Stabilization of zwitterions in solution:: γ-aminobutyric acid (GABA)

The stabilization of gamma-aminobutyric acid (GABA) zwitterions in aqueous solution is investigated at the HF/6-31+G*, B3LYP/6-31+G*, and MP2/6-31+G* levels of theory using explicitly bound water molecules to model short-range solvent effects and the conductor-like screening solvation model (COSMO) to estimate long-range solvent interactions. B3LYP and MP2 yield similar structures, relative energies and overall trends whereas HF theory does not provide a realistic description of GABA. The only approaches yielding zwitterionic structures consistent with experiment are the application of COSMO to GABA.2H(2)O or GABA.5H(2)O. An accurate description of aqueous phase GABA therefore requires both explicit interaction with at least two water molecules and long-range dielectric interactions with the solvent. Both types of interaction preferentially stabilize zwitterionic over neutral structures and stabilize extended zwitterions with respect to folded conformers. No stable neutral GABA.5H(2)O tautomers are obtained, suggesting that only zwitterionic forms of GABA are likely to be present in water. The extended GABA.5H(2)O zwitterions are postulated to be more stable in solution than the folded conformers and it is likely that a number of zwitterionic conformations are stable in water.