DOMAIN COIL-GLOBULE TRANSITION IN HOMOPOLYMERS

The temperature-induced coil-globule transition has been studied in di lute aqueous solutions (with 200 mg/L SDS) for different fractions of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamid e) (PNIPMAM) using scanning microcalorimetry, diffusion, and size-excl usion chromatography (FPLC). It has been shown that both these polymer s undergo a coil-globule transition upon temperature increase. This tr ansition is accompanied by cooperative heat absorption and a decrease of heat capacity, which makes it similar to the cold denaturation of g lobular proteins. The globule-coil transition is an ''all-or-none'' pr ocess only for the fractions with the lowest molecular weights (simila r to 10 x 10(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 th e molecular weight of the polymer. This suggests that the ''domain'' c haracter of cooperative transitions in large proteins does not, in pri nciple, need evolutionary-selected amino acid sequences but can occur even in homopolymers.