Conformational Induction in Block Copolypeptide of γ-Benzyl L-Glutamate and ε-Carbobenzoxy L-Lysine

Abstract

Conformations of triblock copolypeptide [Glu(OBzl)]m—[Lys(Cbz)]n—[Glu(OBzl)]m in m-cresol and in mixtures of dichloroacetic acid (DCA) and 1, 2-dichloroethane (EDC) were studied by optical rotatory dispersion measurements. It was found that the polypeptide was essentially helical in m-cresol, while it underwent a conformational transition from random coil to helix with raising temperature in mixtures of DCA and EDC of appropriate compositions. In order to elucidate the correlation between these transition behaviors of the copolypeptide and those of the constituent homopolypeptides, the Zimm–Bragg–Nagai formalism of helix-coil transition in homopolypeptide has been extended to triblock copolypeptides. The helical fraction is expressed as a function of the transition parameters s and σ of the two residues and the degree of polymerization of each block. The helix formation in m-cresol can be reasonably interpreted by this theory if s and σ are given experimental values of the corresponding homopolypeptides. In mixtures of DCA and EDC, however, the observed helical fraction was always larger than the theoretical value computed by using the homopolypeptide parameters. The copolymer data and the homopolymer data can be explained consistently by this theory if the helical conformation is assumed to be stabilized at each of the block boundaries.