origins of Life. Nature 1992, 355 (6356), 125– 132, DOI: 10.1038/355125a0
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     Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life
     Chyba, Christopher; Sagan, Carl
     Nature (London, United Kingdom) (1992),355 (6356),125-32CODEN:NATUAS; ISSN:0028-0836.
     Sources of org. mols. on the Early Earth can be divided into 3 categories: delivery by extraterrestrial objects, org. synthesis driven by impact shocks, and org. synthesis by other energy sources (such as UV light or elec. discharges). Ests. of these sources for plausible end-member oxidn. states of the early terrestrial atm. suggest that the heavy bombardment before 3.5 Gyr ago either produced or delivered quantities of orgs. comparable to those produced by other energy sources.
101. 101
     Bevan, A. W. R.; Bland, P. A.; Jull, A. J. T. Meteorite Flux on the Nullarbor Region, Australia. In Meteorites: Flux with Time and Impact Effects; Geological Society: London, Special Publications, 1998; Vol. 140, pp 59– 73.
102. 102
     Macke, R. J.; Consolmagno, G. J.; Britt, D. T. Density, Porosity, and Magnetic Susceptibility of Carbonaceous Chondrites. Meteoritics & Planetary Science 2011, 46 (12), 1842– 1862, DOI: 10.1111/j.1945-5100.2011.01298.x
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     102
     Density, porosity, and magnetic susceptibility of carbonaceous chondrites
     Macke, Robert J.; Consolmagno, Guy J.; Britt, Daniel T.
     Meteoritics & Planetary Science (2011),46 (12),1842-1862CODEN:MPSCFY; ISSN:1945-5100. (Wiley-Blackwell)
     We report phys. properties (bulk and grain d., magnetic susceptibility, and porosity) measured using nondestructive and noncontaminating methods for 195 stones from 63 carbonaceous chondrites. Grain densities over the whole population av. 3.44 g cm-3, ranging from 2.42 g cm-3 (CI1 Orgueil) to 5.66 g cm-3 (CB Bencubbin). Magnetic susceptibilities (in log units of 10-9 m3 kg-1) averaged log χ = 4.22, ranging from 3.23 (CV3 Axtell) to 5.79 (CB Bencubbin). Porosities averaged 17%, ranging from 0 (for a no. of meteorites) to 41% (for one stone of the CO Ornans). Notably, we found significant differences in porosity between the oxidized and reduced CV subgroups, with the porosities of CVo averaging approx. 20% and CVr porosities approx. 4%. Overall, porosities of carbonaceous chondrite falls trend with petrog. type, from type 1 (CI) near 35%, type 2 (CM, CR) averaging 23%, type 3 (CV, CO) 21%, to type 4 (CK and some CO) averaging 15%. There is also a significant decrease in porosity between meteorites of shock stage S1 and those of S2, indicative of shock compression.
103. 103
     Pierazzo, E.; Chyba, C. F. Impact Delivery of Prebiotic Organic Matter to Planetary Surfaces. In Comets and the Origin and Evolution of Life; Thomas, P. J., Hicks, R. D., Chyba, C. F., McKay, C. P., Eds.; Advances in Astrobiology and Biogeophysics; Springer: Berlin, Heidelberg, 2006; pp 137– 168.
104. 104
     Keil, K. Mineralogical and Chemical Relationships among Enstatite Chondrites. J. Geophys. Res. 1968, 73 (22), 6945– 6976, DOI: 10.1029/JB073i022p06945
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     Mineralogical and chemical relationships among enstatite chondrites
     Keil, Klaus
     Journal of Geophysical Research (1968),73 (22),6945-76CODEN:JGREA2; ISSN:0148-0227.
     Fifteen of the 16 known enstatite chondrites were studied microscopically in reflected and transmitted light, and their modal compns. were detd. by point-counting techniques. Compns. of clinoenstatite, orthoenstatite, plagioclase, kamacite, taenite, troilite, oldhamite, daubreelite, niningerite, ferroan alabandite, and schreibersite were detd. with the electron microprobe x-ray analyzer. Chem. compn., mineral occurrence, and mineral compn. were found to depend on degree of recrystn. of the chondrites as judged by, for example, distinctness of chondrules and coarseness of silicates. On the basis of these parameters, 3 groups of enstatite chondrites can be distinguished and are referred to as type I, intermediate type, and type II. Differences between types I and II are pronounced, whereas intermediate type is transitional. The suggestion of Van Schmus and Wood that type II enstatite chondrites originated from type I by reheating is reviewed in the light of the new data. Although many of the chem.-mineralogical parameters of type II chondrites could be explained as being the result of reheating of type I chondrites, there are some that would require rather stringent environmental conditions during reheating. For example, lower Fe and S contents in type II chondrites would presumably require reheating of type I chondrites to ≥975°, the lowest temp. at which a melt would appear in the Fe-Ni-S system of type I compn. and at which phys. sepn. of the liq. from the silicates could occur. Differences in Si/μg. ratios would require reheating to even higher temps. and fractionation in an open system. Furthermore, observed differences in N and sinoite contents between type I and type II are difficult to explain unless reheating took place in a closed system, or under O fugacities low enough to allow N to react with SiO2 and Si to form Si2N2O. An alternative model to the one by Van Schmus and Wood is discussed; it assumes that major differences in chem. and mineralogical compns. between type I and type II were essentially established before or during chondrule formation and agglomeration by, e.g., igneous differentiation or fractionation during condensation from a solar nebula, and that differences in texture are due either to different cooling rates of type I and type II chondrites during and after agglomeration of chondrules or to mild reheating to temps. ≤975°. This model does not, however, readily explain why only enstatite chondrites of type II bulk chem. compn. (i.e. low Fe, S) cooled slowly or were reheated, and why chondrites of type I compn. (high Fe, S) were always quenched to temps. low enough to prevent recrystn. and were not reheated.
105. 105
     McDonough, W. F. Compositional Model for the Earth’s Core. Treatise on Geochemistry 2003, 2, 547, DOI: 10.1016/B0-08-043751-6/02015-6
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     Dauphas, N. The Isotopic Nature of the Earth’s Accreting Material through Time. Nature 2017, 541 (7638), 521– 524, DOI: 10.1038/nature20830
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     Isotopic nature of Earth's accreting material through time
     Dauphas, Nicolas
     Nature (London, United Kingdom) (2017),541 (7638),521-524CODEN:NATUAS; ISSN:0028-0836. (Nature Publishing Group)
     The Earth formed by accretion of Moon- to Mars-size embryos coming from various heliocentric distances. The isotopic nature of these bodies is unknown. However, taking meteorites as a guide, most models assume that the Earth must have formed from a heterogeneous assortment of embryos with distinct isotopic compns. High-precision measurements, however, show that the Earth, the Moon and enstatite meteorites have almost indistinguishable isotopic compns. Models have been proposed that reconcile the Earth-Moon similarity with the inferred heterogeneous nature of Earth-forming material, but these models either require specific geometries for the Moon-forming impact or can explain only one aspect of the Earth-Moon similarity (i.e., 17O). Here I show that elements with distinct affinities for metal can be used to decipher the isotopic nature of the Earth's accreting material through time. I find that the mantle signatures of lithophile O, Ca, Ti and Nd, moderately siderophile Cr, Ni and Mo, and highly siderophile Ru record different stages of the Earth's accretion; yet all those elements point to material that was isotopically most similar to enstatite meteorites. This isotopic similarity indicates that the material accreted by the Earth always comprised a large fraction of enstatite-type impactors (about half were E-type in the first 60 per cent of the accretion and all of the impactors were E-type after that). Accordingly, the giant impactor that formed the Moon probably had an isotopic compn. similar to that of the Earth, hence relaxing the constraints on models of lunar formation. Enstatite meteorites and the Earth were formed from the same isotopic reservoir but they diverged in their chem. evolution owing to subsequent fractionation by nebular and planetary processes.
107. 107
     Bermingham, K. R.; Worsham, E. A.; Walker, R. J. New Insights into Mo and Ru Isotope Variation in the Nebula and Terrestrial Planet Accretionary Genetics. Earth Planet Sci. Lett. 2018, 487, 221– 229, DOI: 10.1016/j.epsl.2018.01.017
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     New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics
     Bermingham, K. R.; Worsham, E. A.; Walker, R. J.
     Earth and Planetary Science Letters (2018),487 (),221-229CODEN:EPSLA2; ISSN:0012-821X. (Elsevier B.V.)
     When cor. for the effects of cosmic ray exposure, Mo and Ru nucleosynthetic isotope anomalies in iron meteorites from at least nine different parent bodies are strongly correlated in a manner consistent with variable depletion in s-process nucleosynthetic components. In contrast to prior studies, the new results show no significant deviations from a single correlation trend. In the refined Mo-Ru cosmic correlation, a distinction between the non-carbonaceous (NC) group and carbonaceous chondrite (CC) group is evident. Members of the NC group are characterized by isotope compns. reflective of variable s-process depletion. Members of the CC group analyzed here plot in a tight cluster and have the most s-process depleted Mo and Ru isotopic compns., with Mo isotopes also slightly enriched in r- and possibly p-process contributions. This indicates that the nebular feeding zone of the NC group parent bodies was characterized by Mo and Ru with variable s-process contributions, but with the two elements always mixed in the same proportions. The CC parent bodies sampled here, by contrast, were derived from a nebular feeding zone that had been mixed to a uniform s-process depleted Mo-Ru isotopic compn. Six molybdenite samples, four glacial diamictites, and two ocean island basalts were analyzed to provide a preliminary constraint on the av. Mo isotope compn. of the bulk silicate Earth (BSE). Combined results yield an av. μ97Mo value of +3.2 ± 6.5. This value, coupled with a previously reported μ100Ru value of +1.2 ± 7.2 for the BSE, indicates that the isotopic compn. of the BSE falls precisely on the refined Mo-Ru cosmic correlation. The overlap of the BSE with the correlation implies that there was homogeneous accretion of siderophile elements for the final accretion of 10 to 20 wt% of Earth's mass. The only known cosmochem. materials with an isotopic match to the BSE, with regard to Mo and Ru, are some members of the IAB iron meteorite complex and enstatite chondrites.
108. 108
     Hellmann, J. L.; Hopp, T.; Burkhardt, C.; Becker, H.; Fischer-Gödde, M.; Kleine, T. Tellurium Isotope Cosmochemistry: Implications for Volatile Fractionation in Chondrite Parent Bodies and Origin of the Late Veneer. Geochim. Cosmochim. Acta 2021, 309, 313– 328, DOI: 10.1016/j.gca.2021.06.038
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     Tellurium isotope cosmochemistry: Implications for volatile fractionation in chondrite parent bodies and origin of the late veneer
     Hellmann, Jan L.; Hopp, Timo; Burkhardt, Christoph; Becker, Harry; Fischer-Goedde, Mario; Kleine, Thorsten
     Geochimica et Cosmochimica Acta (2021),309 (),313-328CODEN:GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
     Tellurium stable isotope compns. and abundances (δ128/126Te relative to SRM 3156) are reported for 43 ordinary, enstatite, and Rumuruti chondrites, which together with results from a companion study on carbonaceous chondrites are used to assess the origin of volatile element fractionations in chondrites. Whereas Te isotope variations among carbonaceous chondrites predominantly reflect mixing between isotopically light chondrules/chondrule precursors and CI-like matrix, Te isotope variations among non-carbonaceous chondrites mainly result from Te redistribution during parent body thermal metamorphism. The enstatite chondrites in particular display increasingly heavy Te isotopic compns. and decreasing Te concns. with increasing degree of metamorphism, indicating migration of isotopically light Te from the strongly metamorphosed inner parts towards the cooler outer regions of the parent bodies. By contrast, ordinary and Rumuruti chondrites display less systematic Te isotope variations, implying more localized redistribution of Te during parent body thermal metamorphism.We also report Te stable isotope data for 19 terrestrial mantle-derived rocks. Peridotites with Al2O3 contents close to those inferred for the bulk silicate Earth (BSE) exhibit uniform δ128/126Te values, which we interpret to represent the Te isotopic compn. of the BSE. This compn. overlaps with the Te isotope compn. of some volatile-rich carbonaceous chondrites (most notably CM chondrites), but also with that of enstatite chondrites. Comparison of the Te results to Se isotopes and Se/Te ratios shows that due to uncertainties in the compn. of the BSE and the isotopic compn. of bulk chondrite parent bodies, neither Te isotopes alone nor the combined Se-Te elemental and isotopic systematics can distinguish between a carbonaceous and enstatite chondrite-like late veneer, which is the presumed source of Se and Te in the BSE. Together, the results of this study illustrate that the relative abundances and mass-dependent isotope compns. of volatile elements like Se and Te are modified by phys. and chem. processes occurring after planetary accretion, which severely complicates their use as genetic tracers. A corollary of this is that contrary to prior proposals the Se-Te systematics are not contradicting an inner solar system origin of the late veneer, as it has been inferred using nucleosynthetic isotope anomalies of other elements.
109. 109
     Macke, R. J.; Consolmagno, G. J.; Britt, D. T.; Hutson, M. L. Enstatite Chondrite Density, Magnetic Susceptibility, and Porosity. Meteoritics & Planetary Science 2010, 45 (9), 1513– 1526, DOI: 10.1111/j.1945-5100.2010.01129.x
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     Enstatite chondrite density, magnetic susceptibility, and porosity
     Macke, Robert J.; Consolmagno, Guy J.; Britt, Daniel T.; Hutson, Melinda L.
     Meteoritics & Planetary Science (2010),45 (9),1513-1526CODEN:MPSCFY; ISSN:1945-5100. (Wiley-Blackwell)
     As part of our continuing survey of meteorite phys. properties, we measured grain and bulk d., porosity, and magnetic susceptibility for 41 stones from 23 enstatite chondrites (ECs), all with masses greater than 10 g, representing the majority of falls and a significant percentage of all available non-Antarctic EC meteorites. Our sampling included a mix of falls and finds. For falls, grain densities range from 3.45 to 4.17 g cm-3, averaging 3.66 g cm-3; bulk densities range from 3.15 to 4.10 g cm-3, averaging 3.55 g cm-3; porosities range from 0 to 12% with the majority less than 7%, and magnetic susceptibilities (in log units of 10-9 m3 kg-1) from 5.30 to 5.64, with an av. of 5.47. For finds, weathering reduces both grain and bulk densities as well as magnetic susceptibilities. On av., finds have much higher porosity than falls. The two EC subgroups EH and EL, nominally distinguished by total iron content, exhibit similar values for all of the properties measured, indicating similar metallic iron content in the bulk stones of both subgroups. We also obsd. considerable intra-meteorite variation, with inhomogeneities in bulk and grain densities at scales up to approx. 40 g (approx. 12 cm3).
110. 110
     Lewis, J. S.; Prinn, R. G. Planets and Their Atmospheres: Origin and Evolution; International Geophysics Series; Academic Press Inc.: Orlando, FL, 1984; Vol. 33.
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     Wang, Z.; Mai, K.; Kumar, N.; Elder, T.; Groom, L. H.; Spivey, J. J. Effect of Steam During Fischer–Tropsch Synthesis Using Biomass-Derived Syngas. Catal. Lett. 2017, 147 (1), 62– 70, DOI: 10.1007/s10562-016-1881-8
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     Effect of Steam During Fischer-Tropsch Synthesis Using Biomass-Derived Syngas
     Wang, Zi; Mai, Khiet; Kumar, Nitin; Elder, Thomas; Groom, Leslie H.; Spivey, James J.
     Catalysis Letters (2017),147 (1),62-70CODEN:CALEER; ISSN:1011-372X. (Springer)
     Fischer-Tropsch synthesis (FTS) with biomass-derived syngas was performed using both iron-based 100Fe/6Cu/4K/25Al catalyst and ruthenium-based 5 % Ru/SiO2 catalyst. During FTS, different concns. of steam were co-fed with the biomass-derived syngas to promote the water gas shift reaction and increase the H2/CO ratio. On Fe-based catalysts the increase in steam concn. led to lower conversion, while deactivation is not obsd. on Ru-based catalysts. XRD of the spent iron-based catalyst showed the oxidn. of iron carbides. Adding steam inhibited surface carbon deposition, as measured by temp. programmed hydrogenation. The iron carbide phase could be re-carburized by flowing CO. The addn. of steam had different effects on these two catalysts. Fe-based catalyst showed significantly lower methane selectivity and greater C5+ hydrocarbon selectivity, while on the Ru-based catalyst, adding steam only showed slightly decreased methane selectivity.
112. 112
     Pendyala, V. R. R.; Shafer, W. D.; Jacobs, G.; Davis, B. H. Fischer–Tropsch Synthesis: Effect of Reaction Temperature for Aqueous-Phase Synthesis Over a Platinum Promoted Co/Alumina Catalyst. Catal. Lett. 2014, 144 (6), 1088– 1095, DOI: 10.1007/s10562-014-1247-z
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     Fischer-Tropsch Synthesis: Effect of Reaction Temperature for Aqueous-Phase Synthesis Over a Platinum Promoted Co/Alumina Catalyst
     Pendyala, Venkat Ramana Rao; Shafer, Wilson D.; Jacobs, Gary; Davis, Burtron H.
     Catalysis Letters (2014),144 (6),1088-1095CODEN:CALEER; ISSN:1011-372X. (Springer)
     The effect of reaction temp. on the performance of a traditional Fischer-Tropsch cobalt catalyst (0.5 % Pt-25 % Co/Al2O3) was investigated during aq.-phase Fischer-Tropsch synthesis (AFTS) using a 1 L stirred tank reactor in the batch mode of operation. The CO conversion rate of the catalyst was found to increase monotonically with increasing reaction temp. At lower temps. oxygenate selectivity was high. With increasing the reaction temp., oxygenate selectivity decreased and the selectivity to hydrocarbons increased. Carbon dioxide and methane selectivity also increased with reaction temp. and the corresponding higher hydrocarbon (C5+) selectivity decreased. For comparison, the CO conversion rate of the catalyst was also tested using C30 oil as a solvent, and similar activation and reaction conditions were utilized in the batch mode of operation. Slightly higher CO rate was obsd. with C30 oil as a solvent than with the water.
113. 113
     Navarro-González, R.; Romero, A.; Honda, Y. Power Measurements of Spark Discharge Experiments. Orig Life Evol Biosph 1998, 28 (2), 131– 153, DOI: 10.1023/A:1006577104654
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     Power measurements of spark discharge experiments
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     Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life (1998),28 (2),131-53 ISSN:0169-6149.
     An accurate and precise knowledge of the amount of energy introduced into prebiotic discharge experiments is important to understand the relative roles of different energy sources in the synthesis of organic compounds in the primitive Earth's atmosphere and other planetary atmospheres. Two methods widely used to determine the power of spark discharges were evaluated, namely calorimetric and oscilloscopic, using a chemically inert gas. The power dissipated by the spark in argon at 500 Torr was determined to be 2.4 (+12%/-17%) J s-1 by calorimetry and 5.3 (+/- 15%) J s-1 by the oscilloscope. The difference between the two methods was attributed to (1) an incomplete conversion of the electric energy into heat, and (2) heat loss from the spark channel to the connecting cables through the electrodes. The latter contribution leads to an unwanted effect in the spark channel by lowering the spark product yields as the spark channel cools by mixing with surrounding air and by losing heat to the electrodes. Once the concentrations of the spark products have frozen at the freeze-out temperature, any additional loss of heat from the spark channel to the electrodes has no consequence in product yields. Therefore, neither methods accurately determines the net energy transferred to the system. With a lack of a quantitative knowledge of the amount of heat loss from the spark channel during the interval from ignition of the spark to when the freeze-out temperature is reached, it is recommended to derive the energy yields of the spark products from the mean value of the two methods with the uncertainty being their standard deviation. For the case of argon at 500 Torr, this would be 3.8 (+/-50%) J s-1.
114. 114
     Chiu, R.; Tinel, L.; Gonzalez, L.; Ciuraru, R.; Bernard, F.; George, C.; Volkamer, R. UV Photochemistry of Carboxylic Acids at the Air-Sea Boundary: A Relevant Source of Glyoxal and Other Oxygenated VOC in the Marine Atmosphere. Geophys. Res. Lett. 2017, 44 (2), 1079– 1087, DOI: 10.1002/2016GL071240
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     UV photochemistry of carboxylic acids at the air-sea boundary: A relevant source of glyoxal and other oxygenated VOC in the marine atmosphere
     Chiu, R.; Tinel, L.; Gonzalez, L.; Ciuraru, R.; Bernard, F.; George, C.; Volkamer, R.
     Geophysical Research Letters (2017),44 (2),1079-1087CODEN:GPRLAJ; ISSN:1944-8007. (Wiley-Blackwell)
     Photochem. plays an important role in marine dissolved org. carbon (DOC) degrdn., but the mechanisms that convert DOC into volatile org. compds. (VOCs) remain poorly understood. We irradiated carboxylic acids (C7-C9) on a simulated ocean surface with UV light (<320 nm) in a photochem. flow reactor and transferred the VOC products into a dark ozone reactor. Glyoxal was detected as a secondary product from heptanoic, octanoic, and nonanoic acid (NA) films, but not from octanol. Primary glyoxal emissions were not obsd., nor was glyoxal formed in the absence of ozone. Addn. of a photosensitizer had no noticeable effect. The concurrent detection of heptanal in the NA system suggests that the ozonolysis of 2-nonenal is the primary chem. mechanism that produces glyoxal. This source can potentially sustain tens of parts per trillion by vol. (pptv) glyoxal over oceans, and helps to explain why glyoxal fluxes in marine air are directed from the atm. into the ocean.
115. 115
     Simoneit, B. R. T.; Rushdi, A. I.; Deamer, D. W. Abiotic Formation of Acylglycerols under Simulated Hydrothermal Conditions and Self-Assembly Properties of Such Lipid Products. Adv. Space Res. 2007, 40 (11), 1649– 1656, DOI: 10.1016/j.asr.2007.07.034
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     Abiotic formation of acylglycerols under simulated hydrothermal conditions and self-assembly properties of such lipid products
     Simoneit, Bernd R. T.; Rushdi, Ahmed I.; Deamer, David W.
     Advances in Space Research (2007),40 (11),1649-1656CODEN:ASRSDW; ISSN:0273-1177. (Elsevier Ltd.)
     The abiotic formation of aliph. lipid compds. (i.e., fatty acids, alcs., and acylglycerols) has been reported to occur at elevated temps. and pressures under simulated hydrothermal conditions. Although abiotic synthetic chem. can occur under these conditions, the prebiotic self-assembly of micelles to bilayer to vesicles (protocells) may have occurred elsewhere. Amphiphilic compds. such as fatty acids are important candidates for micelle/bilayer/vesicle formation, because they are abundant products of Fischer-Tropsch-type reactions and are also found in carbonaceous meteorites. Thus, it is of interest to det. whether more complex amphiphilic precursor compds., capable of assembling into stable membrane structures, can be synthesized under hydrothermal conditions. Hydrothermal expts. were conducted to study condensation reactions of model lipid precursors in aq. media, i.e., glycerol and alkanoic acids, to form acylglycerols (glyceryl alkanoates) at elevated temp. under confining pressure. Nine different alkanoic acids ranging from C7 to C16 (except C8) were used in these expts. The condensation products were two isomers each of monoacylglycerols and diacylglycerols, as well as the corresponding triacylglycerol. The results indicated that: (1) condensation (dehydration) reactions are possible under aq. pyrolysis conditions; (2) abiotic synthesis and subsequent condensation reactions of aliph. lipid compds. are possible under hydrothermal conditions; and (3) such mols. have robust properties of self-assembly into membranous structures that would be suitable boundary structures for primitive forms of cellular life.
116. 116
     Rapf, R. J.; Perkins, R. J.; Dooley, M. R.; Kroll, J. A.; Carpenter, B. K.; Vaida, V. Environmental Processing of Lipids Driven by Aqueous Photochemistry of α-Keto Acids. ACS Cent. Sci. 2018, 4 (5), 624– 630, DOI: 10.1021/acscentsci.8b00124
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     Environmental Processing of Lipids Driven by Aqueous Photochemistry of α-Keto Acids
     Rapf, Rebecca J.; Perkins, Russell J.; Dooley, Michael R.; Kroll, Jay A.; Carpenter, Barry K.; Vaida, Veronica
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     Sunlight can initiate photochem. reactions of org. mols. though direct photolysis, photosensitization, and indirect processes, often leading to complex radical chem. that can increase mol. complexity in the environment. α-Keto acids act as photoinitiators for org. species that are not themselves photoactive. Here, we demonstrate this capability through the reaction of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, with a series of fatty acids and fatty alcs. We show for five different cases that a cross-product between the photoinitiated α-keto acid and non-photoactive species is formed during photolysis in aq. soln. Fatty acids and alcs. are relatively unreactive species, which suggests that α-keto acids are able to act as radical initiators for many atmospherically relevant mols. found in the sea surface microlayer and on atm. aerosol particles.
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     Prebiotic phosphorylation of (pre)biol. substrates under aq. conditions is a crit. step in the origins of life. Previous investigations have had limited success and/or require unique environments that are incompatible with subsequent generation of the corresponding oligomers or higher-order structures. Here, we demonstrate that diamidophosphate (DAP) - a plausible prebiotic agent produced from trimetaphosphate - efficiently (amido)phosphorylates a wide variety of (pre)biol. building blocks (nucleosides/tides, amino acids and lipid precursors) under aq. (soln./paste) conditions, without the need for a condensing agent. Significantly, higher-order structures (oligonucleotides, peptides and liposomes) are formed under the same phosphorylation reaction conditions. This plausible prebiotic phosphorylation process under similar reaction conditions could enable the systems chem. of the three classes of (pre)biol. relevant mols. and their oligomers, in a single-pot aq. environment.
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     Abstr.: Protocellular membranes are thought to be composed of mixts. of single chain amphiphiles, such as fatty acids and their derivs., moieties that would have been part of the complex prebiotic chem. landscape. The compn. and physico-chem. properties of these prebiol. membranes would have been significantly affected and regulated by their environment. In this study, pertinent properties were systematically characterized, under early Earth conditions. Two different fatty acids were mixed with their resp. alc. and/or glycerol monoester derivs. to generate combinations of binary and tertiary membrane systems. Their properties were then evaluated as a function of multiple factors including their stability under varying pH, varying Mg2+ ion concns., diln. regimes, and their permeability to calcein. Our results demonstrate how environmental constraints would have acted as important prebiotic selection pressures to shape the evolution of prebiol. membranes. The study also illustrates that compositionally diverse membrane systems are more stable and robust to multiple selection pressures, thereby making them more suitable for supporting protocellular life.
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     Compartmentalization was likely essential for primitive chem. systems during the emergence of life, both for preventing leakage of important components, i.e., genetic materials, and for enhancing chem. reactions. Although life as we know it uses lipid bilayer-based compartments, the diversity of prebiotic chem. may have enabled primitive living systems to start from other types of boundary systems. Here, we demonstrate membraneless compartmentalization based on prebiotically available org. compds., α-hydroxy acids (αHAs), which are generally coproduced along with α-amino acids in prebiotic settings. Facile polymn. of αHAs provides a model pathway for the assembly of combinatorially diverse primitive compartments on early Earth. We characterized membraneless microdroplets generated from homo- and heteropolyesters synthesized from drying solns. of αHAs endowed with various side chains. These compartments can preferentially and differentially segregate and compartmentalize fluorescent dyes and fluorescently tagged RNA, providing readily available compartments that could have facilitated chem. evolution by protecting, exchanging, and encapsulating primitive components. Protein function within and RNA function in the presence of certain droplets is also preserved, suggesting the potential relevance of such droplets to various origins of life models. As a lipid amphiphile can also assemble around certain droplets, this further shows the droplets' potential compatibility with and scaffolding ability for nascent biomol. systems that could have coexisted in complex chem. systems. These model compartments could have been more accessible in a "messy" prebiotic environment, enabling the localization of a variety of protometabolic and replication processes that could be subjected to further chem. evolution before the advent of the Last Universal Common Ancestor.
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     Nature communications (2020),11 (1),5423 ISSN:.
     Wet-dry cycling on the early Earth is thought to have facilitated production of molecular building blocks of life, but its impact on self-assembly and compartmentalization remains largely unexplored. Here, we investigate dehydration/rehydration of complex coacervates, which are membraneless compartments formed by phase separation of polyelectrolyte solutions. Solution compositions are identified for which tenfold water loss results in maintenance, disappearance, or appearance of coacervate droplets. Systems maintaining coacervates throughout the dehydration process are further evaluated to understand how their compartmentalization properties change with drying. Although added total RNA concentrations increase tenfold, RNA concentration within coacervates remains steady. Exterior RNA concentrations rise, and exchange rates for encapsulated versus free RNAs increase with dehydration. We explain these results in light of the phase diagram, with dehydration-driven ionic strength increase being particularly important in determining coacervate properties. This work shows that wet-dry cycling can alter the phase behavior and protocell-relevant functions of complex coacervates.
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     Fatty acid membrane assembly on coacervate microdroplets as a step towards a hybrid protocell model
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     Mechanisms of prebiotic compartmentalization are central to providing insights into how protocellular systems emerged on the early Earth. Protocell models are based predominantly on the membrane self-assembly of fatty-acid vesicles, although membrane-free scenarios that involve liq.-liq. microphase sepn. (coacervation) have also been considered. Here we integrate these alternative models of prebiotic compartmentalization and develop a hybrid protocell model based on the spontaneous self-assembly of a continuous fatty-acid membrane at the surface of preformed coacervate microdroplets prepd. from cationic peptides/polyelectrolytes and ATP or oligo/polyribonucleotides. We show that the coacervate-supported membrane is multilamellar, and mediates the selective uptake or exclusion of small and large mols. The coacervate interior can be disassembled without loss of membrane integrity, and fusion and growth of the hybrid protocells can be induced under conditions of high ionic strength. Our results highlight how notions of membrane-mediated compartmentalization, chem. enrichment and internalized structuration can be integrated in protocell models via simple chem. and phys. processes.

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