Protonation-deprotonation of the glycine backbone as followed by Raman scattering and multiconformational analysis

Because of the absence of the side chain in its chemical structure and its well defined Raman spectra, glycine was selected here to follow its backbone protonation-deprotonation. The scan of the recorded spectra in the 1800-300 cm (1) region led us to assign those obtained at pH 1, 6 and 12 to the cationic, zwitterionic and anionic species, respectively. These data complete well those previously published by Bykov et al. (2008) [16] devoted to the high wavenumber Raman spectra (>2500 cm (1)). To reinforce our discussion, DFT calculations were carried out on the clusters of glycine + 5H(2)O, mimicking reasonably the first hydration shell of the amino acid. Geometry optimization of 141 initial clusters, reflecting plausible combinations of the backbone torsion angles, allowed exploration of the conformational features, as well as construction of the theoretical Raman spectra by considering the most stable clusters containing each glycine species. (C) 2013 Elsevier B.V. All rights reserved.