Redox and complexation chemistry of the Cr super(VI)/Cr super(V)-[sma ll caps]d-glucaric acid system

When an excess of uronic acid over Cr super(VI) is used, the oxidation of d-glucaric acid (Glucar) by Cr super(VI) yields d-arabinaric acid, CO sub(2) and Cr super(III)-Glucar complex as final redox products. The redox reaction involves the formation of intermediate Cr super(IV) and Cr super(V) species. The reaction rate increases with [H super(+)] and [substrate]. The experimental results indicated that Cr super(IV) and Cr super(V) are very reactive intermediates since their disappearance rates are much faster than Cr super(VI). Cr super(IV) and Cr super(V) intermediates are involved in fast steps and do not accumulate in the redox reaction of the mixture Cr super(VI)-Glucar. Kinetic studies show that the redox reaction between Glucar and Cr super(VI) proceeds through a mechanism combining one- and two-electron pathways: Cr super(VI) arrow right Cr super(IV) arrow right Cr super(II) and Cr super(VI) arrow right Cr super(IV) arrow right Cr super(III). After the redox reaction, results show a slow hydrolysis of the Cr super(III)-Glucar complex into [Cr(OH sub(2)) sub(6)] super(3+). The proposed mechanism is supported by the observation of free radicals, CrO sub(2) super(2+) (superoxo-Cr super(III) ion) and oxo-Cr super(V)-Glucar species as reaction intermediates. The continuous-wave electron paramagnetic resonance, CW-EPR, spectra show that five-coordinate oxo-Cr super(V) bischelates are formed at pH less than or equal to 4 with the aldaric acid bound to oxo-Cr super(V) through the carboxylate and the alpha -OH group. A different oxo-Cr super(V) species with Glucar was detected at pH 6.0. The high g sub(iso) value for the last species suggests a mixed coordination species, a five-coordinated oxo-Cr super(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 super(V)-uronic species, and must thus be related to the Glucar acyclic structure. In vitro, our studies on the chemistry of oxo-Cr super(V)-Glucar complexes can provide information on the nature of the species that are likely to be stabilized in vivo.