Thermodynamics and Applications of Elastin-like Polypeptides

Understanding protein stability and folding is of central importance in chemistry, biology, and medicine. Despite its importance, a molecular level understanding of protein stability still remains illusive due to the complexity of the system. In this study, we employed protein-like polypeptides to study several aspects of protein stability in different aqueous environments. The model system employed here is elastin-like polypeptides (ELPs). First, the modulation of the lower critical solution temperature (LCST) of neutral 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. By contrast, 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 also compared to the results previously found for the hydrophobic collapse and aggregation of poly(N-isoproplyacrylamide). A positively charged ELP, ELP KV6-112, was used as a next model system. We observed both inverse and direct Hofmeister effects on LCST with five chaotropic salts. Next, the solvent isotope effects on the LCST of ELPs were investigated as a function of ELP chain length and guest residue chemistry using D2O and H2O. Differences in the LCST values with heavy and light water were correlated with secondary structure formation of the polypeptide chains which was quantified by circular dichroism, FTIR, and differential scanning calorimetry measurements. It was found that there is a great change in the LCST values between H2O and D2O for those polypeptides which form the greatest amount of b-spiral structure. This study suggests that hydrogen bonding rather than hydrophobicity is the key factor in the stabilization of ELPs in D2O over H2O. The phase transition property of ELPs can also be applied to development of stimuli responsive biosensor system. In this study, we employed ELP-conjugate solid supported lipid bilayer as a size selective binding sensor.