HELIX TO COIL TRANSITION IN POLY-L-AMINO ACIDS .3. KINETIC SCHEME FOR POLY-GAMMA-BENZYL-L-GLUTAMATE

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. © 1969, CSIRO. All rights reserved.