Water-Induced Disproportionation of Superoxide Ion in Aprotic Solvents

Abstract

The water-induced disproportionation of the electrogenerated superoxide ion (O2 -) in acetonitrile, dimethylformamide, and dimethyl sulfoxide media containing various concentrations of water as a Brønsted acid has been examined by UV−vis spectroscopy. Analysis of the kinetics as a function of O2 - and water concentrations and of the measurement time demonstrated that the disproportionation of O2 - by water in these media obeys a common mechanism: O2 - + H2O ⇄ HO2 • + OH- (k 1, k -1) HO2 • + O2 - → HO2 - + O2 (k 2) (HO2 •, hydroperoxyl radical; HO2 -, hydrogen peroxide anion). The solvent dependence of the obtained kinetic parameters of (k 1/k -1)k 2, k 1 and k -1/k 2 is discussed in terms of the solvation of O2 - and H2O as well as the effective acidities of H2O in different aprotic solvents.

Role of a distal pocket in the catalytic O 2 reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes

“…This may account for the apparent stabilization of the oxygen complex in 3 and provides a plausible explanation for the selectivity of this catalyst. Despite the fact that free superoxide disproportionates in water at high concentrations (20,21), at low concentrations, superoxide under basic conditions has been spectroscopically identified as superoxide di-, tetra-, and hexa-hydrate clusters (22)(23)(24)(25)(26). This observation is supported by ab initio calculations (22)(23)(24)(25)(26).…”

Immobilized Cytochrome c Sensor in Organic/Aqueous Media for the Characterization of Hydrophilic and Hydrophobic Antioxidants

“…The system containing DMSO showed less noise and a higher steady-state current signal. The latter may be due to the fact that the superoxide radical has a longer lifetime in mixtures containing organic solvents than in aqueous buffer [37].…”

The nature and mechanism of superoxide production by the electron transport chain: Its relevance to aging

“…However, there is a substantial problem with the above model which is that the reduction potential of O~ in the hydrophobic phase (as opposed to aqueous phase) is in the order of -700mV (113). This very low reduction potential is due to the creation of a negative ion and its unfavorable solubility in an aprotic environment (113). Even if we assume the lowest value for the Qo semiquinone, -400mV, it is still too high to reduce O 2 to O~ in the hydrophobic phase.…”

“…The rate constant given for the spontaneous dismutation of HO 2" was determined from measurements in aqueous solution at low pH. Since the dismutation of HO 2" involves proton transfers (113,114), its rate constant is lower in low dielectric media (such as the inner mitochondrial membrane) because proton transfers are energetically unfavorable in that environment. To that effect, the rate constant of the dismutation of HO 2. in the aprotic medium dimethylsulfoxide has been measured at 1.7 x 104 M -~ s ~, 15 times slower than in water (114).…”