High-energy chemistry of formamide: a unified mechanism of nucleobase formation - PubMed (original) (raw)

High-energy chemistry of formamide: a unified mechanism of nucleobase formation

Martin Ferus et al. Proc Natl Acad Sci U S A. 2015.

Abstract

The coincidence of the Late Heavy Bombardment (LHB) period and the emergence of terrestrial life about 4 billion years ago suggest that extraterrestrial impacts could contribute to the synthesis of the building blocks of the first life-giving molecules. We simulated the high-energy synthesis of nucleobases from formamide during the impact of an extraterrestrial body. A high-power laser has been used to induce the dielectric breakdown of the plasma produced by the impact. The results demonstrate that the initial dissociation of the formamide molecule could produce a large amount of highly reactive CN and NH radicals, which could further react with formamide to produce adenine, guanine, cytosine, and uracil. Based on GC-MS, high-resolution FTIR spectroscopic results, as well as theoretical calculations, we present a comprehensive mechanistic model, which accounts for all steps taking place in the studied impact chemistry. Our findings thus demonstrate that extraterrestrial impacts, which were one order of magnitude more abundant during the LHB period than before and after, could not only destroy the existing ancient life forms, but could also contribute to the creation of biogenic molecules.

Keywords: LIDB; asteroid impact; biomolecules; origin of life.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

Impactor flux on the Moon surface according Koeberl et al. (5) is shown in A. It is estimated that Earth was exposed to an impactor flux that was at least 1 order of magnitude higher than that of the Moon. The conversion formula for such an estimate is shown in Upper; E Earth/E Moon is the Earth−Moon impact flux ratio, _Ve_2 is the square of the escape velocity from the target body (Earth’s Ve = 11.2 km/s), and _V_∞ is the square of the impactor velocity (typically 14.5 km/s at the present time but potentially larger during the LHB, as discussed in the Introduction) (10). B shows selected milestones in early Earth history (, –46).

Fig. 2.

Fig. 2.

(A) Experimental set-up used for the formamide irradiation. (B) Laser hall with the sample placed at the end of the beam line.

Fig. 3.

Fig. 3.

The main methods used to study the formamide dissociation in a high-density energy event along with examples of the measured spectra are shown in A. The reactive radical products have been detected using emission spectroscopy. Stable molecular gases formed as the dissociation products of formamide, such as HCN, are fingerprinted by gas-phase IR absorption spectroscopy. Nucleobases, formed from formamide in the high-energy impact event, are detected by GC-MS derivatization technique. (B) Mechanistic model for the transformation of formamide in plasma and its afterglow in a high-density energy event. Formamide reacts with transient species formed from its own dissociation, resulting in all of the canonical nucleobases present in contemporary genetic materials. AICN, 4-amino-5-cyanoimidazole.

Fig. 4.

Fig. 4.

B3LYP/6-311++G(2d,2p) calculated free-energy profile for the conversion of the trisubstituted pyrimidinyl radical product from ref. to 2,4-diaminopyrimidine, i.e., a common precursor of pyrimidine bases.

Comment in

References

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