The Redox Thermodynamics for Dioxygen Species (O2, O2-·, HOO·, HOOH, and HOO-) and Monooxygen Species (O, O-·, ·OH, and -OH) in Water and Aprotic Solvents
Biological systems activate dioxygen (O2) by reduction via electron transfer and H-atom transfer to give intermediates that may be further activated by transition metals in proteins and metabolic co-factors. The reaction thermodynamics for these processes are influenced by the solution matrix and its acidity. Thus, the redox thermodynamics of O2 are directly dependent upon proton activity, O2 + 4H+ + 4e- → 2 H2O E°’ (1) which in turn depends upon the reaction matrix. Table 1 summarizes the pKa’ values for a series of Br0nsted acids in several aprotic solvents and water.1 In acetonitrile the activity values for pKa’ range from -8.8 for (H3O)ClO4 to 30.4 for H2O. This means that the formal potential (E°’) for reaction 1 in acetonitrile (MeCN) is +1.75 V vs. NHE in the presence of 1M(H3O)ClO4 and -0.56 V in the presence of IM (Bu4N)OH.
KeywordsMethyl Viologen Aprotic Solvent Anhydrous Acetonitrile Aprotic Medium Allylic Hydrogen
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- 3.R. Parsons, “Handbook of Electrochemical Constants,” Butterworths Scientific Publications, London (1959).Google Scholar
- 8.D.T. Sawyer, J.L. Roberts, Jr., T. Tsuchiya, and G.S. Srivatsa, Generation of activated oxygen species by electron-transfer reduction of dioxygen in the presence of protons, chlorinated hydrocarbons, methyl viologen, and transition metal ions, in: “Oxygen Radicals in Chemistry and Biology,” W. Bors, M. Saran and D. Tait, eds., Walter de Gruyter and Co., Berlin, (1984).Google Scholar
- 10.J.L. Roberts, Jr. and D.T. Sawyer, Activation of superoxide ion by reactions with protons. Electrophiles, secondary amines, radicals, and reduced metal ions, Israel J. Chem. 23:430 (1983)Google Scholar
- 13.I. Fridovich, “The Biology of Superoxide and of Superoxide Dismutases—In Brief,” in: “Oxygen and Oxy Radicals,” M.A.J. Rodgers and E.L. Powers, eds., Academic Press, New York, (1981).Google Scholar
- 14.J.A. Fee, “Is Superoxide Toxic and are Superoxide Dismutases Essential for Aerobic Life?,” in: “Oxygen and Oxy Radicals,” M.A.J. Rodgers and E.L. Powers, eds., Academic Press, New York, (1981).Google Scholar