Interaction between an amide nitrogen lone pair and a noncontiguous carbonyl carbon and its effect on amide rotation

ALTHOUGH rotational barriers^1-4 about the C-N bond in simple amides have been investigated extensively by means of nuclear magnetic resonance (NMR) spectroscopy, there is no information about the effect of proximal dipolar groups on the rotational process. Information of this type would be particularly relevant to peptide bonds in proteins because of the multiplicity of dipoles with the potential for Coulombic interaction with the amide moiety, and because such interaction may be important in determining the preferred conformation of proteins⁵. We have evidence to suggest that N-CO rotation is significantly increased by orbital overlap between the nitrogen atom of the amide group and a noncontiguous carbonyl function.