Superoxide anion radical rate constants

Danen, W. C., and R. J. Warner The Remarkable Nucleophlicity of Superoxide Anion Radical. Rate Constants for Reaction of Superoxide Ion with Aliphatic Bromides. Tetrahedron Letts 1977, 989. 

In many papers the term "superoxide" (HOO ) is used simultaneously with "superoxide anion radicals" (Oa )- However, this is under the physiological conditions incorrect The pKa value of this acid-base equilibrium is 4.8 [227] and, therefore, there is only a very small contribution of HOO at physiological pH (7.4). Therefore, the term "superoxide anion radical" should be exclusively used. The superoxide anion radical is both, a one-electron oxidant and a one-electron reductant. The reactions of O2 with many different biological substrates were studied in detail by the radiation chemists and a summary of the obtained second order rate constants is provided in [228]. However, not a single carbohydrate is mentioned in this comprehensive survey since no reaction could have been observed [229]. 

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. 

The production of ethylene from methional (3-thiomethylpropanal) was induced by the oxidation of xanthine by dioxygen catalysed by xanthine oxidase The second-order rate constant for the reaction of hydroxyl radicals with methional was estimated by pulse radiolysis to amount to 8.2 x lO s while the superoxide anion reacted more slowly The short lag period of the ethylene production induced by the oxidation of xanthine could be overcome by the addition of small amounts of hydrogen peroxide. The reaction was inhibited by SOD or by catalase, and by scavengers of hydroxyl radicals, so that the Haber-Weiss reaction was implicated... 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

Daasbjerg and Lund provided an interesting probe of the borderline between direct and mediated reactions192. Rate constants ( r) for the S 2 reaction of superoxide (02 ) with alkyl halides (e.g. chlorobutane) were compared with the expected rate constants ( ET) for electron transfer reaction between the same alkyl halide and an aromatic anion radical of... 

Reaction Rate Constants of Superoxide Radical Anion/Perhydroxyl Radical with Amino Acids and Selected Antioxidants0... 

Reaction Rate Constants for Superoxide Radical Anion/Rerhydroxyl Radical with Selected Proteins... 

From the decay kinetics of the 550-nm band, in the absence and presence of oxygen, the rate constant for the process pyranyl radical TPF plus oxygen can be estimated to be ca. 107 dm3 mol-1 s 1. This latter result shows that TPF is far less efficient in generating superoxide ion than several cyanoaromatic radical-anion sensitizers. Finally, cyclic voltammetric experiments, under oxygen atmosphere, clearly show that the reversible CV curves, recorded for the sensitizer under inert atmosphere, become totally irreversible. This typical EC behavior [185] had also been observed in a case previously reported [186]. 

The reactions of superoxide radical anions (O2") with sulfide radical cation complexes might represent an important and efficient reaction pathway for the formation of sulfoxides in peptides and proteins containing methionine residues. Absolute rate constants for two sulfide radical cation complexes (S..S) from 1,5-dithia-3-hydroxycyclooctane and (S..N) from Met-Gly dipeptide with Of were measured using pulse radiolysis. The rate constant for the... 

Although the bimolecular rate constant of radical cations with superoxide anion is higher, this process most often results in electron transfer rather than in oxygenation, see ref. [310]. 

The first reduction step provides superoxide anion, which was shown to react with added silanone precursors as is seen fiom the decrease up to a total disappearance of the oxidation peak of O2. Using the ratio of these signals, /p(02 ) tp(02), and the kinetic treatment proposed in [10] for reversible electron transfer followed by an irreversible reaction of ion-radicals and modifying it to pseudo-first order reactions, we determined absolute rate constants of nucleophilic addition of electrogenerated superoxide anion on several silanone precursors (Scheme 5). 

Methionine. The reaction of superoxide radical anions (02 with sulfide radical cation-nucleophile complexes might represent an efficient sulfoxide-forming process in peptides and proteins containing methionine under conditions where significant amounts of sulfide radical cation complexes and superoxide are formed simultaneously. The rate constant for the reaction of 02 with the (S.-. N)+ complex was found to be ca. 3-fold slower as compared to that ofthe reaction with the (S.-.Sf complex. This drop in reactivity may, in part, reflect the lower probability of 62 to encounter S-atom in the (S.-.N) complex as... 

An important application of Equations 1.45 and 1.46 involves analysis of electron exchange between dioxygen (02) and the superoxide radical anion (02 ) in water. Lind and Merenyi determined the rate constant for this reaction by reacting 3202 with isotopically labeled 02 (Equation 1.47).50 The labeled superoxide anion was generated by /-irradiation of 3602. 

The raised energy of the HOMO also provides an explanation with an SET mechanism, since it allows an electron to be transferred more easily to the LUMO of the electrophile, and the radical pair then couple, as usual. A single electron removed from one of the two pairs will leave behind a stabilised radical, and so the rate constant of a reaction of an a-effect nucleophile ought to be more sensitive to the ionisation potential (LUMO energy) of the electrophile than the rate constant with a normal lone-pair nucleophile.277 This proved to be the case in the rates of N-methylation of a series of N-phenylhydroxylamines compared with the rates in some comparable anilines.278 Further support for an SET mechanism is provided by superoxide anions, RO2 , which are exceptionally powerful nucleophiles benefiting simultaneously from an a-effect and from the availability of an unpaired electron.279... 

Cr (02)4 to Cr04. This was confirmed by Peters et al. (1975). The same decompensation is considered hkely to occur in the reaction of sodium chro-mate(Vl)with H2O2. Equation [140] represents Haber-Weiss reaction that superoxide anion and hydrogen peroxide can combine together directly to generate the hydroxyl radical. The rate constant of this reaction is very small in the absence of matals as iron or copper, however. HO may be formed then a chromium(V)-peroxide complex decompses into Cr(Vl) complex and hydroxyl radical as in the Fenton reaction ... 

The mechanism of addition of the superoxide radical anion to cobalt(n) macro-cyclic complexes, [CoN4] +, has been studied in aqueous media. In the pH range 7—8, reaction of [Coii(4,ll-diene-N4)] and [Co (l,3,8,10-tetraene-N4)] with Oa yielded transient intermediates which have been characterized spectroscopically. The rate constants (1.4 x 10 and 1.6 x 10 1 mol s respectively) rule out the participation of HOa at these pH s. The transients are neither Co nor Co and it is concluded that the Oa adds to the metal centre at a labile axial site. The transient complex [OaCo(4,ll-diene-N4)]+ is also formed by the reaction of the corresponding cobalt(i) species with O2. The fate of these intermediates is still uncertain although one possibility is a dimerization to the dinuclear / -complex. 

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