Copper outer-sphere electron transfer reactions

However, while remaining in the binding site, superoxide accepts its electron when the oxygen is more than 3 A from the copper ion (outer-sphere electron transfer) in the following reaction ... 

Quenching of excited-state [Ru(bipy)3] by reduced blue proteins involves electron transfer from the Cu with rate constants close to the diffusion limit for electron-transfer reactions in aqueous solution. It is suggested that the excited Ru complex binds close to the copper-histidine centre, and that outer-sphere electron transfer occurs from Cu through the imidazole groups to Ru. Estimated electron-transfer distances are about 3.3 A for plastocyanin and 3.8 A for azurin, suggesting that the hydrophobic bipy ligands of Ru " penetrate the residues that isolate the Cu-His unit from the solvent. ... 

In contrast to other systems in which the mechanism (inner-sphere or outer-sphere electron transfer) may be a matter of debate, the reaction between the protein horse heart cytochrome c with anionic Cu" complexes was adjudged to proceed by an outer-sphere mechanism. [170] The copper(ll) complex bis(5,6-bis(4-suphonatophenyl)-3-(2-pyridyl)-l,2,4-triazine)Cu(ll), (the ligand is commonly known as ferrozine) possesses square pyramidal geometry with the two bidentate ligands in the equatorial plane, and the fifth axial position is occupied by a water molecule, in aqueous solution. 

A different mechanism, in which the bridge between copper and zinc is not broken and the protons needed by the reaction are provided by thd bulk solvent, has been proposed from theoretical calculations (246, 335, 336). In this mechanism a superoxide molecule forms a complex with Cu(II) thanks to the stabilizing effect of Arg-143. This stable intermediate can oxidize a second superoxide molecule by an outer-sphere electron transfer originating an E-Cu -02 complex (E = enzyme), which is subject to proton transfer from the solvent followed by electron transfer from copper, giving rise to E-Cu -02H . The hydroperoxide anion readily dissociates from the latter complex, leaving the metal in the oxidized state (246, 335). 

Fig. 55. Reaction scheme of superoxide dismutation following the bovine enzyme numbering. The first O2 molecule binds to Cu(II) and is stabilized by the H bond to Arg-141. A second superoxide molecule then approaches the active site and, by an outer-sphere electron transfer via the Cu-bound first O2 molecule, reduces the copper to Cud) 44) (step III). Alternatively, O2" directly reduces superoxide to peroxide 336) (step IV), leaving as dioxygen. Note that the Cu(I)-superoxide and Cu(II)-peroxide complexes are resonant forms of the same molecular arrangement. The newly formed peroxide is protonated by Arg-141 and leaves as HO2. Arg-141 receives a proton from the solvent, restoring the active enz5Tne (I). These reaction proposals do not require the breaking and reforming of the Cu-His-61 bridge.

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Like all redox reactions those of copper(II) may be divided into two types (a) outer sphere mechanisms involving electron (or proton) transfer between coordination shells that remain essentially intact and (b) inner sphere mechanisms in which the oxidizing and reducing species are connected by a bridging ligand, which is common to both metal ion coordination spheres. ... 

In accord with the qualitative evidence that both reactions proceed at comparable rates, the mechanism proposed involves a rate-determining one-electron transfer to yield Ir and S and is probably outer-sphere. The intermediate is then further oxidized by or is involved in a dimerization to dithionate. Experiments in the presence of copper(ii) show negligible catalytic activity. 

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