Resonance Raman studies of the peptide bond: implications for the geometry of the electronic-excited state and the nature of the vibronic linewidth

Abstract

Recent resonance Raman studies of the peptide bond especially the simple compound N-methylacetamide (NMA) have shown that only a few of the vibrationalmodes of motion are resonantly enhanced with excitation in the region of the ir electronic excitation. The resonance Raman spectra observed for the vapor acetonitrile solution and aqueous solution are quite distinct. These changes in the Raman spectra with changes in solvent and similar changes seen with N-deuteration are consistent with a variation of the form of the normal modes in the ground electronic state. In the case of solutions of NMA in D20 the resonance Raman spectrum contains only a single strong vibration the amide II'' mode. The spectra obtained with 200 run excitation show overtones up to the 0 - 4 transition. This amide II'' vibration shows a 37 cm1 shift upon 13C15N isotopic substitution. This indicates that this mode has an important contribution from C-N stretching but normal mode calculations indicate significant components of other degrees of freedom. The available resonance Raman and absorption spectra provide sufficient data to permit a minimal model of the displacement along the active normal mode with electronic excitation. A fit of this minimal model has been found that is in excellent agreement with the absorption and Raman data. A dis?lacement of L 5 is obtained and the homogeneous linewidth I'' is found to be 1400 cm .