Energetics and Mechanism of Conformational Transitions of Protein-Like NIPAM-Sodium Styrene Sulfonate Copolymers in Aqueous Solutions

摘要

Protein-like and random NIPAM-sodium styrene sulfonate copolymers of similar composition have been prepared by radical polymerization in water at temperatures above and below the LCST of PNIPAM, respectively. Thermal transitions of the copolymers in aqueous solutions have been studied by means of dynamic light scattering, viscometry, and high-sensitivity differential scanning calorimetry. The phase separation or cooperative conformational transitions without phase separation were observed for the random or the protein-like copolymers, respectively. Transition temperature, enthalpy, and heat capacity increment of the protein-like copolymer differed insignificantly from those of the random copolymer of similar composition. The transition heat capacity increments of the protein-like copolymers revealed that only 10-20 of their NIPAM links participate in the formation of a dense water-free globule core. The coil-globule transitions of the protein-like copolymers were described by the thermodynamic three-state model according to the scheme "random coil. condensed coil. globule", which is known to simulate the folding mechanism of globular proteins.