Formation of Glycine and Alanine Zwitter-Ionic Structures Within the Discrete-Continuum Model of a Water Solvent: Intramolecular Proton Transfer

Neutral and zwitter-ionic conformers of glycine (Gly) and alanine (L-Ala) are quantum chemically calculated at the B3LYP+GD3/def2TZVPP level of density functional theory within the discrete-continuum model in the gas phase and with regard to the effect of the aqueous medium. Structures with the minimum dipole moments prove to be the most stable neutral conformers of Gly and Ala in the gas phase. When solvation effects are taken into account within the polarized continuum model, conformers with the maximum dipole moments become energetically favorable due to the dipole-dipole interaction with the medium. Reaction activation barriers of the intramolecular proton transfer in the formation of zwitter-ions studied are calculated depending on the number of water molecules in the first coordination sphere. The inclusion of seven water molecules saturating the hydrogen bond of –\(\text{NH}_{3}^{+}\) and –COO^– functional groups into the discrete-continuum model of a water solvent is shown to significantly decrease the activation barrier to 0.02 kcal/mol for glycine and to 0.09 kcal/mol for alanine.