Dioxygen reduction intermediates

The standard potential of the 02/02 pair is equal to -0.15 V in water and -0.60 V in DMF. Usually, dioxygen easily captures two electrons in the stepwise reaction O2 + e —> O2 , then O2 + e 02 . In DMSO, dioxygen reductions into the superoxide ion and then into the dioxygen dianion are characterized by Ey2 = -0.5 V and Ey = -1.5 V in regard to the saturated calomel electrode (Sawyer and Gibian 1979). The superoxide ion occupies an intermediate position in the following redox triad O2 —> 02 —> In accordance with such a position, the superoxide ion... 

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... 

It was observed in a 2005 article that Co(II) porphyrin-Co(III) corrole dimers are more effective dioxygen reduction electrocatalysts than analogous Co(III)-Co(III) corrole dimers or monomeric Co(III) corroles [145], The heterodimers operated effectively at lower overpotentials and promote complete reduction to water (the average number of electrons transferred per 02 molecule approaches 4 in the best porphyrin-corrole catalyst). It was suggested that the inferior catalytic performance of the corrole homodimers could be due to a reduction in the basicity of the activated intermediate when two Co(III) moieties are involved, leading to a less favorable 4-electron reduction. Heterobimetallic catalysts containing formally Co (IV) corroles were also examined as potential dioxygen reduction catalysts [146]. 

Since dioxygen reduction provides much more energy ( 400 kJ mol-1) than is needed to form a single ATP from ADP + phosphate ( 30 kJ mol-1), it is to the cell s advantage to carry out the reduction stepwise. The actual process yields six ATP (i.e. 45% efficient) per dioxygen. This involves intermediate oxygen reduction products, superoxide (02-) and... 

The true metabolic role of HA per se (and not as mere precursor of other substances such as quinolinic acid and so on) is still awaiting a conclusive definition. However, HA toxicity seems to be related not to the compound itself, but rather to other substances, arising form its (auto)oxidation. As ever, one can speculate about the chemical nature of those species, therefore some evidence exists in favour of the profound involvement of reactive intermediates in dioxygen reduction, namely superoxide, peroxide and hydroxyl, whereas other indications suggest the participation of anthranilyl and/or HA quinoneimine in the toxicity mechanism. 

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