NUCLEAR MAGNETIC RESONANCE STUDY OF AMIDE MONOMERS IN A POLYPEPTIDE HELIX-RANDOM COIL INTERCONVERTING MEDIA

Proton high-resolution nuclear magnetic resonance spectra of N-methylacetamide and N,N-dimethylacetamide have been examined in mixtures of trifluoroacetic acid and chloroform (a solvent system, when appropriately constituted, that is postulated to support helical-random coil or lnterconverting structural forms of certain polypeptides) to order to contrast the spectral behavior of simple amides with that of appropriate polypeptides. Proton spectral displacements for the acid proton of triluoroacetic acid, N-methyl protons, the amide proton, and C-methyl protons are interpreted in terms of equilibria involving the amide, an ion pair consisting of the conjugate acid of the amide and the trifluoracetate ion, and the dissociated ion pair. Concentrations of the various species are functions of the per cent acid; amide Is present in absence of acid, the ion pair is maximized at very nearly 1:1 acid-amide, and the Ion pair completely dissociates as the acid is increased. Kinetic processes, amide-proton exchange, and rotation about the amide-carbon bond are shown to behave consistently with these structural Interpretations.