The thermally induced coil-helix transition of poly(y-benzyl-cglutamate) (PBLG) and poly(y-methyl-Lglutamate) (PMLG) in binary solvent mixtures was investigated by calorimetric and optical rotatory dispersion (ORD) measurements. Dichloroacetic acid was the common active solvent, and the inert solvent was one of the chlorinated hydrocarbons, such as chloroform, 1,3dichloropropane, lchlorobutane, or lchlorooctane.
The thermodynamic parameters characterizing the intramolecular polypeptide and polypeptide-solvent interactions were calculated using the Karasz and Gajnos theoretical model [(1973) J. Phys. Chern. 77, 1139-11451. It was found that the enthalpy (AHJ and entropy (AS,) of helix stabilization in the absence of the active solvent depend on the inert solvent, but only in the case of PBLG. This is explained by the additional helix stabilization achieved by the stacking of the benzyl groups. The stacking is more pronounced in less polar chlorinated hydrocarbons with longer aliphatic chains. The results obtained indicate that the maximum helix stability is reached in chlorinated hydrocarbons with 12 C atoms. In the case PMLG, with an aliphatic ester side group, AH, and AS, are independent of the inert solvent.
The ORD measurements were used to determine the maximum fraction of helicity attained a t constant solvent composition and the transition temperature, Tc, at the point where f H = 0.5. It was found that, for the same solvent composition, T, was higher than the temperature of the midpoint of the calorimetric peak. This is explained by the fact that the maximum fraction of helicity is less than unity. The finite transition width was taken into account by calculating the phase boundaries for different fractions of helicity using the value of u estimated from the calorimetric and van't Hoff enthalpies in the usual manner