Conformations of peptides in solution by nuclear magnetic resonance spectroscopy. Part I. Application of nuclear magnetic double resonance spectroscopy to the determination of cis- and trans-conformations of peptide bonds

Abstract

The 100 MHz 1H n.m.r. spectra of some N-substituted amides and linear peptides have been measured. A small five-bond, long-range spin coupling has been shown to exist between groups antiperiplanar to each other across the amide and peptide bonds. In certain cases, 5J(HH) has been observed between groups with a syncoplanar arrangement such that 5J(HH)anti > 5J(HH)syn. The observation of 5J(HH) was used to assign the cis- and/or trans-conformations of peptide bonds in linear peptides containing N-methylated amino-acids. For unsubstituted peptide bonds the observation of 5J(HH) between the Cα protons of adjacent amino-acids provides a direct method to confirm the presence of the trans-isomer of the peptide bond in linear or cyclic peptides in aqueous solutions.