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

Abstract

When an excess of uronic acid over Cr^VI is used, the oxidation of D-glucaric acid (Glucar) by Cr^VI yields D-arabinaric acid, CO₂ and Cr^III-Glucar complex as final redox products. The redox reaction involves the formation of intermediate Cr^IV and Cr^V species. The reaction rate increases with [H⁺] and [substrate]. The experimental results indicated that Cr^IV and Cr^V are very reactive intermediates since their disappearance rates are much faster than Cr^VI. Cr^IV and Cr^V intermediates are involved in fast steps and do not accumulate in the redox reaction of the mixture Cr^VI–Glucar. Kinetic studies show that the redox reaction between Glucar and Cr^VI proceeds through a mechanism combining one- and two-electron pathways: Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III. After the redox reaction, results show a slow hydrolysis of the Cr^III-Glucar complex into [Cr(OH₂)₆]³⁺. The proposed mechanism is supported by the observation of free radicals, CrO₂²⁺ (superoxo-Cr^III ion) and oxo-Cr^V-Glucar species as reaction intermediates. The continuous-wave electron paramagnetic resonance, CW-EPR, spectra show that five-coordinate oxo-Cr^V bischelates are formed at pH ≤ 4 with the aldaric acid bound to oxo-Cr^V through the carboxylate and the α-OH group. A different oxo-Cr^V species with Glucar was detected at pH 6.0. The high g_iso value for the last species suggests a mixed coordination species, a five-coordinated oxo-Cr^V bischelate with one molecule of Glucar acting as a bi-dentate ligand, using the 2-hydroxycarboxylate group, and a second molecule of Glucar with any vic-diolate sites. At pH 7.5 only a very weak EPR signal was observed, which may point to instability of these complexes. This behaviour contrasts with oxo-Cr^V-uronic species, and must thus be related to the Glucar acyclic structure. In vitro, our studies on the chemistry of oxo-Cr^V-Glucar complexes can provide information on the nature of the species that are likely to be stabilized in vivo.