Helix to coil transition in poly-L-amino acids. I. Viscosity study of poly-gamma-benzyl-L-glutamate.

Abstract

The polyelectrolyte effect has been observed in the viscometry of various synthetic polypeptides, including poly-γ-benzyl-l-glutamate, in dry dichloroacetic acid. The effect can be suppressed by the addition of water (0.83 m) or water plus potassium chloride but not by potassium chloride alone. Its occurrence can only be explained on the basis of charging of the peptide groups of the randomcoil polymers by interaction with DCA. It is also observed in 20% DCA-80% ethylene dichloride for PBG samples of weight-average degree of polymerization (DPw) ≯ 155. The helix is flexible under these conditions, due probably to the occurrence of short, charged, random-coil breaks internally and at both ends (structure 2, Fig. 9). The intrinsic viscosity of PBG samples of a wide range of molecular weights has been determined in mixtures of DCA and EDC. The exponent in the Mark-Houwink equation decreases from 1.65 in the absence of DCA to 1.45 in 20% DCA-80% EDC, 1.39 in 66.7% DCA-33.3% EDC and 0.89 in 83% DCA-17% EDC. This shows that the rigid helix becomes flexible on the addition of DCA, and this structure is maintained up to the helix-coil transition point originally proposed by Doty & Yang (1956). As DPw increases, the transition from coil to flexible helix occurs between DPw 13 and 18 in 50% DCA-50% EDC and between DPw 28 and 40 in 66.7% DCA-33.3% EDC.