Oxidation of Cysteine and Glutathione by Soluble Polymeric MnO 2 JULIAÄ N HERSZAGE AND MARIÄ A DOS SANTOS AFONSO* INQUIMAE and Departamento de Quı ´mica Inorga´nica, Analı ´tica y Quı ´mica Fı ´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria Pabello´n II, (C1428EHA) Buenos Aires, Argentina GEORGE W. LUTHER, III College of Marine Studies, University of Delaware, Lewes, Delaware 19956 The kinetics of reduction of soluble polymeric MnO 2 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 MnO 2 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 MnO 2 and thiol concentration. Eyring plots for both thiols reacting with MnO 2 indicate that the reaction is associative (ΔS q ∼ -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 tMn IV SR - and tMn 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. Introduction Thiols are widespread in the environment. Many have been identified and quantified in a variety of samples from suboxic and anoxic waters and sediments, using electrochemical techniques (1-4) and/or chromatographic techniques (3, 5-9). Particularly in sediments, thiols are found in the nano- to micromolar concentration range (2, 3, 5). Thiols are suspected to affect the bioavailability of essential trace metals (e.g., Cu and Zn) in sulfidic environments where they are strongly bound as solid sulfide minerals (3). Among the nonvolatile thiols reported in the literature, cysteine and glutathione are two of the most frequently found. Glutathione is a tripeptide formed by the amino acids glycine, cysteine, and glutamic acid and is thought to be the most abundant nonprotein thiol in animals, plants, and several bacteria (10, 11). Glutathione is a redox cofactor for proteins involving disulfide bonds (12, 13), works as a cysteine repository, and is possibly involved in the S 0 metabolism (12). The main sources of cysteine, an essential amino acid for protein synthesis, are microbial protein degradation (14) and as- similatory sulfate reduction (15). In aquatic environments, hydrogen sulfide can be readily oxidized by manganese and/or iron oxides, either biotically or abiotically, leading to the formation of several oxidized sulfur species such as sulfite, thiosulfate and polysulfides (16-21). Although the reductive dissolution of different iron oxides with several thiols has been studied in water (22-25), this is not the case for the manganese oxides. To the best of our knowledge, the only report for this kind of reaction in the aqueous phase is included in the study of the reactivity of manganese(III/IV) oxides toward many different organic molecules done by Stone and Morgan (26). The authors used thiosalicylic acid as an example of a thiol and only reported an apparent second-order rate constant. There are a few other reports in the literature for the reductive dissolution of manganese oxides with thiols in organic solvents (27-29) and with metallic oxides other than manganese and iron oxides, such as PbO2, CrO3, and Co2O3 (30). The study of the reductive dissolution of manganese oxides with thiols in water could contribute to a deeper and more complete knowledge of both the Mn and S bio- geochemical cycles. Luther and Church (31) proposed that thiols could be oxidized to disulfides and even to sulfonic acids by reaction with manganese and or iron oxides. Sulfonates have been confirmed in sediments in an XANES study (32). According to the results reported for the reaction of thiols with iron oxides in water, the only oxidation products are the corresponding disulfides (24, 25). The same products were reported in the oxidation with manganese oxides in organic solvents (27-29). In this work the study of the reduction of soluble polymeric manganese(IV) dioxide by cysteine and glutathione has been undertaken to find a rate law, identify the reaction products, and elucidate the elementary steps involved in this process over the pH range of 4.0-9.0. Experimental Section The soluble manganese dioxide used was a stable colloidal suspension, which we term polymeric. It was prepared following the technique described by Perez-Benito et al. (33) by mixing the appropriate amounts of KMnO4 (Aldrich) and Na2S2O3 (Aldrich) stock solutions, according to the following stoichiometry: Although this soluble phase is unlikely to exist in marine environments for extended periods, it is likely present in freshwater systems (34, 35). The polymeric manganese dioxide was electrochemically inactive (35) but showed a large absorption band covering the whole visible region of the spectrum with absorbance uniformly increasing with decreasing wavelength that gave a broad maximum at 300- 400 nm (Figure 1). Dilutions of this polymeric oxide followed the Lambert-Beer law in the concentration range used (400nm ) 1.77 × 10 4 M -1 cm -1 ). The UV-Vis spectra were performed using a Hewlett-Packard 8453 diode array spectrophotometer. The stoichiometry of the oxide was determined to be MnO2 (average of three replicates) by iodometric techniques and flame atomic absorption spectroscopy using a Varian AAS5 instrument for total Mn measurement. The point of zero charge (pzc) of the oxide was determined to be 1.93 by potentiometric titrations using a 716 Metrohm Titrino equipped with a Metrohm pH glass combination electrode. * Corresponding author fax: +5411-4576-3341; e-mail: dosantos@ qi.fcen.uba.ar. 3S 2 O 3 2- + 8MnO 4 - + 2H + f 8MnO 2 + 6SO 4 2- + H 2 O Environ. Sci. Technol. 2003, 37, 3332-3338 3332 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 15, 2003 10.1021/es0340634 CCC: $25.00  2003 American Chemical Society Published on Web 06/26/2003