Solvent effects on protonation and complexation of histidine with molybdenum (VI) at different aqueous solutions of methanol

The formation constants of the species formed in the systems H+ + Mo(VI) + histidine and H+ + histidine have been determined at different aqueous solutions of methanol (0 - 45 % v/v) at 25 °C and constant ionic strength (0.1 mol dm-3 sodium perchlorate), using a combination of spectrophotometric and potentiometric techniques. The composition of the complex species was determined by the continuous variations method (Job). It was shown that molybdenum (VI) forms a mononuclear 1:1 complex with histidine of the type MoO3L- at pCH 5.8. The protonation of histidine and the formation constant of the formed complex species in various media were analyzed in terms of Kamlet, Abboud, and Taft (KAT) parameters. Single-parameter correlation of the formation constant versus (hydrogen-bond donor acidity), (hydrogen-bond accepter basicity) and * (dipolarity/polarizability) are poor in all solutions, but dual-parameter correlation represents significant improvement with regard to the single or multi-parameter models. Linear correlation is observed when the experimental log KS values are plotted versus the calculated ones while the KAT parameters are considered. Finally, the results are discussed in terms of the effect of solvent on protonation and complexation.