Protonation Constants of Valin in Different Aqueous Solutions of Dimethylsulfoxide

The protonation constants of valin (K1 and K2) was determined in binary mixtures of water with dimethylsulfoxide containing (0, 10, 20, 30, 40, and 50) % (v/v) using a combination of potentiometric method at 25 °C and constant ionic strength (0.1mol ·dm-3 sodium perchlorate). The protonation constants were analyzed using the normalized polarityparameter (ETN) and Kamlet, Abboud, and Taft (KAT) parameters. A very good linear correlation of log K versus the normalized polarity parameter was obtained. Dual-parameter correlation of log K versus π*(dipolarity/polarizability) and R (hydrogen-bond donor acidity) as well as π* and (hydrogen-bond acceptor basicity) also gives good results in various aqueous organic solvent mixtures. Finally, the results are discussed in terms of the effect of the solvent on the protonation constants.