Conformations of peptides in solution by nuclear magnetic resonance spectroscopy. Part II. Homoallylic coupling in cyclic dipeptides

Abstract

Using the crystal structure data of numerous cyclic dipeptides the scope of analysing ⁵,J(HH) of peptides in terms of homoallylic coupling is discussed. From the observed correlation between ³J(HNCH) and ⁵J(HC^αNC′C^αH) of cyclic dipeptides in DMSO and CDCl₃ solutions it is shown that five bond coupling can be rationalised in terms of homoallylic coupling. Assuming the same angular dependence for ⁵J(HH) for peptides in all solvents, the conformational properties of the substituted diketopiperazine rings are determined for cyclic dipeptides in different solvents; in particular, detailed conformational properties are discussed for cyclic dipeptides in D₂O solutions where ³J(HNCH) cannot be observed. The analysis is extended to explain changes with conformation in observed ⁵J(α-CH,NR) of N-substituted cyclic dipeptides (R = CH₃ or δ-CH₂ of proline).