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

摘要

Recent resonance Raman studies of the peptide bond, especially the simple compound N-methylacetamide (NMA), have shown that only a few of the vibrational modes of motion are resonantly enhanced with excitation in the region of the $pi$pi$+*$/ 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 D$-2$/O the resonance Raman spectrum contains only a single strong vibration, the amide II' mode. The spectra obtained with 200 nm excitation show overtones up to the 0 $YLD 4 transition. This amide II' vibration shows a 37 cm$+$MIN@1$/ shift upon $+13$/C-$+15$/N 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 displacement of $Delta $EQ 2.5 is obtained and the homogeneous linewidth $Gamma is found to be 1400 cm$+$MIN@1$/. The possible role of inhomogeneous broadening in the analysis of this data is discussed. The geometry change upon electronic excitation is discussed in relation to calculations of the excited state geometry using semiempirical methods.