Superoxide anion disproportionation reaction

The mechanism of the electron transfer from electron donor such as MV to producing molecular hydrogen and catalyzed by Au [187] or Pt clusters [188], and that of the disproportionation reaction of the superoxide radical anion O2 by Pt clusters in solution or supported on colloidal Ti02 particles [189], have been studied by pulse radiolysis. 

The enzyme copper, zinc superoxide dismutase (Cu,Zn-SOD, EC 1.15.1.1) catalyzes the disproportionation of superoxide anion to dioxygen and hydrogen peroxide (equations 1 and 2). Crystallographic data can be found in References 41-46. This antioxidant enzyme is present in the cytosol and mitochondrial intermembrane space of eukaryotic cells and in the periplasmic space of bacterial cells as a homodimer of 32 kDa. Each monomer binds one copper and one zinc ion. The reaction mechanism involves the... 

Hydrogen abstraction by RH02 could also participate in the process of initiating a chain of thermal oxidation reactions (pathy). In aqueous systems, cations will further react by solvolysis, and superoxide anion will readily disproportionate to yield H202 (path i). This is in contrast to the fate of superoxide anions in ozonation advanced oxidation processes (AOPs), where they react primarily with ozone to produce hydroxyl radical. This description of the chemical pathways of UV/H202 oxidation of organics illustrates that, when oxygen is present, the major paths directly or indirectly create more... 

The use of Co + ions as a radical trap and the flash photolysis data obtained for such a system [381] indicate that during the oxidation of [Co(sep)]2+ cation the superoxide radical O2 anion is formed as an intermediate and its reduction even in acidic media (Reaction 137) is more favourable than disproportionation (Reaction 138). 

Che et al. [7] performed a comprehensive analysis of the kinetic parameters of the superoxide anion in different aprotic solvents (e.g. DMSO, ACN and DME) and in the presence of water. Evidence for a reaction between the superoxide anion, 02, and the hydroperoxyl, HO2" (Eq. 18.11), was found rather than the disproportionation reaction of HO2" radicals (Eq. 18.10). 

Dehydroascorbic acid exists in a variety of forms with the hemiketal being favored in aqueous solution. The delocalized nature of the unpaired electron in the ascorbyl radical makes it relatively unreactive, and thus it may disproportionate as shown in Fig. 4. Under aerobic conditions a variety of reactive species derived from oxygen may potentially be involved with ascorbic acid. However, the reactions of the hydroperoxyl radical (HO2) and the superoxide anion radical (02 0 with ascorbic acid have been studied using pulse radiolysis and photolysis and are probably the two most likely to be found under normal physiological conditions (Fridovich, 1976). 

This is achieved upon dismutation or disproportionation of superoxide anion (Oj), according to the reaction mentioned before ... 

The enzyme superoxide dismutase is essential for protecting subcellular structures from damage by oxidative reactions (free radicals), as it catalyses the disproportionation of superoxide anion radicals (202 ), which are toxic products of oxygen metabolism (see Section 3.8.1.13) ... 

At pH values below 4.7— the pATa of HO2 —disproportionations of 02 and HO2 were orders of magnitude slower than reactions of superoxide anion or its protonated form with a second equivalent of POMie. This was true even at the lowest pH values studied, at which disproportionations of superoxide (and of... 

Under physiological conditions, the disproportionation of superoxide radical is very fast and involves reaction of the radical anion 02 with a molecule of the neutral radical, H02 (Eq. 2, Scheme 8.36). Disproportionation yields one molecule of molecular oxygen and one molecule of hydrogen peroxide. 

The reactivity of O2 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution (Chapter 7).23-25,45 These and subsequent kinetic studies confirm that the reaction order is primary>secondary >tertiary and I>Br>Cl >F for alkyl halides, and that the attack by O2-- results in inversion of configuration (Sn2). Superoxide ion also reacts with CCl4,25,26 Br(CH2)2Br,46 C6Cl6, 2,48 and esters -Sl in aprotic media. The reactions are via nucleophilic attack by O2-- on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxy anions, water suppresses the nucleophilicity of 02 - (hydration energy, 100 kcal)52 and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO- and ROO ). 

Our knowledge of the structural properties and enzymic function of a large number of copper proteins has accumulated during the last few decades. The main results have been comprehensively reviewed 1—4) or presented at symposia (5—54). This survey is devoted to erythro-cuprein, one of the most actively studied copper proteins. Erythrocu-prein is sometimes called haemocuprein, hepatocuprein, cerebrocuprein, cytocuprein, or erythro-cupro-zinc protein. Alternatively, the name superoxide dismutase has been suggested as descriptive of its activity the enzyme-catalyzed disproportionation of anionic monovalent superoxide radicals. However, whether or not the enzymic reaction is specific for Oi- still needs to be investigated1). Thus, the name erythrocuprein is used throughout this review. 

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