The effects of hydration on the structure and vibrational force field of the peptide group were examined by performing ab initio molecular orbital (MO) calculations on N-methylacetamide (NMA) and its clusters with up to three water molecules. The dielectric solvent effect was taken into account by the self-consistent reaction field (SCRF) method. It is shown that the wavenumbers of NMA are strongly affected by both the dielectric effect and formation of hydrogen bonds. The vibrational force field of NMA in aqueous solution is well described by taking into account the hydrogen bonding of three water molecules and the dielectric effect of the surrounding water solvent. The wavenumber shifts of the amide I, II and III bands induced by formation of hydrogen bonds are approximately additive. There is a strong correlation among the C=O stretching force constant, the C=O bond length and the amide I wavenumber. Calculated changes in the structural parameters induced by hydrogen-bond formation are consistent with changes in the resonance Raman intensities of the amide I, II and III bands. The changes in the C—N and C=O bond lengths induced by hydrogen-bond formation in the excited electronic (21A′) state are opposite from those in the ground electronic (11A′) state. These opposite structural changes between the two electronic states are rationalized by using a simple Hamiltonian based on a two-state model, which describes the vibronic interaction between the two states involving a mode consisting of a linear combination of the C=O stretching and the C—N contraction.
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Torii et al. (1998) studied this question.