Redox and complexation chemistry of the CrVI/CrV/CrIV-d-glucuronic acid system

Abstract

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, CrO₂²⁺ (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 ≤ 4 with the uronic acid bound to Cr^V through the carboxylate and the α-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.