Structure and Stability of γ-Aminobutyric acid-(H2O)n(n = 0-5) Clusters: Zwitterionic vs. Canonical forms

Abstract

Calculations are presented for the γ-aminobutyric acid-(H 2 O) n (n = 0-5) clusters in both canonical and zwitterionic forms. We examine the effects of microsolvation on the structures and transformation between the canonical and zwitterionic forms. The canonical forms are predicted to be more stable for n = 0-4. With five microsolvating water molecules, the two forms of γ-aminobutyric acid become quasidegenerate, with the energies of zwitterionic forms slightly (by 1 - 3 kcal/mol) higher. The lowest energy zwitterionic conformer of γ-aminobutyric acid-(H 2 O) 5 cluster is calculated to isomerize to canonical form through a barrier-less proton transfer process and is thus predicted to be kinetically unstable. Therefore, we predict that the canonical conformers of y-aminobutyric acid should be observed predominantly in the gas phase at low temperature in presence of up to five water molecules.