The temperature-induced coil-globule transition has been studied in dilute aqueous solutions (with 200 mg/L SDS) for different fractions of polyW-isopropylacrylamide) (PNIPAM) and poly-CZV-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 fractions with the lowest molecular weights (~10 x 103) 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.

Methanol-induced conformational transitions in cytochrome c have been studied by near- and far-UV circular dichroism, Trp fluorescence, microcalorimetry, and diffusion measurements. The existence of at least two cooperative stages of transition has been shown. At the first stage, the native protein is transformed into an intermediate which has only traces of tertiary structure, but has a native-like secondary structure content and is relatively compact; i.e., it has properties of the molten globule state. On the second stage, the alcohol-induced molten globule is transformed into a more helical state, typical of proteins at high alcohol concentrations. The conditions at which the alcohol-induced molten globule exists (moderately low pH and moderately low dielectric constant) could be similar to those existing near negatively charged membrane surfaces. Consequently, these results might explain how the molten globule state can be achieved under physiological conditions.

The polycationic polyelectrolyte poly(methacryloylethyltrimethylammonium methyl sulfate) (PMETMMS) of high molecular weight (ca. 26 × 10 6 g mol -1 ) in dilute solutions with acetone-water mixtures displays a coil-globule transition above a certain threshold of acetone. SANS observations of this system indicate that the collapsed state of this molecule consists of a sequence of dense spheres with smooth surfaces connected by regions of loose polymer. The radius of the spheres, 28 nm, is smaller than either the hydrodynamic radius (RH ) 39 nm) or the radius of gyration (RG ) 42 nm). The number of such collapsed regions is small (3-4), the volume fraction of polymer inside them being about 8%.