The isotropic and anisotropic Raman spectra of neat N-methylacetamide (NMA) at different temperatures between -10 and 60 °C and NMA in acetonitrile were measured in order to spectroscopically compare and characterize the crystallized (T < 28 °C) and liquid states. These plus infrared data were subjected to a self-consistent component band analysis. We found that the amide I band is composed of two subbands in the solid phase and three in the liquid phase. For the former, the subbands at 1633 and 1656 cm -1 arise from transition dipole coupling interactions associated with the A g and B 2g species of the crystal unit cell. Depolarization ratio measurements suggest a departure from strict D 2h symmetry. The three subbands in the liquid phase reflect different aggregate structures. The lowest frequency band at 1634 cm -1 results from an NMA oligomer exhibiting a structure similar to that observed in the ordered crystal phase. The most intense subband shows a significant negative noncoincidence effect, its isotropic component appearing at 1650 cm -1 and its anisotropic part at 1655 cm -1 . This subband is interpreted as resulting from locally ordered short oligomeric hydrogen-bonded structures. The third subband is at 1675 cm -1 and results from isolated nonhydrogen-bonded NMA molecules or from amide I modes of the terminal groups of the above oligomers. Amide III shows a small but detectable positive noncoincidence effect in the liquid phase (2 cm -1 ), which is also assignable to transition dipole coupling between adjacent molecules in a locally ordered environment. The Raman bands arising from the symmetric bending modes of the two methyl groups are significantly affected by crystallization; the CCH 3 symmetric bending mode becomes depolarized and less intense while the NCH 3 symmetric bending mode gains intensity and becomes polarized. Ab initio calculations of torsional distortions of the CH 3 groups, caused by interactions between adjacent non-hydrogen-bonded NMA molecules in the crystal, qualitatively reproduce these effects.

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 Cα methylene group and internal coordinates which normally give rise to the amide III vibration (i.e., CN and Cα 1C 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.

We have measured polarized Raman spectra of the model peptides glycylglycine and N-acetylglycine in aqueous solution with different excitation wavelengths. The spectra in the region between 1500 and 1800 cm-1 were consistently analyzed by using the same band shapes, halfwidths, and frequencies to fit the profiles of corresponding Raman bands. The quality and the statistical significance of the fits were judged by their residuals and reduced χ2 numbers. This strategy enables us to obtain reliable spectral parameters of even strongly overlapping bands. Our experimental results show that the amide I bands of glycylglycine and N-acetylglycine in H2O are composed of two subbands, whereas the corresponding amide I‘ band of glycylglycine in D2O can be fitted by one single band. Moreover, we found that the amide I band region in the Raman spectra of glycylglycine in different H2O/D2O mixtures (i.e., 25%/75%, 50%/50%, and 75%/25%) significantly deviate from the weighted superposition of the corresponding spectra of glycylglycine in pure H2O and D2O. These results are rationalized by invoking vibrational coupling between the amide I mode and the bending modes of the surrounding water molecules which provide a continuum of vibrational states. This coupling is absent in D2O because deuteration causes a downshift of the water's bending mode. In H2O/D2O mixtures the undeuterated species exhibits a reduced splitting of its amide I band due to the lower density of H2O molecules. Hence our results show that peptides and their aqueous environment form a dynamic entity. For glycylglycine the analysis of amide II also reveals a splitting into two subbands which most likely results from two different conformers with respect to the orientation of the carboxyl group.