Oxidizing sulfur-specific

The catalytic activities and substrates of the oxidative sulfur-specific pathway enzymes are unique, yet related enzymes and catalytic mechanisms can be found in a variety of enzymes operating on different substrates. 

Microcoulometric titration is used as the detection mode in some commercial sulfur-specific analysers. Sulfur in PP and waxes (range from 0.6 to 6 ppm S) were determined by means of an oxidative coulometric procedure [537]. The coulometric electrochemical array detector was used for determining a variety of synthetic phenolic antioxidants (PG, THBP, TBHQ, NDGA, BHA, OG, Ionox 100, BHT, DG) in food and oils [538],... 

Rhodococcus sp. Strain WU-K2R A Rhodococcus strain capable of sulfur-specific desulfurization of benzothiophene, naphthothiophene (NT), and some of their alkyl derivatives was reported [35]. The metabolites of BT desulfurization were BT sulfone, benzo[c][l,2]oxanthiin S-oxide, benzo[c][l,2]oxanthiin S,S-dioxide, o-hydroxystyrene, 2,(2 -hydroxyphenyl)ethan-l-al, and benzofuran. The NT metabolites were NT sulfone, 2 -hydroxynaphthyl ethene, and naphtho[2,l-b]furan [35], The exact biochemical pathway was not determined, however, part of the pathway for BT desulfurization was speculated to be similar to Paenibacillus All-2. 

More specifically, the invention involves the use of Thiobacillus denitrificans under anaerobic conditions to oxidize sulfur compounds such as hydrogen sulfide to sulfate. The process may be carried out in various ways such as in a batch or a continuous bioreactor system using a suspended or an immobilized biocatalyst. The method is particularly applicable to treating natural gas containing hydrogen sulfide and producing a biomass byproduct. 

Sulfate is typically found in all types of wastewater in concentrations greater than 5-15 gS nr i.e., in concentrations that are not limiting for sulfide formation in relatively thin biofilms (Nielsen and Hvitved-Jacobsen, 1988). In sewer sediments, however, where sulfate may penetrate the deeper sediment layers, the potential for sulfate reduction may increase with increasing sulfate concentration in the bulk water phase. Under specific conditions, e.g., in the case of industrial wastewater, it is important that oxidized sulfur components (e.g., thiosulfate and sulfite) other than sulfate may act as sulfur sources for sulfate-reducing bacteria (Nielsen, 1991). 

Bacteria may catalyze and considerably enhance the oxidation of pyrite and Fe(II) in water, especially under acidic conditions (Welch et al., 2000, 597). Many microbial species actually oxidize only specific elements in sulfides. With pyrite, Acidithiobacillus thiooxidans is important in the oxidation of sulfur, whereas Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans (formerly Thiobacillus fer-rooxidans) oxidize Fe(II) (Gleisner and Herbert, 2002, 140). Acidithiobacillus ferrooxidans obtain energy through Reaction 3.45 (Gleisner and Herbert, 2002, 140). The bacteria are most active at about 30 °C and pH 2-3 (Savage, Bird and Ashley, 2000, 407). Acidithiobacillus sp. and Leptospirillum ferrooxidans have the ability to increase the oxidation of sulfide minerals by about five orders of magnitude (Welch et al., 2000, 597). 

Inorganic peroxygens have also been used to oxidize sulfur centres, particularly sodium perborate, which is excellent for oxidizing electron-deficient sulfides to sulfones.412 Sodium perborate tetrahydrate has been used to stereo-specifically oxidize a-methylbenzylamine thiols in acetic acid.413 The use of organic peracids such as MCPBA was not found to be stereospecific for the above mentioned oxidation. 

Margalith et al. (1966) demonstrated that T. ferrooxidans grown on iron could oxidize sulfur but the rate of oxidation was lower than with iron however, sulfur-grown cells of T. ferrooxidans could also oxidize sulfite, dithionate, thiosulfate, tetrathionate and sulfide but not thiocyanate (Silver, 1970). Details of the specific metabolic reactions for sulfur oxidation by T. ferrooxidans have been outlined in an earlier publication (Lundgren et al., 1974). 

Initially, fuel sulfur was regulated to reduce emissions of the oxides of sulfur, which contribute to acid rain, ozone, and smog. The recent and stricter round of sulfur specifications, however, are an effort to reduce automobile emissions of the oxides of nitrogen (NO c) and particulate matter (PM). For example, the 15 ppmw diesel sulfur limit follows from the USEPA s parallel program of rule making that seeks to reduce automobile NO and PM emissions by 95% and 90%, respectively, by 2007. Automobile manufacturers are demanding ultra-low-sulfur fuels because only then would their advanced, sulfur-sensitive after-treatment technologies achieve such drastic reductions in NO and PM emissions. 

The significance of the total sulfur content of diesel fuel cannot be overestimated and is of great importance because of the production of sulfur oxides that contaminate the surroundings. Generally, only slight amounts of sulfur compounds remain in diesel fuel after refining, and the diesel fuel must meet sulfur specification. However, with the planned reduc-... 

In 1881, Camille Faure coated the lead plates with a paste of red lead oxide, sulfuric acid and water, and then charged them to form Pb and Pb02 active masses. The specific energy of the battery increased to 8 Wh kg at 10 hours discharge rate [12]. 

Cys-11, Asp-14, Cys-17, and Cys-56. One consequence of the partial noncysteinyl ligation to the 4Fe cluster is that the Fe atom that would be normally coordinated by a cysteinyl sulfur is more easily removed when coordinated by a carboxyl group of Asp. This means that the ferredoxin can be quantitatively converted to the 3Fe form by chemical oxidation that specifically removes the Asp-coordinated Fe atom. The P. furiosus protein also contains two other Cys residues, at positions 21 and 48 (Fig. 1), and these are redox active and can form a disulfide bond. The 4Fe center is also redox active, and like most other clusters of this type, it undergoes one-electron redox chemistry with a low midpoint potential (—370 mV, 23°). Hence, P. furiosus ferredoxin can exist in four formal redox states, where the cluster is either reduced (Fdred) or oxidized (Fdox), and the two Cys residues 21 and 48 are either in the form of a disulfide (form A) or exist as free thiols (form B). These four distinct redox states of the protein are stable and can be prepared at room temperature. The lack of rapid equilibration between the different states appears to be due to the high stability of the protein. For example, the wild-type ferredoxin exhibits limited degradation even when incubated at 95° for many days. 

Characterization of the Sulfur-Specific Pathway. The enzymatic pathway for oxidative desulfurization of DBT and related organosulfur compounds has proven to involve unique biochemistry. 

Table 2. Representative substrates and products of sulfur specific oxidative desulfurization...

Sulfur-specific desulfurization of DBTs and other organosulfur compounds is best characterized in the bacterial genus Rhodococcus and exemplified by R. erythropolis strain IGTS8 and involves a series of oxidations of the sulfur moiety followed by a hydrolytic release of sulfite. This and related pathways have been shown to desulfurize a wide range of DBTs, BTHs, and sulfides. Moreover, deep desulfurization to low ppm levels of sulfur has been demonstrated with a variety of hydrotreated diesel range oils. 

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