Conformational dependence of anharmonic vibrations in peptides: amide-I modes in model dipeptide.

The conformational dependence of a set of anharmonic vibrational parameters for the amide-I modes in peptides has been examined at the level of Hartree-Fock theory. By using glycine dipeptide as a model molecule, the phi-psi maps of diagonal and off-diagonal anharmonicities, local mode mixing angle, zero-order local mode frequencies, as well as intermode coupling, were calculated, and all were found to exhibit certain conformational sensitivities. The characteristics of the phi-psi maps of the diagonal and off-diagonal anharmonicities were found to be complementary to each other, and the latter was found to correlate well with that of the mixing angle, reflecting the fact that these anharmonic parameters are interconnected and all determined by the same set of underlying anharmonic force field. The mean values of the diagonal and off-diagonal anharmonicities were found to be 15.7 cm(-1) and 9.9 cm(-1) respectively. The mean value for the two local mode frequency differences was estimated to be 9.7 cm(-1), showing a nondegenerate local mode picture. For significant peptide conformations, the calculated anharmonic parameters were found to be in reasonable agreement with values obtained at the level of density functional theory as well as values obtained with recent two-dimensional infrared experiments.