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

Abstract

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...