Conformations of peptides in solution by nuclear magnetic resonance spectroscopy. Part 5. Homoallylic proton spin coupling in linear peptides

Abstract

Five bond proton spin coupling, ⁵J(HH), has been observed in some linear di- and tri-peptides with trans peptide bonds. Magnitudes of ⁵J(HH) were analysed in terms of homoallylic coupling using N-methylacetamide and NN-dimethylacetamide as standard compounds for groups antiperiplanar across peptide bonds. Together with ³J(HNCH) magnitudes the results for ⁵J(HH) can be used to limit the range of conformations (ϕ,ψ) for peptides in solution. Attention has been focused on two peptide conformations studied by 100 MHz ¹H n.m.r. measurements of N-acetyl-L-alanyl-N-methylamide (C₇ structure) and N-acetyl-L-valyl-glycyl-N-methylamide (β-turn) in different solvents. The conformational properties are compared with previous studies using X-ray crystallography, theoretical calculations, and spectroscopy (n.m.r., i.r.).