Masumeh Jabbari - Department of Chemistry,Ardabil Branch, Islamic Azad University, Ardabil, Iran
Farrokh Gharib - Department of Chemistry, Shahid Beheshti University, G. C., Tehran, Evin, Iran
Ali Shamel - Department of Chemistry,Ardabil Branch, Islamic Azad University, Ardabil, Iran

The acid-base equilibria of valin have been studied in different aqueous solutions of dimethylsulfoxide (0-50 % by v/v) using potentiometric method. Inthis study, the macro and micro protonation constants of the amino acid and its tautomeric constant have been determined at 25 C andconstant ionic strength 0.1 mol dm-3 (NaCl). Theprotonation and the tautomeric constants of valin in different binary mixtures were analyzed in terms ofKamlet, 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 singleand 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.

Valin; Micro and macro protonation and tautomeric constants; dimethylsulfoxide; Solvent effect