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Tautomeric and microscopic protonation equilibria of some alpha-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.