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 andconstant ionic strength (0.1 mol dm-3 sodium perchlorate), using a combination of spectrophotometricand potentiometric techniques. The composition of the complex species was determined by thecontinuous variations method (Job). It was shown that molybdenum (VI) forms a mononuclear 1:1complex with histidine of the type MoO3L- at pCH 5.8. The protonation of histidine and the formationconstant 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 withregard to the single or multi-parameter models. Linear correlation is observed when the experimentallog 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.