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 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.

Conformational analysis of tetrapeptides by exploiting the excitonic coupling between amide I modes

“…The analysis of vibrational spectra of cationic and zwitterionic di-and tripeptides in D 2 O revealed amide I wavenumbers around 1675 cm 1 for the N-terminal peptide group ( 1 in our notation). 10,22 For the sake of simplicity and in close agreement with experimental results, 18 we assumed an identical splitting  0 between the uncoupled amide I modes, so that the wavenumbers of 2 and 3 are 1675 cm 1  0 and 1675 cm 1 2 0 , respectively. Finally, we neglected the possibility that like amide III 23 the intrinsic frequency of amide I might be dependent on the dihedral angles owing to the admixture of C˛H in-plane bending, as suggested by theoretical calculations.…”

“…peptide or to an electronic transition of the adjacent amino acid residue. 22,24 The parameters in Eqn (9) are not independent. The tensor element d can be obtained as a function of a and b by rotating S 1 into the principal axis system, in which the Raman tensor is diagonal.…”

Visible and UV‐resonance Raman spectroscopy of model peptides

“…44 It was assigned to different orientations of the carboxylate group. Together with the structure sensitivity observed for the AII of NMA this result suggests that the AII wavenumber might depend on the dihedral angle .…”

“…Additionally, we introduce a non-amide mode at 1400 cm 1 which has been proved to be a very useful tool for the structure analysis of proteins. 3,6 Figures 8 and 9 show the isotropic and anisotropic visible Raman spectra of diglycine 44 and dialanine (F. Eker and R. Schweitzer-Stenner, unpublished work) between 1200 and 1450 cm 1 measured at neutral pH. In this region these spectra are much more complex than those of aqueous NMA.…”

Eco-Technologies for Obtaining Aminoacid Based Surfactants

“…The FTIR spectrum of glycylglycine presents characteristic absorption bands of amides: the amide I appears as doublet bands at 1674 cm -1 and 1660 cm -1 and amide II at 1482 cm -1 and 1442 cm -1 [14]. Dodecandioyl dichloride presents characteristic absorption bands in FTIR spectrum: CH2 group stretching vibration at 2929 cm -1 and 2857 cm -1 , CH2 vibration deformation at 1465 cm -1 and 1404 cm -1 , (-(CH2)n-,n>3) at 722 cm -1 , corresponding to long chain hydrocarbon.…”