Amide I vibrational modes in glycine dipeptide analog: <i>Ab initio</i> calculation studies

Abstract

Coupling between two local amide I vibrational motions of peptides has been quantitatively estimated by assuming that the two peptides interact with each other via dipole–dipole interaction, the so-called transition dipole coupling (TDC) model. The TDC theory has been applied to describing amide I IR and Raman band envelopes and further used to interpret the two-dimensional IR pump–probe and photon echo spectra of polypeptides recently. In order to quantitatively test the validity of the TDC model or in general dipole–dipole interaction model for a dipeptide, we carry out systematic investigations, by using both the ab initio calculation methods and extended TDC theory, on the potential energy surface, vibrational frequencies of symmetric and antisymmetric amide I vibrational normal modes, transition dipole and transition polarizability, IR and Raman intensities of the two modes, IR-Raman noncoincidence phenomena in the full Ramachandran space for a model dipeptide, glycine dipeptide analog. It is found that the spectroscopic properties of dipeptide can be quantitatively well described by the TDC model, but the quantities related to the potential energy surface such as absolute magnitudes of vibrational frequencies and frequency splitting between the two normal modes cannot be accounted for by using the TDC model. A further investigation of dimeric system with two formamide molecules is presented and the applicability of the TDC model to through space vibrational interaction as a function of intermolecular distance between the two peptides is examined.