On the Ratio of Zwitterion Form to Uncharged Form of Glycine at Equilibrium in Various Aqueous Media

The ratios of the zwitterion form to the uncharged form of glycine, KD=[NH3+CH2COO−]/[NH2CH2COOH], were determined in mixtures of water with methanol, ethanol, t-butyl alcohol, dimethyl sulfoxide, acetonitrile, 1,4-dioxane, N,N-dimethylformamide, and tetrahydrofuran. The value of In KD decreases and, therefore, ΔG298° increases, with the increase in the mole fractions of the organic solvents. The plots of ΔG° vs. reciprocal of the dielectric constants of the alcoholic mixtures are simply linear indifferent to the kind of alcohols, indicating that this relation obeys the Scatchard equation and that the free energy change of the process of the intramolecular protonation in glycine is controlled mainly by electrostatic interaction, especially by the entropy change associated with the preferential solvation around the charged amino and carboxyl groups.