Yanjie Zhang - Academia.edu (original) (raw)

Papers by Yanjie Zhang

Surface Science, 2002

An azobenzene derivative 4-hydroxy-3 0 -trifluoromethyl-azobenzene (FAzoH) was synthesized and th... more An azobenzene derivative 4-hydroxy-3 0 -trifluoromethyl-azobenzene (FAzoH) was synthesized and the physisorbed monolayers of the molecule at the liquid/solid interface were investigated by scanning tunneling microscopy (STM). The acquired STM images exhibit the molecular packing of the monolayers of FAzoH in trans conformation. Following the UV light (365 nm) induced trans-cis isomerization, stable domains of pure cis isomers have been identified by STM. The results suggest that a strong hydrogen bonds network existing in the monolayer stabilizes the domains of the cis isomer. Ó

Thin Solid Films, 2003

The interfacial-dependent self-organization of a chiral amino acid amphiphile was observed on var... more The interfacial-dependent self-organization of a chiral amino acid amphiphile was observed on various substrates with different surface hydrophobicity. Three kinds of organizations are induced at different substrates, which appear as helical aggregates on hydrophobic surface (e.g. highly oriented pyrolytic graphite), molecular flat layer on hydrophilic surface, (e.g. mica) and a transition state between them on silicon wafer. The strength of the interactions between the helical aggregates formed in solution and the substrate is the key factor to the self-organization of the chiral molecule. The results reported here provide insight into how the surface hydrophobicity of substrates affects the self-organization of chiral molecules. ᮊ

Holographic grating recorded by He–Ne laser operating at 632.8 nm in polymer film containing push–pull azo dye

Optics Communications, 2003

Holographic gratings were optically recorded by two beams from a He–Ne laser operating at 632.8 n... more Holographic gratings were optically recorded by two beams from a He–Ne laser operating at 632.8 nm in poly (methyl methacrylate) (PMMA) film containing push–pull azo dye. The holographic characteristics of the recorded gratings were dependent on the polarization direction of the recording beams and the relationship between the diffractive signals and the power density of recording beams was investigated. The

Photochromism and holographic recording in polymer film containing chiral azo molecules derived from amino acid

Optical Materials, 2003

... At λ=632.8 nm, the transmission is due to the photoinduced birefringence which is given by I ... more ... At λ=632.8 nm, the transmission is due to the photoinduced birefringence which is given by I ⊥ =I 0 sin 2 (πΔnd/λ), where I ⊥ is the intensity of the transmitted light through the crossed polarizers, I 0 is the intensity of the light passing through the pair of parallel polarizers before ...

Productivity Change and Policy Reform Effects in China's Agriculture since 1979

Abstract During China’s rural reform period, adjustments of policies occur frequently. Most of po... more Abstract During China’s rural reform period, adjustments of policies occur frequently. Most of policies are in favour,of continuation and deepening of reform. But some of them contradict each other, or even lead to regress of reform process. It is useful to examine,in which ways rural reforms did affect China’s agricultural production in the past. This paper aims to estimate productivity

High-Throughput Studies of the Effects of Polymer Structure and Solution Components on the Phase Separation of Thermoresponsive Polymers

Macromolecules, 2004

... Similar effects have been reported in previous light scattering 27 and pressure perturbation ... more ... Similar effects have been reported in previous light scattering 27 and pressure perturbation calorimetry 15 measurements using ... precise, and high-throughput way to study the effects of polymer microstructure and solution composition of ... 13) Mao, H.; Li, C.; Zhang, Y.; Bergbreiter ...

Journal of The American Chemical Society, 2009

The direct binding mechanism for urea-based denaturation of proteins was tested with a thermoresp... more The direct binding mechanism for urea-based denaturation of proteins was tested with a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM). Thermodynamic measurements of the polymer's hydrophobic collapse were complemented by Fourier transform infrared (FTIR) spectroscopy, Stokes radius measurements, and methylated urea experiments. It was found that the lower critical solution temperature (LCST) of PNIPAM decreased as urea was added to the solution. Therefore, urea actually facilitated the hydrophobic collapse of the macromolecule. Moreover, these thermodynamic measurements were strongly correlated with amide I band data which indicated that the decrease in the LCST was coupled to the direct hydrogen bonding of urea to the amide moieties of the polymer. In addition, the hydrogen bonding was found to be highly cooperative, which is consistent with a cross-linking (bivalent binding) mechanism. Cross-linking was confirmed by Stokes radius measurements below the polymer's LCST using gel filtration chromatography. Finally, phase transition measurements with methylurea, dimethylurea, and tetramethylurea indicated that these substituted compounds caused the LCST of PNIPAM to rise with increasing methyl group content. No evidence could be found for the direct binding of any of these methylated ureas to the polymer amide moieties by FTIR. These results are inconsistent with a direct hydrogen-bonding mechanism for the urea-induced denaturation of proteins.

Current Opinion in Chemical Biology, 2006

The Hofmeister series, first noted in 1888, ranks the relative influence of ions on the physical ... more The Hofmeister series, first noted in 1888, ranks the relative influence of ions on the physical behavior of a wide variety of aqueous processes ranging from colloidal assembly to protein folding. Originally, it was thought that an ion's influence on macromolecular properties was caused at least in part by 'making' or 'breaking' bulk water structure. Recent timeresolved and thermodynamic studies of water molecules in salt solutions, however, demonstrate that bulk water structure is not central to the Hofmeister effect. Instead, models are being developed that depend upon direct ion-macromolecule interactions as well as interactions with water molecules in the first hydration shell of the macromolecule.

Journal of Physical Chemistry B, 2008

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides... more The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V 5 A 2 G 3 -120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V 5 -120. It was also found that larger anions could bind with ELP V 5 A 2 G 3 -120 more readily in comparison with ELP V 5 -120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.

Measuring LCSTs by Novel Temperature Gradient Methods: Evidence for Intermolecular Interactions in Mixed Polymer Solutions

Journal of The American Chemical Society, 2003

Herein we describe studies of molecular interactions in thermoresponsive polymers as they go thro... more Herein we describe studies of molecular interactions in thermoresponsive polymers as they go through phase transitions in aqueous solutions. By using our recently reported linear temperature gradient setup for studying the effects of temperature on chemical processes, we demonstrate the ability to probe lower critical solution temperature (LCST) behavior with excellent precision. This method also provides a simple and convenient way to assay the LCST of solutions containing more than one polymer and follow the clouding kinetics of polymer mixtures in real time.

Biomacromolecules, 2006

The kinetics of aqueous two-phase system (ATPS) formation for elastin-like polypeptides (ELP) wit... more The kinetics of aqueous two-phase system (ATPS) formation for elastin-like polypeptides (ELP) with defined chemical composition and chain length was investigated by dark field microscopy in an on-chip format with a linear temperature gradient. Scattering intensities from peptide solutions in the presence and absence of sodium dodecyl sulfate (SDS) were recorded as a function of temperature and time, simultaneously. It was found that the formation of the ATPS for three ELPs of different molecular weights (36 075, 59 422, and 129 856 Da) in the absence of SDS followed a coalescence mechanism, and the rate constant and activation energy were independent of chain length. With the introduction of SDS into the ELP solutions, the rate constants were attenuated more strongly with increasing chain length. Moreover, the coalescence process in the presence of SDS showed non-Arrhenius kinetics as a function of temperature. For the two shorter ELPs, ATPS formation occurred via coalescence at all SDS concentrations and temperatures investigated. On the other hand, the coalescence process was greatly suppressed for the longest ELP at elevated temperatures and higher SDS concentrations. Under these circumstances, ATPS formation was forced to proceed via a mixed Ostwald ripening and coalescence mechanism.

Langmuir, 2010

Vibrational sum frequency spectroscopy (VSFS) was used to explore specific ion effects on interfa... more Vibrational sum frequency spectroscopy (VSFS) was used to explore specific ion effects on interfacial water structure adjacent to a bovine serum albumin (BSA) monolayer adsorbed at the air/water interface. The subphase conditions were varied by the use of six different sodium salts and four different pH values. At pH 2 and 3, the protein layer was positively charged and it was found that the most chaotropic anions caused the greatest attenuation of water structure. The order of the salts followed an inverse Hofmeister series. On the other hand, when the protein layer was near its isoelectric point (pH 5), the most chaotropic anions caused the greatest increase in water structure, although the effect was weak. In this case, a direct Hofmeister series was obeyed. Finally, virtually no effect was observed when the protein layer was negatively charged with a subphase pH of 9. For comparison, similar experiments were run with positively charged, negatively charged, and zwitterionic surfactant monolayers. These experiments gave rise to nearly the identical results as the protein monolayer which suggested that specific anion effects are dominated by the charge state of the interfacial layer rather than its detailed chemical structure. In a final set of experiments, salt effects were examined with a monolayer made from an elastin-like polypeptide (ELP). The peptide consisted of 120 pentameric repeats of the sequence Val-Pro-Gly-Val-Gly. Data from this net neutral biopolymer followed a very weak, but direct Hofmeister series. This suggested that direct anion binding to the amide groups in the backbone of a polypeptide is quite weak in agreement with the BSA data. The results from the variously charged protein, surfactant, and polymer monolayers were compared with a modified Gouy-Chapman-Stern model. The agreement with this simple model was quite good.

Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series

Journal of The American Chemical Society, 2005

Aqueous processes ranging from protein folding and enzyme turnover to colloidal ordering and macr... more Aqueous processes ranging from protein folding and enzyme turnover to colloidal ordering and macromolecular precipitation are sensitive to the nature and concentration of the ions present in solution. Herein, the effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temperature gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, analysis of solvent isotope effects and of the changes in LCST with ion concentration and identity showed multiple mechanisms were at work. In solutions containing sufficient concentrations of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding.

Chemistry of Hofmeister Anions and Osmolytes

Annual Review of Physical Chemistry, 2010

The study of the interactions of salts and osmolytes with macromolecules in aqueous solution orig... more The study of the interactions of salts and osmolytes with macromolecules in aqueous solution originated with experiments concerning protein precipitation more than 100 years ago. Today, these solutes are known to display recurring behavior for myriad biological and chemical processes. Such behavior depends both on the nature and concentration of the species in solution. Despite the generality of these effects, our understanding of the molecular-level details of ion and osmolyte specificity is still quite limited. Here, we review recent studies of the interactions between anions and urea with model macromolecular systems. A mechanism for specific ion effects is elucidated for aqueous systems containing charged and uncharged polymers, polypeptides, and proteins. The results clearly show that the effects of the anions are local and involve direct interactions with macromolecules and their first hydration shell. Also, a hydrogen-bonding mechanism is tested for the urea denaturation of proteins with some of these same systems. In that case, direct hydrogen bonding can be largely discounted as the key mechanism for urea stabilization of uncollapsed and/or unfolded structures.

Effects of end group polarity and molecular weight on the lower critical solution temperature of poly(N-isopropylacrylamide

Journal of Polymer Science Part A-polymer Chemistry, 2006

The lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N-isopropy... more The lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N-isopropylacrylamide) (PNIPAM) were studied to determine the effect of polymer molecular weight on the LCST using a high throughput temperature gradient apparatus. PNIPAM fractions prepared by a conventional radical polymerization using azoisobutyronitrile (AIBN) as the initiator had LCSTs that were largely invariant with molecular weight or dispersity. Only slight deviations were noted with lower molecular weight samples. An 18-kDa sample had a 0.6 °C higher LCST. A 56-kDa sample had a 0.2 °C higher LCST. PNIPAM derivatives prepared with a triphenylmethyl (trityl) functionalized azo initiator were also prepared and mass fractionated. These samples' LCSTs were identical to those of PNIPAM samples prepared using AIBN initiation when higher molecular weight samples were compared. The trityl-containing PNIPAM fractions' LCSTs varied when the molecular weight decreased below 100 kDa. Acidolysis of the trityl end groups provided a third set of PNIPAM derivatives whose LCST differed only with samples with Mw values < 60 kDa. These results show there is no effect of molecular weight on LCST until the degree of polymerization is such that end group structure becomes significant. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1492–1501, 2006

Journal of The American Chemical Society, 2007

The present studies explore multivalent ligand-receptor interactions between pentameric cholera t... more The present studies explore multivalent ligand-receptor interactions between pentameric cholera toxin B subunits (CTB) and the corresponding membrane ligand, ganglioside GM1. CTB binding was monitored on supported phospholipid bilayers coated on the walls and floors of microfluidic channels. Measurements were made by total internal reflection fluorescence microscopy (TIRFM). Apparent dissociation constants were extracted by fitting the binding data to both the Hill-Waud and Langmuir adsorption isotherm equations. Studies of the effect of ligand density on multivalent CTB-GM 1 interactions revealed that binding weakened with increasing GM1 density from 0.02 mol % to 10.0 mol %. Such a result could be explained by the clustering of GM1 on the supported phospholipid membranes, which in turn inhibited the binding of CTB. Atomic force microscopy (AFM) experiments directly verified GM1 clustering within the supported POPC bilayers.

Proceedings of The National Academy of Sciences, 2009

Anion effects on the cloud-point temperature for the liquid؊liquid phase transition of lysozyme w... more Anion effects on the cloud-point temperature for the liquid؊liquid phase transition of lysozyme were investigated by temperature gradient microfluidics under a dark field microscope. It was found that protein aggregation in salt solutions followed 2 distinct Hofmeister series depending on salt concentration. Namely, under low salt conditions the association of anions with the positively charged lysozyme surface dominated the process and the phase transition temperature followed an inverse Hofmeister series. This inverse series could be directly correlated to the size and hydration thermodynamics of the anions. At higher salt concentrations, the liquid-liquid phase transition displayed a direct Hofmeister series that correlated with the polarizability of the anions. A simple model was derived to take both charge screening and surface tension effects into account at the protein/water interface. Fitting the thermodynamic data to this model equation demonstrated its validity in both the high and low salt regimes. These results suggest that in general positively charged macromolecular systems should show inverse Hofmeister behavior only at relatively low salt concentrations, but revert to a direct Hofmeister series as the salt concentration is increased.

Journal of Physical Chemistry C, 2007

The effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(... more The effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(Nisopropylacrylamide), PNIPAM, was investigated as a function of molecular weight and polymer concentration with a temperature gradient microfluidic device under a dark-field microscope. In solutions containing sufficient concentrations of kosmotropic anions, the phase transition of PNIPAM was resolved into two separate steps for higher molecular weight samples. The first step of this two step transition was found to be sensitive to the polymer's molecular weight and solution concentration, while the second step was not. Moreover, the binding of chaotropic anions to the polymer was also influenced by molecular weight. Both sets of results could be explained by the formation of intramolecular and intermolecular hydrogen-bonding between polymer chains. By contrast, the hydrophobic hydration of the isopropyl moieties and polymer backbone was found to be unaffected by either the polymer's molecular weight or solution concentration.