"Domain" Coil-Globule Transition in Homopolymers

Abstract

The temperature-induced coil-globule transition has been studied in dilute aqueous solutions (with 200 mg/L SDS) for different fractions of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamide) (PNIPMAM) using scanning microcalorimetry, diffusion, and size-exclusion chromatography (FPLC). It has been shown that both these polymers undergo a coil-globule transition upon temperature increase. This transition is accompanied by cooperative heat absorption and a decrease of heat capacity, which makes it similar to the cold denaturation of globular proteins. The globule-coil transition is an "all-or-none" process only for the fraction with the lowest molecular weight (~10x10^3), while fractions of larger molecular weights behave as if they consist of quasi-independent cooperative units, the "domains". The number of "domains" in a macromolecule is proportional to the molecular weight of the polymer. This suggests that the "domain" character of cooperative transitions in large proteins does not, in principle, need evolutionary-selected amino acid sequences but can occur even in homopolymers.