Different Conformers and Protonation States of Dipeptides Probed by Polarized Raman, UV−Resonance Raman, and FTIR Spectroscopy

We have measured the polarized nonresonance and resonance Raman as well as FTIR spectra of the model peptides glycylglycine and N-acetylglycine in H₂O and D₂O at pH/pD values between 1.5 and 12.0 with visible, near UV, and far UV excitation wavelengths. The spectra were self-consistently analyzed to obtain reliable spectral parameters of even strongly overlapping bands. Additionally, we have analyzed the polarized nonresonance and preresonance Raman spectra of glycylglycine single crystals. The most important result of this analysis is that for glycylglycine all amide bands as well as the symmetric carboxyl stretch band at ca. 1400 cm^-1 are doublets. As shown in an earlier study (Sieler, G.; Schweitzer-Stenner, R. J. Am. Chem. Soc. 1997, 119, 1720) the amide I doublet results from vibrational coupling of the delocalized H₂O bending mode with internal coordinates of the amide I mode. The amide III doublet is interpreted to result from vibrational coupling between the twisting mode of the C_α methylene group and internal coordinates which normally give rise to the amide III vibration (i.e., CN and C_α1C stretching). In contrast, the amide II and carboxylate subbands are assigned to different conformers with respect to the torsional coordinate of the carboxylate group. While the higher frequency subband of the amide II and carboxylate bands may reflect a parallel orientation of the latter with respect to the peptide, which could be stabilized by hydrogen bonding to NH, the lower frequency band may reflect different orientations in which the carboxylate is hydrogen bonded to water. For N-acetylglycine we also observe two subbands underlying amide I and the carboxyl symmetric stretch band, which again reflects vibrational mixing with water and multiple rotational substates of the carboxylate, respectively.