Abstract
The solvent quality determines the collapsed or the expanded state of a polymer. For example, a polymer dissolved in a poor solvent collapses, whereas in a good solvent it opens up. While this standard understanding is generally valid, there are examples when a polymer collapses even in a mixture of two good solvents. This phenomenon, commonly known as co-non-solvency, is usually associated with a wide range of synthetic (smart) polymers. Moreover, recent experiments have shown that some biopolymers, such as elastin-like polypeptides (ELPs) that exhibit lower critical solution behavior Tl in pure water, show co-non-solvency behavior in aqueous ethanol mixtures. In this study, we investigate the phase behavior of elastin-like polypeptides (ELPs) in aqueous binary mixtures using molecular dynamics simulations of all-atom and complementary explicit solvent generic models. The model is parameterized by mapping the solvation free energy obtained from the all-atom simulations onto the generic interaction parameters. For this purpose, we derive segment-based (monomer level) generic parameters for four different peptides, namely proline (P), valine (V), glycine (G), and alanine (A), where the first three constitute the basic building blocks of ELPs. Here, we compare the conformational behavior of two ELP sequences, namely -(VPGGG)- and -(VPGVG)-, in aqueous ethanol and -urea mixtures. Consistent with recent experiments, we find that ELPs show co-non-solvency in aqueous ethanol mixtures. Ethanol molecules have preferential binding with all ELP residues, with an interaction contrast of 6–8 kBT, and thus driving the coil-to-globule transition. On the contrary, ELP conformations show a weak variation in aqueous urea mixtures. Our simulations suggest that the glycine residues dictate the overall behavior of ELPs in aqueous urea, where urea molecules have a rather weak preferential binding with glycine as observed from the all atom simulations, i.e., less than kBT. This weak interaction dilutes the overall effect of other neighboring residues and thus ELPs exhibit a different conformational behavior in aqueous urea in comparison to aqueous ethanol mixtures. While the validation of the latter findings will require a more detailed experimental investigation, the results presented here may provide a new twist to the present understanding of cosolvent interactions with peptides and proteins.
Highlights
Solvation of macromolecules in water and especially in a mixture of solvents is of central relevance for many areas of chemical physics, polymer physics, soft matter science, and material research.[1−7] solvation effects are the driving force underlying various macromolecular processes ranging from the responsiveness of hydrogels to external stimuli or concentration gradients of the solvents (“smart polymers”) to denaturation of proteins
To investigate the coil-to-globule transition of elastin-like polypeptides (ELPs) in aqueous ethanol mixtures, we consider that the ELP chain is under a good solvent condition in pure water, i.e., when T < Tl
To obtain model parameters that reasonably satisfy the good solvent condition of ELPs in pure water, we have estimated the possible number of hydrogen bonds (H-bonds) between an amino acid and water molecules Nw from the all-atom simulations
Summary
Solvation of macromolecules in water and especially in a mixture of solvents is of central relevance for many areas of chemical physics, polymer physics, soft matter science, and material research.[1−7] solvation effects are the driving force underlying various macromolecular processes ranging from the responsiveness of hydrogels to external stimuli or concentration gradients of the solvents (“smart polymers”) to denaturation of proteins. The flexibility of macromolecules provides a suitable platform for the tunable design of advanced functional materials.[11−16] because of the carbon-based microscopic architectures, they often create severe environmental problems To circumvent this problem, recent interests have been directed toward the “so-called” green chemistry,[17] making use of macromolecular structures that are biocompatible[18] and/or biodegradable[19] and at the same time are thermal,[16,18] (co-)solvent,[20−30] and photoresponsive.[31,32] While most of these systems are homopolymers, recent interest has been directed to a variety of copolymer architectures.[33−42] Here, polypeptides and synthetic peptide-based polymers have attracted great inter-
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