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

GONZÁLEZ, J. C.; GARCÍA, S.; BELLÚ, S.; SALAS PEREGRIN, J. M.; ATRIA, A. M.; SALA, L. F.; SIGNORELLA, S.

DALTON TRANSACTIONS

ROYAL SOC CHEMISTRY

Lugar: Cambridge; Año: 2010 vol. 2010 p. 2204 - 2217

1477-9226

When excess uronic acid over CrVI is used, the oxidation of D-glucuronic acid (Glucur) by CrVI yields D-glucaric acid (Glucar) and CrIII as final products. The redox reaction involves the formation of intermediate CrIV and CrV species, with CrVI and CrV reacting with Glucur at comparable rates. The rate of disappearance of CrVI, and CrV increases with [H+] and [substrate]. The experimental results indicated that CrIV is a very reactive intermediate since its disappearance rate is much faster than CrVI/CrV and decreases when [H+] rises. Even at high [H+] CrIV intermediate was involved in fast steps and does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucur and CrVI proceeds through a mechanism combining one- and two-electron pathways for the reduction of intermediate Cr(IV) by the organic substrate: CrVI CrIV CrII and CrVI CrIV CrIII. The mechanism is supported by the observation of free radicals, CrO22+ (superoxoCrIII ion) and CrV as reaction intermediates. The EPR spectra show that five-co-ordinate oxo-CrV bischelates are formed at pH 4 with the uronic acid bound to CrV through the carboxylate and the -OH group of the furanose form. Five-co-ordinated oxo-CrV monochelates are observed as minor species in addition to the major five-co-ordinated oxo-CrV bischelates. At pH 7.5 the EPR spectra show the formation of a CrV complex where the cis-diol groups of Glucur participate in the bonding to CrV.In vitro, our studies on the chemistry of CrV 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-CrV species can be used as a finger print to identify CrV complexes formed in biological systems