Oxidation of Sulfite

In Starkeya novella, sulfite is oxidized to sulfate by the catalysis of sulfite-cytochrome c oxidoreductase [reaction (4.5)]. The enzyme catalyzes the reduction with sulfite of not only native ferricytochrome c-550 but also horse ferricytochrome c and ferricyanide (Charles and Suzuki, 1966b Yamanaka et al., 1971, 1981b). The enzyme with a molecular mass of 40 kDa has a cytochrome c-551 subunit (23 kDa) (Yamanaka et al., 1981b) and molybdenum (Toghrol and Southerland, 1983). Recently, Kappler et al. (2000) has reported that the molecular mass of the enzyme is 46 kDa and has cytochrome c subunit of 8.8 kDa. 

Similar enzymes are known to occur in Paracoccus (Thiobacillus) versutus (Lu and Kelly, 1984a,b) and T. thioparus (Lyric and Suzuki, 1970b). The P. versutus enzyme has a molecular mass of 44 kDa and contains cytochrome c-551. The T. thioparus enzyme has one atom each of nonheme iron and molybdenum (Kessler and Rajagopalan, 1972). A membrane-bound type sulfite dehydrogenase has been obtained from Thiobacillus (Acidithiobacillus) thiooxidans JCM 7814. The enzyme has the molecular mass of 400 kDa and catalyzes the reduction of horse ferricytochrome c with sulfite (Nakamura et al., 1995, 2001). Also from Paracoccus (Thio-sphaera) pantotrophus GB17, sulfite dehydrogenase has been obtained. Its molecular mass is 190 kDa (2 x 47 kDa + 2 x 50 kDa) and it has 4 heme C molecules and 1-2 atoms of molybdenum (Quentmeier et al., 2000). Furthermore, 

The sulfite oxidation pathway other than that mentioned above occurs in some thiobacilli sulfite reacts with AMP by the catalysis of adenylylsulfate reductase (APS reductase) to form APS and then APS reacts with diphosphate by the catalysis of sulfate adenylyltransferase to produce sulfate and ATP (Lyric and Suzuki, 1970c Stille and Triiper, 1984), or APS reacts with orthophosphate by the catalysis of sulfate adenylyltransferase (ADP) and ATP is formed from resulting ADP by the catalysis of adenylate kinase (Zimmermann et al., 1999). 

Adenylylsulfate reductase of T. thioparus with the molecular mass of 170 kDa has one molecule of FAD, 8-10 atoms of non-heme iron, and 4-5 atoms of inorganic sulfide in the molecule (Adachi and Suzuki, 1977). 

Starkeya novella cytochrome c-550 was first partially purified by Charles and Suzuki (1966b), and thereafter highly purified by Yamanaka et al. (1971, 1991b). The cytochrome functions as the electron acceptor for sulfite-cytochrome c oxidoreductase as mentioned above, and as the electron donor for cytochrome c oxidase (Yamanaka and Fujii, 1980). It resembles mitochondrial cytochrome c in that it 

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Although the usual product of oxidation of sulfite by air is sulfate, at high pH substantial amounts of the strong oxidant Na2SO can form (335). 

The deleterious effect of sulfur dioxide and sulfites in domestic water is increased corrosivity owing to the lowered pH. However, oxidation of sulfite to sulfate in aqueous solutions uses dissolved oxygen, and lliis may retard corrosion. While llte oxichition of sulfite and sulfiirous acid to sulfate and sulfuric acid in the atmosphere is an environmental concern, this reaction is too... 

Mammalian sulfite oxidase is the last enzyme in the pathway for degradation of sulfur-containing amino acids. Sulfite oxidase (SO) catalyzes the oxidation of sulfite (SO ) to sulfate (S04 ), using the heme-containing protein, cytochrome c, as electron acceptor ... 

Sulfonic acids containing nitrogen have long been implicated as essential intermediates in the synthesis of H2SO4 by the lead-chamber process (p. 708) and, as shown by F. Seel and his group, the crucial stage is the oxidation of sulfite ions by the nitrosyl ion NO+ ... 

Sulfite Oxidation Investigated in Micro Reactors Cas/liquid reaction 27 [CL 27] Oxidation of sulfite to sulfate... 

R. Ojani, J.B. Raoof, and A. Alinezhad, Catalytic oxidation of sulfite by ferrocenemonocarboxylic acid at the glassy carbon electrode. Application to the catalytic determination of sulfite in real sample. Electroanalysis 14, 1197-1203 (2002). 

Electrocatalysis in oxidation has apparently first been shown for ascorbic acid oxidation by Prussian blue [60] and later by nickel hexacyanoferrate [61]. More valuable for analytical applications was the discovery in the early 1990s of the oxidation of sulfite [62] and thiosulfate [18, 63] at nickel [62, 63] and also ferric, indium, and cobalt [18] hexacyanoferrates. More recently electrocatalytic activity in thiosulfate oxidation was shown also for zinc [23] hexacyanoferrate. Prussian blue-modified electrodes allowed sulfite determination in wine products [64], which is important for the wine industry. 

Z.P. Wang, S.F. Wang, M. Jiang, and X.Y. Zhou, Electrocatalytic oxidation of sulfite at nickel hexacyanoferrate-film modified electrode and its application. Fenxi-Shiyanshi 12, 91-93 (1993). 

Any sulfate ions resulting from oxidation of sulfite should be removed otherwise they will be converted to sulfuric acid in the subsequent procedure and destroy sultone. Ordinarily, 12-13 g. of barium chloride dihydrate is required. The end point of the addition is conveniently determined with tetrahydroquinone indicator used in spot tests on filter paper [cf. Ind. Eng. Chem., Anal. Ed., 9, 331 (1937)]. 

Sulfite oxidase is a dimetallic enzyme that mediates the two-electron oxidation of sulfite by the one-electron reduction of cytochrome c. This reaction is physiologically essential as the terminal step in oxidative degradation of sulfur compounds. The enzyme contains a heme cofactor in the 10 kDa N-terminal domain and a molybdenum center in the 42 kDa C-terminal domain. The catalytic cycle is depicted in Fig. 9. 

The two-electron oxidation of sulfite generates a Molv Fem state, which converts to a Mov-Fen state. Cytochrome c then oxidizes this state to Mov-Fem, which then undergoes another internal electron transfer (k3) to form MoVI-Fen. A second oxidation by cytochrome c forms MoVI-Fem, which completes the catalytic cycle. 

Fig. 9. Postulated oxidation state changes occurring at the Mo and Fe centers of SO during the catalytic oxidation of sulfite, and concomitant reduction of cyt c. The one-electron reduction shown with a dashed arrow connecting MoVIFem and MoVIFen can be initiated with a laser pulse, in a solution containing 5-deazariboflavin (dRF) and a sacrificial electron donor (AH2). Reprinted with permission from Ref. (20). Copyright 1999, Society of Biological Inorganic Chemistry.

The oxidation of sulfite and thiosulfate becomes facile in the presence of iodide and novel disposable microband sensor electrodes have been developed by Williams and coworkers [187] to allow fast amperometric determination. A similar approach was proposed for the determination of sulfite in wine [188]. In this method, a coulometric titration is carried out in which S(IV) is indirectly oxidized to S(VI). Speciation of SO2 and sulfite was achieved down to micromolar levels. Sulfide and hydrogen sulfide can be determined elec-trochemically in the presence of an iodide mediator [189]. This process may be further enhanced at elevated temperatures. 

Oxidation of sulfite 59 using RuCh and NaI04 yielded the cyclic sulfate 60 which is now susceptible to nucleophilic attack. Treatment of 60 with NaN3 followed by acid hydrolysis of the intermediate 2-sulfate afforded the trans-i-azido-2-hydroxy derivative 61 (Scheme 3) <1997JOC4277>. 

This includes the oxidation of sulfite (56) and of hydrogen peroxide (45, 65), as well as the oxidation of organic ligands like acetylacetonate (16, 72). The reaction of chromic ions with hydrogen peroxide has been also claimed to proceed via Mechanism 5 (93). Analogous to the behavior of ferric ions (56), the reduction of Cu+2 by sulfite ions (89) very likely proceeds by... 

We have found that HCrOr" in dilute solutions will undergo condensation with hydrogen phosphate, bisulfate, and even hydrochloric acid in water to form these mixed anhydrides like CrSCb-2. We have been able to measure equilibrium constants, and I wondered if this wouldn t prove fruitful in studying such reactions as the oxidation of sulfite with chromate. Sulfite is oxidized to a mixture of dithionate and sulfate. 

Oxidation of sulfite to sulfate within cells occurs by a pathway through adenosine 5 -pHosphosulfate (APS, adenylyl sulfate). Oxidation via APS (Eq. 18-22) provides a means of substrate-level phosphorylation,... 

Sulfite oxidase catalyzes the oxidation of sulfite to sulfate, with cytochrome c as the ultimate physiological electron acceptor. Sulfite is oxidized at the molybdenum site, which is in turn reoxidized by the heme. 

In agreement with the statements of Trueper (1) one can say that principally different dissimilatory sulfur metabolic pathways exist in Anoxyphotobacteria for the oxidation of sulfite to sulfate (via APS or directly), the utilization of thiosulfate (splitting or formation of tetrathionate), and the oxidation of sulfide or elemental sulfur (by a "reverse" siroheme sulfite reductase or other mechanisms). 

The most widely used method for the preparation of 1,3,2-dioxathiolane. Y-oxides (cyclic sulfites) 65 bearing C-linked substituents is the reaction of the corresponding 1,2-diols with thionyl chloride in presence of pyridine or Et3N (Scheme 18). More reactive 1,3,2-dioxathiolane. Y,.Y-dioxidcs (cyclic sulfates) 66 are usually obtained by oxidation of sulfites 65 with sodium periodate, which is mediated by mthenium tetroxide generated in situ from a catalytic amount of ruthenium trichloride. Numerous derivatives 65 and 66 were obtained via this approach and its modifications for further transformations, mostly as the synthetic equivalents of epoxides <1997AHC89, 2000T7051> (see also Sections 6.05.5 and 6.05.6, and Tables 1-7). 

We have found that sulfite and bisulfite undergo one-electron oxidation by many free radicals to produce S03. Rate constants determined for selected radicals are given in Table 1. Measurement of the rate constant over a wide range of pH has, in some cases, allowed the separate determination of rate constants for the oxidation of sulfite and bisulfite. The very strong oxidant OH reacts very rapidly and oxidizes bisulfite faster than sulfite, possibly due to a contribution from hydrogen atom abstraction. For radicals which are relatively weaker oxidants, the reaction with sulfite is the faster. For example, with Br2 the ratio of rate constants for sulfite to bisulfite is about 4 for the even weaker... 

The results we have obtained on the one-electron oxidation of sulfite and bisulfite by free radicals also are important in understanding possible chain-initiation and chain-carrying steps in S02 autoxidation. This is particularly true in systems which do not consist simply of S02 and a catalyst. For example, the production and subsequent reaction of halide free radicals could be important in any system containing halogen ions, and subsequent reactions of organic radicals with sulfite will be required to describe completely the effect of organic inhibitors. 

Another inorganic reaction that can be catalyzed by supported porphyrins is the air oxidation of sulfite to sulfate. Sulfite is formed upon absorption of SO2 from flue gases. For this reaction, a 2-V-methylpyridiniumyl-substituted... 

Sulfite Oxidase. This enzyme, isolated from bovine (26, 27) and chicken liver (28), catalyzes the oxidation of sulfite to sulfate. This is possibly a crucial function in animals as S032" (or S02, its gaseous precursor) is toxic while S042" is relatively innocuous. For example, one of the first signs of molybdenum deficiency in rats is a greatly increased susceptibility to S02 poisoning (28). In addition, a human child bom without sulfite oxidase activity did not survive for very long (29). 

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