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.

JOURNAL OF THE CHEMICAL SOCIETY, DALTON TRANSACTIONS

Royal Society of Chemistry

Año: 2009

0300-9246

When excess of the uronic acid over CrVI is used, the oxidation of D-glucuronic acid (Glucur) by CrVI yields the 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 glucuronic acid 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+] arises. 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 D-glucuronic acid and CrVI proceeds through a mechanism combining CrVI ® CrIV ® CrII and CrVI ® CrIV ® CrIII pathways. The mechanism is supported by the observation of free radicals, CrO22+ 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 a-OH group of the furanose form. Penta-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 D-glucuronic acid participate in the bonding to CrV.