Sequential polypeptides. 2. Helix--coil transition in poly(gamma-benzyl epsilon-carbobenzoxy-L-lysyl-L-glutamate)

Helix--coil transitions of an alternating copolypeptide poly(gamma-benzyl epsilon-carbobenzoxy-L-lysyl-L-glutamate) (poly[Lys(Z)-Glu(OBzl)]) in binary mixtures of dichloroacetic acid (DCA) and ethylene dichloride (EDC) were investigated by optical rotatory dispersion (ORD). The ORD data obtained were analyzed to evaluate the equilibrium constant s for helix formation and the cooperativity parameter sigma. The values of sigma1/2 are substantially independent of temperature and solvent composition, yielding 0.4 X 10(-2) on the average. The transition enthalpy deltaHc at the transition point decreases from 291 to 154 cal mol-1 as the DCA content is increased from 56 to 64 vol%. At about 20 degrees C and at 58 vol% DCA, poly[Lys(Z)-Glu(OBzl)] in binary mixtures of DCA and EDC undergoes a solvent-induced transition. This DCA content is intermediate between those for poly(gamma-benzyl L-glutamate) and poly(epsilon-carbobenzoxy-L-lysine) in the same binary solvent and close to that of an equimolar random copolymer of the two peptides. However, the s of poly[Lys(Z)-Glu(OBzl)] is not a geometrical mean of those of the parent homopolypeptides. This fact suggests that the helical conformation of the copolypeptide is stabilized by a conformational induction operative between adjacent residues.