The Microscopic and Tautomeric Protonation Constants of Some α-Amino Acids in Dioxane–Water Mixtures

Knowledge of the protonation constants of α-amino acids is important, interesting and necessary for complete understanding of the physiochemical behavior of the acids. The acid–base chemistry of zwitterionic compounds such as α-amino acids has been characterized in terms of the macroscopic constants; K1, K2; the microscopic constants; k11, k12, k21, k22 and the tautomeric constant; kz. The compounds may exist in four different microforms; a cation, zwitterion (dipolarion), neutral species and anion. In this study, both to demonstrate whether the predominant species are zwitterion or the neutral form and to predict the change of dipolar form to neutral form ratio in dioxane–water mixtures, the macroscopic protonation constants of the α-amino acids, glycine, DL-alanine, DL-valine, L-leucine, DL-phenylalanine and L-serine, and their esters, were determined potentiometrically using a combined pH electrode system calibrated as the concentration of hydrogen ion and later the microscopic and tautomeric constants of the amino acids were calculated in dioxane–water mixtures (20, 40, and 60% dioxane (v/v)). Titrations were performed at 25.0 ± 0.1 °C and the ionic strength of the medium was maintained at 0.10 mol·dm−3 using sodium chloride. The protonation constants were influenced by changes in solvent composition and the effects of solvent composition on the protonation constants are discussed. Also, the variation of microscopic constants and the ratio of zwitterionic form to neutral form are discussed on the basis of solute–solvent interactions of amino acids in dioxane–water mixtures.