Helix-Coil Stability Constants for the Naturally Occurring Amino Acids in Water. 15. Arginine Parameters from Random Poly(hydroxybutylglutamine-co-L-arginine)

Abstract

Water-soluble copolymers containing L-arginine and N5-(4-hydroxybutyl)-L-glutamine were prepared by copolymerization of the N-carboxy-alpha-amino acid anhydrides of Ndelta-tert-butyloxycarbonyl-L-ornithine and gamma-benzyl L-glutamate, followed by aminolysis with 4-amino-1-butanol, by removal of the tert-butyloxycarbonyl protecting group, and by treatment with O-methylisourea. The copolymers were fractionated and characterized, and the thermally induced helix-coil transitions of these copolymers were studied in water at neutral pH in the presence and in the absence of KCl. The Zimm-Bragg parameters sigma and s for the helix-coil transition in poly(L-arginine) in aqueous solution were deduced from an analysis of the melting curves of the copolymers in the manner described in earlier papers. The computed values of s indicate that L-arginine is a weak helix-making residue at low temperature and a weak helix-breaking residue at high temperature in aqueous solution. The results were found to be in good agreement with those obtained earlier in conformational analyses of arginyl residues in proteins.