Sulfite from sulfide

Anoxyphotobacteria which do not possess a "reverse" sulfite reductase are nevertheless able to form sulfite from sulfide, if they contain a flavocytochrome and a thiosulfate reductase. Therefore Trueper and Fischer (22) assume that in these organisms the following reaction mechanism might be possible ... 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

The sulfite reductase from the hyperthermophilic methanogen Methanocaldococcus jannashii is able to reduce the otherwise toxic sulfite to sulfide that is required for growth. In contrast to most organisms that use nicotinamides and cytochromes as electron carriers, this organism uses a coenzyme p42o-dependent reductase (Johnson and Mukhopadhyay 2005). 

Photocatalytic Hj by Direct Sunlight from Sulfide/Sulfite Solution... 

Cerveramarch, S.C., Borrell, L., Gimenez, J., Simarro, R., Andujar, J.M. 1992. Solar hydrogen photoproduction from sulfide sulfite substrate. Int J Hydrogen Energy 17 683-688. 

Koca, A., Sahin, M. 2002. Photocatalytic hydrogen production by direct sun light from sulfide/ sulfite solution. Int J Hydrogen Energy 27 363-367. 

Dilute acids precipitate sulfur from thiosulfates (difference from sulfides and sulfites),... 

E reduction from sulfur dioxide/sulfite is much more plausible, Le. the direct, ght-dependent reduction of sulfite to sulfide (see Figure 1, pathway 3). 

Sulfate. As for the production of hydrogen sulfide from sulfur dioxide/sulfite at least three possible pathways for the light-dependent synthesis of hydrogen sulfide in response to sulfate can be assumed, i.e. first the light-dependent reduction of sulfate to carrier-bound sulfide followed by a release of the sulfide moiety from its carrier second the light-dependent reduction of sulfate to carrier-bound sulfide followed by an incorporation of the sulfide moiety into cysteine and subsequent degradation of cysteine third the release of sulfite from carrier-bound sulfite followed by reduction of free sulfite to sulfide (see Figure 1). 

Apart from hydrogen evolution, the electrons of reduced ferredoxin can take alternative routes leading to biosynthesis. In anaerobic bacteria, reduced ferredoxin can be used directly for the reduction of pyridine nucleotides (Tagawa and Arnon (99) Valentine, Brill and Wolfe (107) Fredericks and Stadtman (44)) for the reduction of hydroxyla-mine to ammonia (Valentine, Mortenson, Mower, Jackson, and Wolfe (109) for COa fixation in the reductive carboxylation of acetyl-CoA to pyruvate (Bachofen, Buchanan, and Arnon (13) Raeburn and Rabino-witz (83) Andrews and Morris (3) Stern (98)) for the reduction of sulfite to sulfide (Akagi (1)) and, in the presence of ATP, it can be used for the reduction of N2 to NH3 (Mortenson (72,73) D Eustachio and Hardy (40)). The role of ferredoxin in these reactions as well as in the oxidative degradative reactions discussed above is summarized in Fig. 10. 

Sulfur can be effectively removed from a compound, organic or inorganic, by contact with Raney nickel. The action has been described by Aubry (118) as being noncatalytic in nature. One atom of sulfur is removed from sodium thiosulfate in the cold, yielding sodium sulfite from which the sulfur can be completely removed at 100°. Although sulfur can be completely removed from stannous sulfide, it can be only partially removed from antimony sulfide. Following are some of the inorganic compounds from which sulfur removal by use of Raney nickel has been observed (119). 

Addition Reaction. The double bond of dehydroalanine and e-methyl dehydroalanine formed by the e-elimination reaction (Equation 6) is very reactive with nucleophiles in the solution. These may be added nucleophiles such as sulfite (44). sulfide (42), cysteine and other sulfhydryl compounds (20,47), amines such as a-N-acetyl lysine (47 ) or ammonia (48). Or the nucleophiles may be contributed by the side chains of amino acid residues, such as lysine, cysteine, histidine or tryptophan, in the protein undergoing reaction in alkaline solution. Some of these reactions are shown in Figure 1. Friedman (38) has postulated a number of additional compounds, including stereo-isomers for those shown in Figure 1, as well as those compounds formed from the reaction of B-methyldehydroalanine (from 6 elimination of threonine). He has also suggested a systematic nomenclature for these new amino acid derivatives (38). As pointed out by Friedman the stereochemistry can be complicated because of the number of asymmetric carbon atoms (two to three depending on derivative) possible. 

The initial six-electron oxidation of sulfide to sulfite is catalysed by a soluble, dis-similatory sulfite reductase that contains siroheme and at least one iron-sulfur center as prosthetic groups [83-85]. While similar enzymes in plants and in non-pho-totrophic bacteria usually function to reduce sulfite to sulfide in assimilatory pathways, the enzyme in photolithoautotrophically grown C. vinosum appears to function in the reverse direction, with the electrons from sulfide oxidation being delivered to an as yet unidentified acceptor. Evidence is also available for the... 

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. 

More recently, in the determination of cyanide with a metallic silver-wire electrode Frenzel ei al.[38] have shown that relatively non-selective potentiometric sensors may be used to make selective determinations by enhancing the selectivity using gas-diffusion separations. Potential interferences from sulfide, sulfite and nitrite which may form interfering gaseous species were removed by a pre-oxidation with permanganate and dichromate. 

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 ... 

Sulfite reductase features a siroheme-containing active site and catalyzes the six-electron reduction of sulfite to sulfide at the heme iron [31,61]. A detailed account of each step of the mechanism has recently been provided as detailed in Fig. 6, based on DPT data. Thus, the catalytic cycle would be initiated with a Fe(in)-S03H adduct, from which protonation and water elimination lead to a... 

Unwanted carbonyl compounds, which can produce an off-flavor in stored beer, are bound by sulfite derived from yeast metabolism. Yeast reduces the sulfate present in the wort to sulfite and sulfide, which is then consumed in the biosynthesis of sulfur-containing amino acids. If the growth of yeast comes to a standstill, excess sulfite is eliminated, increasing the stability of the beer to oxidative processes. 

Priya R, Kanmani S (2009) Batch slurry photocatalytic reactors for the generation of hydrogen from sulfide and sulfite waste streams under solar irradiation. Solar Energy 83 1802-1805... 

Polythiodipropionic acids and their esters are prepared from acryUc acid or an acrylate with sulfur, hydrogen sulfide, and ammonium polysulfide (32). These polythio compounds are converted to the dithio analogs by reaction with an inorganic sulfite or cyanide. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

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