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(HCαNC′CαH) 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).