Redox and complexation chemistry of the Cr(VI)/Cr(V)/Cr(IV)-D-glucuronic acid system

When excess uronic acid over Cr(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr(VI) yields D-glucaric acid (Glucar) and Cr(III) as final products. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and Cr(V) reacting with Glucur at comparable rates. The rate of disappearance of Cr(VI), and Cr(V) increases with [H(+)] and [substrate]. The experimental results indicated that Cr(IV) is a very reactive intermediate since its disappearance rate is much faster than Cr(VI)/Cr(V) and decreases when [H(+)] rises. Even at high [H(+)] Cr(IV) intermediate was involved in fast steps and does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucur and Cr(VI) proceeds through a mechanism combining one- and two-electron pathways for the reduction of intermediate Cr(IV) by the organic substrate: Cr(VI) --> Cr(IV) --> Cr(II) and Cr(VI) --> Cr(IV) --> Cr(III). The mechanism is supported by the observation of free radicals, CrO(2)(2+) (superoxoCr(III) ion) and Cr(V) as reaction intermediates. The EPR spectra show that five-co-ordinate oxo-Cr(V) bischelates are formed at pH < or = 4 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form. Five-co-ordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-co-ordinated oxo-Cr(V) bischelates. At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the cis-diol groups of Glucur participate in the bonding to Cr(V). In vitro, our studies on the chemistry of Cr(V) complexes can provide information on the nature of the species that are likely to be stabilized in vivo. In particular, the EPR pattern of Glucur-Cr(V) species can be used as a finger print to identify Cr(V) complexes formed in biological systems.