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Oxidation of Cysteine and Glutathione by Soluble Polymeric MnO2

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 μM. In this pH range, the reaction products were identified...

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Bibliographic Details
Published in:Environmental science & technology 2003-08, Vol.37 (15), p.3332-3338
Main Authors: Herszage, Julián, dos Santos Afonso, María, Luther, George W
Format: Article
Language:English
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Summary: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 μM. In this pH range, the reaction products were identified as Mn(II) and the corresponding disulfides (cystine and glutathione disulfide). Cysteic or cysteinesulfonic 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 (ΔS ⧧ ∼ −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 ⋮MnIVSR- and ⋮MnIVSR 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.
ISSN:0013-936X
1520-5851
DOI:10.1021/es0340634