The First Step in Glycine Solvation: The Glycine-Water Complex

The jet-cooled spontaneous Raman spectrum of a glycine-water complex (Gly + H2O), the first step in amino acid hydration, is reported. The low-frequency vibrational spectrum (below 500 cm(-1)) of the solvated molecule is recorded and assigned using quantum chemical data calculated from ab initio (MP2) and DFT (B3LYP, BLYP, PBE0 = PBE1PBE). Anharmonic corrections or Raman and infrared (IR) active vibrations are calculated using second-order perturbation theory at the MP2/6-31+G(d) level. The acquired spectra at medium resolution (hwhm of similar to 4 cm(-1)) allow different conformers of the glycine-water heterodimer to be distinguished. Three different dimer conformations are observed and identified; selective collision-induced relaxation processes are used to estimate their relative stability. The results are compared with recent theoretical predictions and microwave (MW) spectroscopy data. The premise that the acidic character of the OH group of the carboxylic acid dominates the interaction between water and glycine is confirmed. The addition of a water molecule is found to greatly change the potential energy surface and conformational preferences of H2NCH2COOH. Water stabilizes conformations in which formation of a closed-ring, H-bonded structure is possible. Simultaneous participation by the carboxyl oxygen of the amino acid in two hydrogen bonds is found to be unfavorable. It may be expected that the addition of extra water molecules could ultimately lead to the stabilization of the Gly zwitterion.