Conformational Transition in Oligopeptides: An NMR Spectroscopic Study

We examined the 220-MHz NMR spectra for a series of oligopeptides derived from gamma-ethyl L-glutamate, an octapeptide based on beta-methyl L-aspartate, and a low molecular weight L-glutamate polymer ([unk]DP = 20) in deuterochloroform-trichloroacetic acid. A definite transition from folded to nonhelical forms was established for the glutamate systems. The aspartate behavior is explained by a progressive solvation of a peptide chain in a disordered conformation. In all the solvent systems studied, separate peaks for each N-H are observed for the glutamate and aspartate oligomers. Thus, end effects and polydispersity are important in interpreting NMR spectra of partially helical polypeptides. Slow exchange between NH groups on the same chain does not appear to play a significant role in the helix-coil transition.