Electron-transfer mechanism uptake

Reviews of kinetics and mechanisms deal with long-range electron transfer and with the uptake and release of dioxygen species, especially uptake from organic peroxides. " " The photochemistry of iron porphyrin complexes has been documented." ... 

There has been some discussion about the detailed mechanism of the reduction of aromatic nitro compounds. In acid solution the slow step in the four-electron reduction has been found [96,98] to be the uptake of the second electron, the reduction of ArN02H to the dihydroxylamine, ArN(OH)2. According to some authors, ArN02H [96,98,99] is formed on protonation of the radical anion, but there are also strong proponents for its formation by electron transfer to a preprotonated nitro compound [100,101]. In water the pKa of the protonated radical anion, ArN02H, for most aromatic nitro compounds, is between 2 and 4 [102]. 

Studies of intramolecular ET in oxidases provide interesting examples of how pulse radiolysis is employed to obtain insights into both (1) these enzymes respective mechanisms of action and (2) electron transfer along protein polypeptide matrices that were most probably selected by evolution (9,10, 30-32). Thus, early attempts to study the electron uptake mechanism by the blue oxidase, ceruloplasmin, showed that a diffusion-controlled decay process of the eaq in solutions of this protein is paralleled by the formation of transient optical absorptions due to electron adducts of protein residues, primarily of cystine disulfide bonds (30). The monomolecular decay of the latter absorption was found to have the same rate constant as that at which the type 1 Cu(II) absorption band was reduced. These results were interpreted as being the combined result of the high reactivity of the e q and the relatively inaccessible type 1 Cu(II) site, yielding an indirect, intramolecular electron transfer pathway from surface-exposed residues (30). 

The state of the catalytic site formed by the reduction of Pm is denoted F. Transfer of an additional electron to the catalytic site is again coupled to the uptake of two protons and the release of one pumped proton. This process results in the formation of the oxidized catalytic site, denoted OH, where the subscript denotes an activated high-potential state in which it is postulated that Cub has a very high electrochemical potential. The next two electron transfer reactions convert the Oh state to the E state (Cub reduced) and, further to the R state (heme and Cub both reduced). Each of these steps is also thought to be coupled to the uptake of two protons and release of one proton. However, little is known about the nature of the activated Oh state, or the On tE and E R steps of the reaction. For the purposes of this discussion it is assumed that each of the electron transfer steps to the catalytic site. Oh t E, E —> R, Pm F, and F —> Oh, is associated with proton pumping by the same mechanism, but much more needs to be done experimentally to test this assumption. 

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