The models of the denaturation of globular proteins. III. The intramolecular β‐structure coil transitions in poly‐S‐carbobenzoxymethyl‐l‐cysteine

Abstract

AbstractConformational transitions in synthetic poly‐S‐carbobenzoxymethyl‐l‐cysteine (PCBMC) have been investigated in order to model the processes of the denaturation of globular proteins. The solutions of some samples of PCBMC in the mixtures of nonpolar solvents (dichloroethylene, chloroform) with highly polar solvents (dichloroacetic or trifluoroacetic acids) have been studied by viscosimetric, polarimetric, spectroscopic, and dielectric methods, as well as by light scattering and electronic microscopy. Intrinsic viscosity and levorotation increase sharply when solvent breaking hydrogen bonds are added in the narrow range of the solvent composition, the molecular weight (measured by light scattering) remaining constant. It means that intramolecular cooperative conformational transition occurs in the PCBMC molecules. The dispersion of optical activity shows the absence of the helical structures in PCBMC molecules for any solvent composition. Infrared spectra show that the PCBMC molecules possess β‐structure in inert solvents and in films; and viscometric and polarimetric data suggest that this structure is an intramolecular one and corresponds to the compact shape of the PCBMC molecules in solution. The compact structure of PCBMC molecules in nonpolar solvents is also confirmed by dielectric data. The experimental results permit us to draw the conclusion that cooperative conformational transition involving intramolecular β‐structure statistical coil occurs in PCBMC when solvent breaking hydrogen bonds are added. The position and the degree of sharpness of the transition do not depend on the molecular weight of the sample in the interval investigated (from 15,000 to 91,000). The “plate” model of the intramolecular β‐structure has been proposed which does not contradict the results of the electron microscopic study. An assumption is made that owing to the two dimensional character of the intramolecular cross‐β‐structure its melting occurs in accordance with the “all‐or‐none” principle.