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

Abstract

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