Helix to coil transition in Poly-L-amino acids. III. Kinetic scheme for Poly-γ-benzyl-L-glutamate

Abstract

Double resonance experiments show that the α-CH proton magnetic resonance in poly-γ-benzyl-L-glutamate (PBLG) is not coupled to the ¹⁴N nucleus but is coupled to the NH and β-CH₂ protons. The breadth of the resonance in the random coil form of the polypeptide is due to this coupling and is independent of degree of polymerization.     The coupling of NH and α-CH protons and the occurrence of NH proton resonances are indicative of slow exchange of NH protons with solvent. In addition, the occurrence of two α-CH resonances indicates slow exchange between the two-proton magnetic states. However, the rate of the helix to coil transition in PBLG obtained by other workers is about 10⁴ times as fast as these exchange processes.     The experimental data are rationalized by a kinetic scheme which envisages a rapid equilibrium between helical and coil residues, and a slow equilibrium involving protonation of the peptide groups by the strong organic acid. The upfield α-CH resonance is attributed to the uncharged helical and coil residues and the downfield resonance to the charged helical (if present) and charged coil residues.