Solvent effect on tautomeric and microscopic protonation constants of valine in different mixtures of acetonitrile and water

The acid-base equilibria of valine have been studied in different aqueous solutions of acetonitrile (0-50 % by v/v) using a potentiometric method. In this study, the macro and micro protonation constants of the amino acid and its tautomeric constant have been determined at 25 C and constant ionic strength (0.1 mol dm-3 NaClO4). The protonation and the tautomeric constants of valine in different binary mixtures were analyzed in terms of Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the constants versus  (hydrogen-bond donor acidity),  (hydrogen-bond acceptor basicity) and * (dipolarity/polarizability) are poor in all solutions. Multi-parameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single- and multi-parameter models. Linear correlation is observed when the experimental protonation constant values are plotted versus the calculated ones, while the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on protonation and tautomeric constants.