Superoxide addition reactions

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

Deeble DJ, von Sonntag C (1992) Decarboxylation of 3,4-dihydroxymandelic acid induced by the superoxide radical anion a chain reaction. Int J Radiat Biol 62 105 Deeble DJ, Parsons BJ, Phillips GO (1987) Evidence for the addition of the superoxide anion to the anti- oxidant -propyl gallate in aqueous solution. Free Rad Res Commun 2 351-358 Deeble DJ, Parsons BJ, Phillips GO, Schuchmann H-P, von Sonntag C (1988) Superoxide radical reactions in aqueous solutions of pyrogallol and n-propyl gallate the involvement of phenoxyl radicals. A pulse radiolysis study. Int J Radiat Biol 54 179-193 Denisov ET, Denisova TG (1993) The polar effect in the reaction of alkoxy and peroxy radicals with alcohols. Kinet Catal 34 738-744... 

These radicals initiate chain reactions to produce the final low-weight products. In the presence of oxygen, additional reactions generating the superoxide radical are possible ... 

The reaction continuum for HO can be subdivided into three discrete categories that are outlined in Scheme 14 (a) displacement reactions in which the leaving group departs with an electron supplied by HO (polar-group transfer), (b) addition reactions in which a covalent bond is formed (polar-group addition), and (c) simple electron-transfer reactions in which HO acts as an electron donor (single-electron transfer). This view of the chemistry of HO also applies to the reactions of superoxide ion (O2 ) and other nucleophilic oxyanions (Table 16). 

The most common addition reaction is to a carbonyl group without an adequate leaving group. Examples include the reaction of HO and 02 with CO2 and quinones. A common feature of these reactions is the formation of an adduct that is sufficiently stable to be isolated or characterized (see Scheme 18). The same orange-colored species results from the reaction of solid tetramethylammonium superoxide with gaseous CO2 and with neat CCLt, and is believed to be an anion radical, 000(0)0 . ... 

Reactions of free radicals with molecules (or ions) can occur via an addition reaction, for instance the addition of a C-centered radical to oxygen (4.2), hydrogen atom abstraction (4.3), or electron transfer mechanism (e.g., oxidation of CO " by SO" ). Since the total number of electrons is odd, one of the products is a new radical (e.g., (4.2) and (4.3)), except for the reactions with transition metal ions, such as the oxidation of superoxide radicals by Fe3+ ions ... 

Brunmark and Cadenas (27A15) reviewed the major mechanisms that are involved in quinone-induced cytotoxicity in 1989. The redox chemistry of quinoid compounds was surveyed in terms of (1) reactions involving only electron transfers, such as those accomplished during the enzymatic reduction of quinones and nonenzymatic interaction with redox couples generating semiquinones, and (2) nucleophilic addition reactions. In their explanation of the mechanisms involved, quinone is reduced to the hydroquinone or semiqui-none radical by cellular reductase. The semiquinone radical then undergoes rapid autooxidation with the generation of the parent quinone and concomitant formation of superoxide. The hydroquinone reacts rapidly with superoxide to form H2O2 and the semiquinone. 

The earlier references, which state that this powerful oxidant is stable when pure, but explosive when formed as a layer on metallic potassium [1,2], are not wholly correct [3], because the superoxide is manufactured uneventfully by spraying the molten metal into air to effect oxidation [4], Previous incidents appear to have involved the explosive oxidation of unsuspected traces of mineral oil or solvents [3]. However, mixtures of the superoxide with liquid or solid potassium-sodimn alloys will ignite spontaneously after an induction period of 18 min, but combustion while violent is not explosive [3], The additional presence of water (which reduces the induction period) or hydrocarbon contaminant did produce explosion hazards under various circumstances [5], Contact of liquid potassium with the superoxide gives no obvious reaction below 117°C and a controlled reaction between 117 and 177°C, but an explosive reaction occurs above 177°C. Heating at 100°C/min from IT caused explosion at 208°C [6],... 

Figure 1 shows the graphs of the PCL that were recorded with riboflavin as the photosensitizer and luminol as the detector for free radicals [21], The course of the PCL reaction has two maxima at approximately 30 s and 3 min after the start of irradiation. It has been demonstrated by analysis of kinetics after addition of the reactants at varying times that the first maximum is riboflavin-dependent. Luminol is needed only for visualization of the superoxide radicals. 

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