Conformation-dependent solution properties of macromolecules

Abstract

In part A, an expression was derived which correlates the unperturbed mean-square radius of gyration of an intermediate polypeptide chain $<s^2>_{oI}$ with the helical content $f_H$. The unperturbed state corresponding to the $-helix is obtained by converting the helix to a tortuous random-coil with the bond-rotation angle of $+100^\circ$ or $-100^\circ$ (in cis-convention) per residue, or per"virtual bond," and with the bond angle which satisfies the experimental value of 8.8 for the characteristic ratio of the random-coil of PBLG. The ``quasi`` linear expansion factor $\alpha_h$ of a helical sequence in an intermediate chain was evaluated by using Zimm-Bragg-Nagai theory in collaboration with our expression concerning $<s^2>_{oI}$. Thus the mean-square radius of gyration $<s^2>_I$ of polypeptides in the helix-coil transition region was calculated and compared with experiment. Our theoretical results agreed well with experiment. In part B, the theory presented in the preceding paper for the molecular dimensions, i.e., $<s^2>_{oI}$ and $<s^2>_I$, of polypeptides in the helix-coil transition region was applied here to the determination of the intrinsic viscosity $[\eta]_I$ (the $[\eta]$ in the transition region) as a function of the helical content $f_H$. By using the Kurata-Yamakawa theory, the linear expansion factor $\alpha_s$ was converted to the hydrodynamic expansion factor $\alpha_\eta$. Thus, by using the $\alpha_{\eta I}$ (the $\alpha_{\eta}$ in the transition region) and $<s^2>_{oI}$, the $[\eta]_I$ ``s of polypeptides in the helix-coil transition region were calculated, and compared with experimental results for various kinds of polypeptide-solvent systems with good agreement. The relation of the intrinsic viscosity for a perfectly rigid α-helix form of PBLG with the molecular weight, was theoretically obtained. The intrinsic viscosity of PBLG in helicogenic solvents was also calculated, which is applicable to a broken rod; the calculated results we...