To sulfites and

Sulfur Compounds. Aqueous sulfide and H2S, an odiferous compound in some waters, are oxidized rapidly (initially to sulfite and sulfurous acid) the rate constants ate 3x10 and 3 X 10 , respectively. Thiocyanate is oxidized by ozone to cyanide and sulfate via the intermediate formation of sulfite (47). 

The anhydrous monoclinic crystalline form has a density of 1.679 g/cm (59) no hydrates are known. SolubiUty in water is given in Table 4. Ammonium thiosulfate solutions decompose slowly below 50°C and more rapidly at higher temperatures. The anhydrous salt decomposes above 100°C to sulfite and sulfur (60) ... 

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

At the second stoichiometric point, all of the sulfiirous acid has been converted to sulfite, and the pH is determined using 2 For th calculation, we need the concentration of SO3, for which we need to know the volume of NaOH solution required to reach the second stoichiometric point. Start by finding the number of moles of sulfurous acid ... 

The possible roles in sulfur (and sulfide or sulfane-) oxidation of a sulfur dioxygenase or of electron-transport-linked hydration/dehydrogenation are outlined above, but the fate of the sulfite product may be more complex than previously considered. Vishniac and Santer (1957) showed that S-labeled sulfide was rapidly oxidized first to thiosulfate (and polythionates) and then to sulfate by T. thioparus. This observation was incorporated into the original Peck scheme (Eqs. 15.13-15.17) by Peck and Fisher (1962), who realized that the complete oxidation of thiosulfate (after reductive scission to sulfite and sulfide Eq. 15.3) could be explained if there was recycling of sulfide to produce thiosulfate ... 

Gunnison, A. F., and E. D. Palmes. Persistence of plasma 5-sulfonates following exposure of rabbits to sulfite and sulfur dioxide. Toxicol. Appl. Pharmacol. 24 266-278. 1973. 

This enzyme [EC 4.1.1.12], also known as desulfinase, catalyzes the conversion of aspartate to alanine and carbon dioxide. Pyridoxal phosphate is a required cofactor. The enzyme will also catalyze the decarboxylation of aminomalonate as well as the desulfination of 3-sulfino-alanine to sulfite and alanine. 

Cysteine not only is an essential constituent of proteins but also lies on the major route of incorporation of inorganic sulfur into organic compounds.443 Autotrophic organisms carry out the stepwise reduction of sulfate to sulfite and sulfide (H2S). These reduced sulfur compounds are the ones that are incorporated into organic substances. Animals make use of the organic sulfur compounds formed by the autotrophs and have an active oxidative metabolism by which the compounds can be decomposed and the sulfur reoxidized to sulfate. Several aspects of cysteine metabolism are summarized in Fig. 24-25. Some of the chemistry of inorganic sulfur metabolism has been discussed in earlier chapters. Sulfate is reduced to H2S by sulfate-reducing bacteria (Chapter 18). The initial step in assimilative sulfate reduction, used by... 

Sulfur dioxide is a colorless, water-soluble irritant gas (Costa and Amdur 1996). It can be detected by taste at concentrations of 0.35-1.05 ppm (parts per million) and has an immediate pungent irritating odor at a concentration of 3.5 ppm (WHO 1984). It has been termed a mild irritant (Amdur 1969). Ambient sulfur dioxide can react with oxygen to form sulfur trioxide, which then reacts with water (on moist surfaces) to produce sulfuric acid. Sulfur dioxide also can react with water to form sulfurous acid, which dissociates to sulfite and bisulfite ions. The chemical and physical properties of sulfur dioxide are presented in Table 9-1. 

The second pathway by which sulfate is reduced is the dissimilatory pathway in which sulfate is the terminal electron acceptor and leads to the formation of large quantities of H2S. During the dissimilatory reduction of sulfate, APS is formed as in Eq. (6). Then APS is reduced directly to sulfite and AMP by the enzyme APS-reductase. Table XVIII shows the data of Peck 373) on the pathway of sulfate reduction in various microorganisms. 

As pointed out in the preceding section, sulfate assimilation in yeast has been shown to involve the activation of sulfate by ATP successively to adenosine 5 -phosphosulfate and once again to 3 -phosphoadenosine 5 -phosphosulfate. The latter is then reduced in the presence of NADPH to sulfite and 3, 5 -diphosphoadenosine (37 ). Enzymes catalyzing the... 

The archetype Fe(II)/aKG hydroxylase is taurine/ q KG dioxygenase (TauD), an Escherichia coli enzyme that catalyzes the conversion of taurine (2-aminoethanesulfonic acid) to sulfite and aminoacetaldehyde, as illustrated in Scheme 1. TauD catalyzes the hydroxylation of a C H bond on the carbon adjacent to the sulfonate group of taurine. The product of this reaction then decomposes to yield hydrogen sulfite, which serves as an important source of sulfur for many microorganisms. A catalytic mechanism that has been proposed for these enzymes is provided as Scheme 2. Prior to the activation and hydroxylation of the Ci carbon on taurine, q KG binds to the Fe(II) center as a chelate, displacing two of the coordinated waters. Taurine then binds to the enzyme in the vicinity of the Fe(II) center, displacing the remaining water. 

The most important single cause of the instability of neutral or slightly basic thiosulfate solutions is bacteria that metabolize thiosulfate ion to sulfite and sulfate ions as well as to elemental sulfur. To minimize this problem, standard solutions of the reagent are prepared under reasonably sterile conditions. Bacterial activity appears to be at a minimum at a pH between 9 and 10, which accounts, at least in part, for the reagent s greater stability in slightly basic solutions. The presence of a bactericide, such as chloroform, sodium benzoate, or mercury(II) iodide, also slows decomposition. 

In Fig. 4.2, the oxidation mechanisms of lactate with sulfate in the bacteria of Desulfovibrio genus is shown. In the reduction of sulfate, first the compound is changed into adenylylsulfate (APS) using ATP. APS is reduced by the catalysis of adenylylsulfate reductase to sulfite and AMP. The sulfite formed is reduced to... 

Wines manufactured in the U8 carry a contains sulfites statement on the label. 8ome people are allergic to sulfites, and one possible substitute for 8O2 is the enzyme lysoz5me. Lysozyme attacks lactic bacteria, and is used in cheese manufacture. However, it is not able to act as an antioxidant. A possible solution (not yet adopted by the wine industry) would be to mount a combined offensive adding lysozyme and a reduced level of 8O2. 

The naphthylamine is treated with a four- to eight-fold excess of sodium hydrogen sulfite in an aqueous medium at 90-150° for 6-30 hours, the time required varying from case to case this affords the l,2,3,4-tetrahydro-4-oxo-2-naphthalenesulfonic acid which is stable in an acid or neutral medium but decomposes in alkali to sulfite and the naphthol 565 yields are almost quantitative. Moreover, 5-quinolinamine has been converted into 5-quinolinol by this route.566 The reaction fails, however, if there is a sulfo group in the meta-position to the amino group. 

The conversion of serine to cysteine involves some interesting reactions. The source of the sulfur in animals differs from that in plants and bacteria. In plants and bacteria, serine is acetylated to form O-acetylserine. This reaction is catalyzed by serine acyltransferase, with acetyl-GoA as the acyl donor (Figure 23.13). Conversion of O-acetylserine to cysteine requires production of sulfide by a sulfur donor. The sulfur donor for plants and bacteria is 3 -phospho-5 -adenylyl sulfate. The sulfate group is reduced first to sulfite and then to sulfide (Figure 23.14). The sulfide, in the conjugate acid form HS", displaces the acetyl group of the O-acetylserine to produce cysteine. Animals form cysteine from serine by a different pathway because they do not have the enzymes to carry out the sulfate-to-sulfide conversion that we have just seen. The reaction sequence in animals involves the amino acid methionine. 

As discussed above, sulfate is activated by ATP to form APS. Electrons from donors are transferred to APS. The sulfate ion of APS is reduced to sulfite and then to sulfide, which is excreted from cells. The first step, sulfate to sulfite reduction, involves two-electron transfer followed by six-electron transfer during the sulfite to sulfide reduction. The bacteria are capable of performing catabolic sulfate reduction. 

The elemental sulfur becomes colloidally stable in Sg molecules but can also add to sulfite and form thiosulfate S + S (0)3 S - S (0)3 (S203 ), which also loses the sulfur via disproportionation into H2S and SO4 . In hydrometeors and interfacial waters, however, sulfur reacts with oxygen (reactions 5.265 and 5.262) via SO2 to form hydrogen sulfite HSO3. Thiosulfate is an important intermediate in biological sulfur chemistry from both sulfate reduction and sulfide oxidation. Many hypothetical so-called lower sulfuric acids (Table 5.19) might appear as intermediates or in the form of radicals (such as SOH, HSO, HSO2, HSS and HS as seen from the structure formulas) in the oxidation chain from sulfide to sulfate ... 

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