Reduction superoxide

The kinetics and the mechanism of superoxide reduction by SORs have been studied by several researchers. It was suggested that SORs react with superoxide via an inner-sphere mechanism, binding superoxide at ferrous center to form a ferric hydroperoxo intermediate [46,48 50]. The rate constant for this reaction is equal to 108 109 1 mol-1 s-1 [46,49], This... 

There is still a possibility that SOR activity of the enzymes studied could be adventitious [50], but on the whole, the experimental results are very convincing [51,52]. However, the questions remained related to catalytic mechanism of SORs. In contrast to SODs, these enzymes are unable to complete the catalytic cycle by themselves because they need reductants for reducing the oxidized form of an enzyme. Therefore, it is possible that the mechanism of superoxide reduction by SORs may change from catalytic to stoichiometric one depending on the presence of additional reductants. 

Rbo is a homodimeric protein, each subunit of which contains two distinct mononuclear nonheme iron centers in separate domains (Fig. 10.4) (Coehlo et al. 1997). Center I contains a distorted rubredoxin-type [Fe(SCys)4] coordination sphere. [Fe(SCys)4] sites in proteins are known to catalyze exclusively electron transfer, which is, therefore, the putative function for center I. Center II contains a unique [Fe(NHis)4(SCys)] site that is rapidly oxidized by 0, and is, therefore, the likely site of superoxide reduction (Lombard et al. 2000). A blue nonheme iron protein, neelaredoxin (Nlr) from Desulfovibrio gigas (Silva et al. 1999), contains an iron center closely resembling that of Rbo center II (Table 10.1). The blue color is due to the oxidized (i.e., Fe(III)) form [Fe(NHis)4(SCys)] site, which, in both Nlr and Rbo, has a prominent absorption feature at -650 nm. Reduction of center II to its Fe(II) form fully bleaches its visible absorption. These absorption features have been used to probe the reactivity of Rbo with superoxidie. 

Fujita et al. [49] further tested the long-term structural integrity of cyt c by superoxide reduction activity assay after storing cyt c in ILs for 3 weeks. They observed a complete loss of activity of cyt c in [BMIM][MeSO ] and [BMIM][lactate], whereas [choline] [H PO ] retained the activity of cyt c as good as the fresh buffer solution. The observed activity correlated well with the secondary structure information obtained from ATR-FTIR. No change in amide I bands was observed for cyt c in [choline] [H PO ] which is indicative of the intact native structure of cyt c even after... 

Stability and activity of cyt c were also investigated in various ILs with various kosmotropic and chaotropic ions by superoxide reduction assay [49], It was observed that activity and thermodynamic stability of cyt c were reduced in the following order [BMIM][MeSO ]<[BMIM][lactate]<[BMIM][acetate]<[cho-line][Bu2PO ]<[BMPY][HjPO ]<[choline][H2PO ]. This trend agrees well with the kosmotropicity order of the anions present in the ILs indicated by the viscosity B coefficient of the anions [116] [MeSOJ < [lactate] < [acetate] <[BUjPO ] <[H,POJ-<[H/OJ-. 

Reaction of superoxide with a reduced metal-ion complex to give oxidation of the complex and release of hydrogen peroxide (analogous to Reaction 5.97) has been observed in the reaction of Fe EDTA with superoxide. Reduction of a Co superoxo complex by free superoxide to give a peroxo complex (analogous to Reaction 5.99) has also been observed. ... 

The effect of proton-coupled, that is, proton-assisted, reduction of superoxide is obvious and can be quantified (Eqs. (6) and (7)). However, how redox potentials for superoxide reduction and oxidation are changed upon its coordination to a metal center is not known and is usually completely neglected. It is of course not... 

Molten Salt-induced Corrosion of Metals (Hot Corrosion) 1603 superoxide reduction steps ... 

Copper proteins are involved in a wide range of biologieal oxidation-reduetion proeesses. These include long-range electron transfer, dismutation of superoxide, reduction of nitrite and nitrous oxide, and reversible binding, transport, activation. 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

However, under anhydrous conditions and in the absence of catalytic impurities such as transition metal ions, solutions can be stored for several days with only a few per cent decomposition. Some reductions occur without bond cleavage as in the formation of alkali metal superoxides and peroxide (p. 84). 

Under aqueous conditions, flavonoids and their glycosides will also reduce oxidants other than peroxyl radicals and may have a role in protecting membranal systems against pro-oxidants such as metal ions and activated oxygen species in the aqueous phase. Rate constants for reduction of superoxide anion show flavonoids to be more efficient than the water-soluble vitamin E analogue trolox (Jovanovic et al, 1994), see Table 16.1. 

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