Superoxide anion radical redox potential

A semiquinone can be readily oxidized to the parent compormd by molecular oxygen and can then re-enter the reductase-catalyzed reaction. The enzymatic reduction and autoxidation of quinones rmder aerobic conditions generates superoxide anion radicals, and this process is known as redox cycling (Figure 2). Flydroquinones are less prone to transfer electrons to oxygen, because the second-electron potential is often too high. 

Tertiary amine oxides and hydroxy la mines are also reduced by cytochromes P-450. Hydroxylamines, as well as being reduced by cytochromes P-450, are also reduced by a flavoprotein, which is part of a system, which requires NADH and includes NADH cytochrome b5 reductase and cytochrome b5. Quinones, such as the anticancer drug adriamycin (doxorubicin) and menadione, can undergo one-electron reduction catalyzed by NADPH cytochrome P-450 reductase. The semiquinone product may be oxidized back to the quinone with the concomitant production of superoxide anion radical, giving rise to redox cycling and potential cytotoxicity. This underlies the cardiac toxicity of adriamycin (see chap. 6). 

H—A—H -1- 02 -> Products Whether or not the radical anion B will reduce oxygen can be readily determined from standard tables of redox potentials. Another possible route to a radical cation-superoxide anion radical pair is by excitation of charge-transfer complexes and some of these processes were detailed in an earlier review (Davidson, 1975). The propensity for excited states to undergo redox... 

In the late 70s, the interest of many groups was aroused by cyanoaromatic compounds (Sens) because they felt that, by proper choice of sensitizer redox potential, the cyanoaromatic radical anions (SensT) should be re-oxidized by molecular oxygen to produce, by a second exergonic electron-transfer process, superoxide ion OJ [Eq. (6)] [84]. 

Nearly at the same time, the same group reported a study dealing with the electron-transfer initiated oxidation of trans-stilbene (TS) 10a sensitized by the singlet excited states of both DCA and methylene blue (MB+) [124]. The authors proposed that, although the initial electron-transfer step was identical for the two systems, the subsequent steps leading to products (predominantly benzaldehyde) must be different. In fact, dicyanoanthracene radical anion DCAT reduces molecular oxygen to superoxide, whereas reduced methylene blue MB°, owing to its lower redox potential (Ered = —0.25 V vs SCE), doesn t. 

Superoxide radical anion, 02 , a weak reductant, is also an extremely weak oxidant as its standard redox potential of reduction to peroxide, 02 , is about -2.4 V relative to FeCp2 [245]. First made by Gay Lussac almost two centuries ago by reaction of K with O2, KO2 is insoluble except in DMSO in the presence of 18-crown-6. Alternatively, soluble 02 salts are generated cathodically in pyridine, DMF or DMSO in the presence of a solubilizing electrolyte of the type [R4N]C1 (R = Me or -Bu), which provides the superoxide salt [R4N][02] [245]. 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

Solutions of dicyclohexyl-18 crown 6 in DMSO have been used to prepare pale yellow 0.15 mol P solutions of KO2 which contain the O2 anion in approximately the same concentration. The specificity of the superoxide dismutase enzyme was used to show that the solution did indeed contain the superoxide ion in solution. I hc redox potentials of the superoxide and hydroperoxy free radicals, O2 and HO2, have been measured by the fast reaction technique of pulse radiolysis and kinetic absorption spectrophotometry. A d.t.a. study has shown that, during the heating of LiC104-K02 mixtures containing <30% KO2, a eutectic melt occurred at 100—250 C with the loss of superoxide oxygen. At 250—300 °C the mixture melted with loss of peroxide oxygen and at 360— 500 °C the perchlorate decomposed with the loss of all the perchlorate oxygen. 

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