Herszage Julián, dos Santos Afonso María, Luther George W
INQUIMAE and Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pabellón II, (C1428EHA) Buenos Aires, Argentina.
Environ Sci Technol. 2003 Aug 1;37(15):3332-8. doi: 10.1021/es0340634.
The kinetics of reduction of soluble polymeric MnO2 by cysteine and glutathione has been studied in the pH range of 4.0-9.0. The concentration of thiols was varied between 1 and 2 mM, while the MnO2 concentration was varied between 2 and 12 microM. In this pH range, the reaction products were identified as Mn(II) and the corresponding disulfides (cystine and glutathione disulfide). Cysteic or cysteine sulfonic acid was formed only when pH < 2. Experimental data indicate that the rate law over the pH range of 4-9 is first-order in both MnO2 and thiol concentration. Eyring plots for both thiols reacting with MnO2 indicate that the reaction is associative (deltaS(double dagger) approximately -160 J mol(-1) K(-1)) and proceeds via an inner-sphere redox process. The reaction proceeds via the formation of two different inner-sphere complexes [triple bond]Mn(IV)SR- and [triple bond]Mn(IV)SR and their further reaction to products. Both surface species are linked to each other via acid-base equilibria, and the rate constant decreases as pH increases. The presence of two ligand surface species is determined using surface complexation modeling. A reaction mechanism in agreement with the experimental results is proposed.
在pH值为4.0 - 9.0的范围内，研究了半胱氨酸和谷胱甘肽对可溶性聚合二氧化锰的还原动力学。硫醇的浓度在1至2 mM之间变化，而二氧化锰的浓度在2至12 μM之间变化。在此pH范围内，反应产物被鉴定为Mn(II)和相应的二硫化物（胱氨酸和谷胱甘肽二硫化物）。仅当pH < 2时才会形成半胱磺酸或半胱氨酸磺酸。实验数据表明，在4 - 9的pH范围内，速率定律对二氧化锰和硫醇浓度均为一级。两种硫醇与二氧化锰反应的艾林曲线表明该反应是缔合反应（ΔS‡约为 -160 J mol⁻¹ K⁻¹），并通过内球氧化还原过程进行。反应通过形成两种不同的内球配合物[≡Mn(IV)SR⁻]和[≡Mn(IV)SR]及其进一步反应生成产物。两种表面物种通过酸碱平衡相互关联，并且速率常数随着pH值的增加而降低。使用表面络合模型确定了两种配体表面物种的存在。提出了一种与实验结果一致的反应机理。
