Dioxygen electron transfer

Kamitaka Y, Tsujimura S, Setoyama N, Kajino T, Kano K. 2007. Fructose/dioxygen biofuel cell based on direct electron transfer-type bioelectrocatalysis. Phys Chem Chem Phys 9 1793-1801. 

Fukuzumi S, Mochizuki S, Tanaka T. 1990. Efficient catal)dic systems for electron transfer from an NADH model compound to dioxygen. Inorg Chem 29 653. 

The above reaction scheme was established by a combination of uv-visible absorption and fluorescence, ir isotopic substitution, esr and kinetic measurements (37). The important point to note here is that in 02 rich Xe matrices, ground state Cu(2Sj/2) cannot avoid reactive encounters with 02 to form Cu(02)2 and Cu(02) dioxygen complexes,whereas it is proposed that the formation of CuO, Cu(03) and 03 in dilute 02/Xe matrices arises from the reaction of a long lived mobile excited state Cu(2D) with 02. On the other hand the reactions of photoexcited Ag(2P) with 02 are different (37), electron transfer being favoured to form Ag 02. ... 

Metal complexes of the porphyrins have been studied for many years. Such attention is not surprising, since particular derivatives play a central role in photosynthesis, dioxygen transport and storage as well as other fundamental processes such as electron transfer (Smith, 1975 Dolphin, 1978-9). Indeed, there are few compounds found in nature which can compare with the diversity of biochemical functions exhibited by the porphyrins. 

Ketones are resistant to oxidation by dioxygen in aqueous solutions at T= 300-350 K. Transition metal ions and complexes catalyze their oxidation under mild conditions. The detailed kinetic study of butanone-2 oxidation catalyzed by ferric, cupric, and manganese complexes proved the important role of ketone enolization and one-electron transfer reactions with metal ions in the catalytic oxidation of ketones [190-194],... 

E0 = 40 kJ mol-1 at AH=0) is substituted by a few consecutive fast reactions with electron transfer. Russel [284-291] studied a few reactions of oxidation of alkylaromatic hydrocarbons in the presence of strong bases. He proved the chain mechanisms of these reactions. One of them includes a few stages with addition of dioxygen to carbanion. Another includes the electron transfer from carbanion to dioxygen. 

This reaction was found to be accelerated by the addition of electron acceptors such as nitrobenzene and m-trifluoromethylnitrobenzene. These electron acceptors accelerate the electron transfer from the carbanion to dioxygen. 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

Reaction (2) is an outer-sphere exothermic process (AE° is about —0.4 V) and therefore, the equilibrium of this reaction is completely shifted to the right, i.e., the reoxidation of reduced cytochrome c by dioxygen is impossible. However, the rate constant for Reaction (2) (2.6 + O.lxlO5 1 moR1 s 1) is unexpectedly low for the exothermic one-electron transfer... 

It should be mentioned that Spasojevic et al. [57] recently determined the two-electron reduction potential of lucigenin in water as —0.14 V. As this value is close to the one-electron reduction potential of dioxygen °[02 702] = — 0.16 V, these authors regarded their finding as a support for lucigenin redox cycling. However, it has been demonstrated long ago that two-electron reduction potentials cannot be used for the calculation of equilibrium for one-electron transfer processes [58]. 

The reduction of dioxygen to its fully reduced form, H20, requires the transfer of 4 electrons, and the transfer may proceed via a series of intermediate oxidation states, such as 02 /H00, HOO /HOOH, 0 /OH. These reduced forms of oxygen exhibit different redox properties and in the presence of substrate(s) and/or catalyst(s) may open different reaction paths for the electron transfer process. Fast proton transfer reactions between the corresponding acid-base pairs can introduce composite pH dependencies into the kinetic and stoichiometric characteristics of these systems. 

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