Thermal Behavior and Kinetic Analysis of the Chain Unfolding and Refolding and of the Concomitant Nonpolar Solvation and Desolvation of Two Elastin-like Polymers

Abstract

Water-induced chain dynamics alterations are of paramount importance in many protein- based polymers because they determine and affect to a great extent the temperature dependence of the end properties. In this study, the thermal behavior of the reversible unfolding and refolding of poly(Val- Pro-Gly-Val-Gly) and poly(Val-Pro-Ala-Val-Gly) and of their concurrent dehydration and hydration processes has been studied by differential scanning calorimetry (DSC) and turbidimetry. Contrary to the good reversibility shown by poly(Val-Pro-Gly-Val-Gly), the substitution of glycine by alanine in poly(Val- Pro-Ala-Val-Gly) perturbed to a large extent the process of chain unfolding. For the latter polymer, it was found that both chain unfolding and rehydration processes take place at large undercoolings, suggesting that both events occur far from equilibrium conditions and, therefore, are strongly dominated by kinetics. In this context, the existence of an hydration excess with a kinetic rather than a thermodynamic nature is a remarkable observation. The kinetics offolding and unfolding were also studied by using an isoconversional method of kinetic analysis, i.e., the model-free Friedmand’s isoconversional method. As expected, the kinetics of the solvation of nonpolar moieties for both polymers indicated a complex and multistep process. Again, poly(Val-Pro-Ala-Val-Gly) showed a quite different pattern characterized by an acute hysteresis behavior which seems to govern the hydration process for this polymer. The differences observed between both polymers have been interpreted in terms ofthe hindrance provided by the methyl group in alanine during temperature-induced chain dynamics.