Instituto de Química de Rosario-CONICET, Universidad Nacional de Rosario, UNR, Suipacha 531, S2002LRK, Rosario, Argentina.
Dalton Trans. 2010 Mar 7;39(9):2204-17. doi: 10.1039/b915652f. Epub 2010 Jan 5.
When excess uronic acid over Cr(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr(VI) yields D-glucaric acid (Glucar) and Cr(III) as final products. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and Cr(V) reacting with Glucur at comparable rates. The rate of disappearance of Cr(VI), and Cr(V) increases with [H(+)] and [substrate]. The experimental results indicated that Cr(IV) is a very reactive intermediate since its disappearance rate is much faster than Cr(VI)/Cr(V) and decreases when [H(+)] rises. Even at high [H(+)] Cr(IV) intermediate was involved in fast steps and does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucur and Cr(VI) proceeds through a mechanism combining one- and two-electron pathways for the reduction of intermediate Cr(IV) by the organic substrate: Cr(VI) --> Cr(IV) --> Cr(II) and Cr(VI) --> Cr(IV) --> Cr(III). The mechanism is supported by the observation of free radicals, CrO(2)(2+) (superoxoCr(III) ion) and Cr(V) as reaction intermediates. The EPR spectra show that five-co-ordinate oxo-Cr(V) bischelates are formed at pH < or = 4 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form. Five-co-ordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-co-ordinated oxo-Cr(V) bischelates. At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the cis-diol groups of Glucur participate in the bonding to Cr(V). In vitro, our studies on the chemistry of Cr(V) 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-Cr(V) species can be used as a finger print to identify Cr(V) complexes formed in biological systems.
当使用过量的糖醛酸超过六价铬时，六价铬会氧化 D-葡萄糖醛酸（Glucur）生成 D-葡萄糖二酸（Glucar）和三价铬作为最终产物。该氧化还原反应涉及中间态 Cr(IV) 和 Cr(V) 物种的形成，其中六价铬和五价铬与 Glucur 的反应速率相当。Cr(VI) 和 Cr(V) 的消失速率随[H(+)]和[底物]的增加而增加。实验结果表明，Cr(IV) 是一种非常活泼的中间产物，因为它的消失速率比六价铬/五价铬快得多，并且当[H(+)]升高时会降低。即使在高[H(+)]下，Cr(IV) 中间产物也参与快速步骤，不会在反应中积累。动力学研究表明，Glucur 和六价铬之间的氧化还原反应通过一种机制进行，该机制结合了有机底物对中间态 Cr(IV) 的一电子和两电子途径还原：Cr(VI) --> Cr(IV) --> Cr(II) 和 Cr(VI) --> Cr(IV) --> Cr(III)。该机制得到了自由基、CrO2(2+)（超氧 Cr(III) 离子）和 Cr(V) 作为反应中间体的观察结果的支持。EPR 谱表明，在 pH < 或 = 4 时，形成五配位的氧代 Cr(V) 双螯合物，糖醛酸通过羧酸盐和呋喃糖形式的α-OH 基团与 Cr(V) 结合。除了主要的五配位氧代 Cr(V) 双螯合物外，还观察到少量的五配位氧代 Cr(V) 单螯合物。在 pH 7.5 时，EPR 谱表明形成了一种 Cr(V) 配合物，其中 Glucur 的顺二醇基团参与与 Cr(V) 的键合。在体外，我们对 Cr(V) 配合物化学的研究可以提供有关可能在体内稳定的物种性质的信息。特别是，Glucur-Cr(V) 配合物的 EPR 模式可用作识别生物系统中形成的 Cr(V) 配合物的指纹。
