Tautomeric and microscopic protonation equilibria of some α-amino acids

Abstract

The acid–base equilibria of several amino acids, namely glycine, l-alanine, l-valine, l-leucine, l-phenylalanine, l-serine, l-methionine, and l-isoleucine, have been characterized in terms of microscopic protonation constants and tautomeric ratios. In this study, the microscopic protonation constants and tautomeric ratios of eight amino acids in different ethanol–water mixtures have been determined. The variation of microscopic constants of amino acids is discussed on the basis of solute–solvent interactions, and the ratio of dipolar ionic form to neutral form is determined in ethanol–water mixtures (20/80% ethanol by volume). The dipolar ionic form still greatly predominates, even in 80% ethanol, as estimated from potentiometric titration data.