Hydrophobic-Radical Influence on the Structure and Vibrational Spectra of Zwitterionic Glycine and Alanine in the Condensed State

The structure and vibrational spectra of zwitterionic glycine and alanine in aqueous solution and the solid state were calculated in the B3LYP/6-311++G(d,p) approximation. The environment infl uence was taken into account by two methods, i.e., the self-consistent reaction fi eld (SCRF) model and a clear accounting for the effects of hydrogenbonds (complexes with water molecules). The geometric, energetic, and spectral characteristics required to establish that a hydrophobic radical affected the ability of glycine and alanine to form H-bonds were determined. It was found by comparison with experiment that zwitterionic glycine and alanine in the condensed states had to be surrounded with three water molecules, one of which was situated between the N⁺H₃ and COO– ions, in order to calculate their vibrational (IR and Raman) spectra. The formation energies of the alanine complexes with water were 56.47 kcal/ mol and 12.55 kcal/mol greater than those of glycine for formation of a complex with one water molecule situated between the ionized groups and with three water molecules, respectively.