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.

Abstract

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.