Anharmonic C-H and C=O interactions in peptide and sugar

C-H and C=O stretching modes are two among many structural and dynamic probes of proteins and peptides in condensed phases. Anharmonic properties of these two modes in peptide and sugar have been examined using a second-order perturbative vibrational approach. High order force constants were obtained and examined to find how crucial they are in determining the degree of mode localization and the nature of mode anharmonicity of the two stretching modes. It is found that the C-H mode is highly localized, and its diagonal anharmonicity is mainly determined by the mode itself. However, the C=O mode is largely delocalized, and the diagonal anharmonicity involves contributions from other modes. The off-diagonal anharmonicity between C-D and C=O modes is found to be negative in deuterated species, differing from those of the non-deuterated ones. It is also found that inter-mode interaction between each of the two modes with low-frequency modes contribute significantly to the off-diagonal anharmonicity. These low-frequency modes give rise to a network of energy relaxation or intramolecular vibrational energy redistribution pathways which can be used to examine temporal behavior of intramolecular vibration energy flow, provided a femtosecond broadband two-dimensional infrared spectroscopy is available.