Conformational dependence of anharmonic NH stretch vibration in peptides

Conformational dependence of the fundamental and overtone vibrational transition frequencies and anharmonicities for the NH stretch mode in glycine dipeptide has been examined theoretically using a second-order vibrational perturbation approach. It is found that unlike the diagonal and off-diagonal anharmonicities, both the harmonic and anharmonic vibrational frequencies exhibit clear sensitivity to peptide conformation, convincingly showing that the NH stretch mode can be used as peptide local structural probe. These results reflect the intrinsic localized nature of the NH stretch mode. Weak vibrational coupling is predicted between the two adjacent NH stretch modes, even though they are highly anharmonic.