Tautomeric and microscopic protonation equilibria of some α-amino acids

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. (c) 2007 Elsevier Inc. All rights reserved.