Four-electron dioxygen reduction

Interpreting these results on a detailed molecular basis is difficult because we have at present no direct structural data proving the nature of the split Co(IIl/lI) voltammetry (which seems critical to the electrocatalytic efficacy). Experiments on the dissolved monomeric porphyrin, in CH-C solvent, reveal a strong tendency for association, especially for the tetra(o-aminophenyl)porphyrin. From this observation, we have speculated (3) that the split Co(III/II) wave may represent reactivity of non-associated (dimer ) and associated forms of the cobalt tetra(o-aminophenyl)porphyrins, and that these states play different roles in the dioxygen reduction chemistry. That dimeric cobalt porphyrins in particular can yield more efficient four electron dioxygen reduction pathways is well known (24). Our results suggest that efforts to incorporate more structurally well defined dimeric porphyrins into polymer films may be a worthwhile line of future research. 

Dioxygen reduction with dicopper(I) complexes that affords dicopper(III) bis- i-oxo species (Equation 4.30) can be considered formally as a four-electron process. The four-electron dioxygen reduction with bimetallic systems affording high-valent diamond core products or intermediates is also known for transition metals other... 

Anson and co-workers have shown that two Co ions were not necessary for four-electron 02 reduction.266 The mew-substituted complex porphyrin Co(TPyP) (42) complex bears four active pyridyl donors which readily react with four equivalents of [Ru(NH3)5(OH2)]2+ to produce the tetra-ruthenated derivative. The four Ru centers are sufficiently remote that their RuIII/n potentials coincide. Under steady state conditions [Co(TPyP)] Ru(NH3)5 4]8+ (43) adsorbed onto a pyrolytic graphite working electrode catalyzes the reduction of dioxygen (Figure 6). 

Collman JP, Hendricks NH, Kim K, Bencosme CS. 1987. The role of Lewis acids in promoting the electtocatal3dic four-electron reduction of dioxygen. J Chem Soc Chem Commun 1537. 

Fukuzumi S, Okamoto K, Gros CP, Guilard R. 2004. Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins. J Am Chem Soc 126 10441. 

Park GJ, Nakajima S, Osuka A, Kim K. 1995. Electrocatalytic four-electron reduction of dioxygen by 1,2-phenylene-bridged dicobalt diporphyrins. Chem Lett 255. 

Shi C, Mak KW, Chan K-S, Anson EC. 1995. Enhancement by surfactants of the activity and stability of iridium octaethyl porphyrin as an electrocatalyst for the four-electron reduction of dioxygen. J Electioanal Chem 397 321. 

ELECTROCATALYTIC REDUCTION AND ACTIVATION OF DIOXYGEN 9.10.7.1 Electrocatalytic Four-electron Reduction of Dioxygen... 

The four-electron reduction of dioxygen in water (Equation (70)) is an essential reaction carried out by aerobic organisms during the energy production cycle in the mitochondria. 

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

D. Dolphin Collman and Anson have found with their five-atom bridged amide systems that the reduction of dioxygen is partitioned roughly one third undergoing a four-electron reduction to water and the rest a two-electron reduction to peroxide. We see the same result indeed the 5-5 systems of ours and that of Collman-Anson are indistinguishable. Neither type of cofacial dimers with chains greater than five show any four-electron reduction. 

In a subsequent study of this type (Durand, Bencosme, Collman Anson, 1983), dimers of type (143) were investigated as potential redox catalysts for the four-electron reduction of dioxygen to water (via peroxide). The Co(ii)/Co(ii) dimer is an effective catalyst for the electrochemical reduction of dioxygen once again the dioxygen binds... 

Cytochrome c oxidase. Cytochrome c oxidase is an enzyme that occurs in the inner mitochondrial membrane and, as we have seen, catalyzes the four-electron reduction of dioxygen to water as the final reaction in the... 

As described earlier, superoxide is a well-proven participant in apoptosis, and its role is tightly connected with the release of cytochrome c. It has been proposed that a switch from the normal four-electron reduction of dioxygen through mitochondrial respiratory chain to the one-electron reduction of dioxygen to superoxide can be an initial event in apoptosis development. This proposal was supported by experimental data. Thus, Petrosillo et al. [104] have shown that mitochondrial-produced oxygen radicals induced the dissociation of cytochrome c from bovine heart submitochondrial particles supposedly via cardiolipin peroxidation. Similarly, it has been found [105] that superoxide elicited rapid cytochrome c release in permeabilized HepG2 cells. In contrast, it was also suggested [106] that it is the release of cytochrome c that inhibits mitochondrial respiration and stimulates superoxide production. 

Clearly, the S0 state must have one Mn(II), one Mn(III) and two Mn(IV) while the S4 must have three Mn(IV) and one Mn(V)—but where they are located in the tetra-Mn centre and how their four-electron reduction is coupled with water splitting and dioxygen evolution remains to be established. (For recent reviews see Goussias et al., 2002 Ferreira et al., 2004 Rutherford and Boussac, 2004 Iverson, 2006.)... 

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