摘要 : When excess uronic acid over Cr~(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr' yieldsD-glucaric acid (Glucar) and Cr"' as final products. The redox reaction involves the formation ofintermediate Cr~(VI) and Cr~V ... 展开 When excess uronic acid over Cr~(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr' yieldsD-glucaric acid (Glucar) and Cr"' as final products. The redox reaction involves the formation ofintermediate Cr~(VI) and Cr~V species, with Cr~(VI) and Cr~Vreacting with Glucur at comparable rates. The rateof disappearance of Cr~(VI), and Cr~V increases with[H~+]and [substrate]. The experimental resultsindicated that Cr~(VI) is a very reactive intermediate since its disappearance rate is much faster thanCr /Cry and decreases when [H~+]rises. Even at high[H*]Cr~(VI) intermediate was involved in fast stepsand does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucurand Cr"' proceeds through a mechanism combining one- and two-electron pathways for the reductionof intermediate Cr(iv) by the organic substrate: Cr~(VI) →Cr~(IV)→Cr~(II) and Cr~(VI) → Cr~(IV) Cr~(III). Themechanism is supported by the observation of free radicals, CrO_2~(2+)(superoxoCrion) and Cr~V asreaction intermediates. The EPR spectra show that five-co-ordinate oxo-Cr~Vbischelates are formed atpH < 4 with the uronic acid bound to Cr~V through the carboxylate and the a-OH group of the furanoseform. Five-co-ordinated oxo-Cr~Vmonochelates are observed as minor species in addition to the majorfive-co-ordinated oxo-Cry bischelates. At pH 7.5 the EPR spectra show the formation of a Crcomplexwhere the cis-diol groups of Glucur participate in the bonding to Cr. In vitro, our studies on the chemistry of Cr~V complexes can provide information on the nature of thespecies that are likely to be stabilized in vivo. In particular, the EPR pattern of Glucur-Cr~V species canbe used as a finger print to identify Cr~Vcomplexes formed in biological systems. 收起