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 fa r UV excitation wavelengths. The spectra were self-consistently analyzed to obtain reliable spectral parameters of even strongly overlapping bands. Ad ditionally, we have analyzed the polarized nonresonance and preresonance Ra man spectra of glycylglycine single crystals. The most important result of this analysis is that for glycylglycine all amide bands as well as the symm etric carboxyl stretch band at ca. 1400 cm(-1) are doublers. As shown in an earlier study (Sieler, G.; Schweitzer-Stenner, R. J, Am. Chern. Sec. 1997, 119, 1720) the amide I doublet results from vibrational coupling of the de localized H2O bending mode with internal coordinates of the amide I mode. T he amide III doublet is interpreted to result from vibrational coupling bet ween the twisting mode of the C-alpha methylene group and internal coordina tes which normally give rise to the amide III vibration (i.e., CN and Calph a 1C stretching). In contrast, the amide II and carboxylate subbands are as signed to different conformers with respect to the torsional coordinate of the carboxylate group. While the higher frequency subband of the amide II a nd 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 observ e 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.