Tautomeric and Microscopic Protonation Constants of Alanine and Valine in Different Aqueous Solutions of Methanol and Tetrahydrofuran

The solute–solvent interactions of alanine and valine were investigated potentiometrically in different aqueous solvent mixtures (0–50 % v/v) of methanol (an amphiprotic solvent) and tetrahydrofuran (a dipolar aprotic solvent). The macroscopic and microscopic protonation constants as well as the tautomeric constants were determined and correlated with either a macroscopic parameter such as dielectric constant or a microscopic parameter such as a Kamlet, Abboud, and Taft (KAT) solvatochromic parameter. The obtained results show that the protonation constants of the amino group (in log10 scale) are linearly related to the reciprocal of the dielectric constants of the water–organic solvent mixtures but this is not true for the protonation of the carboxylic acid group of the amino acids in both systems. Finally, the results are discussed in terms of the effect of solvent on the protonation and tautomeric constants.