Superoxide intermediates

Li X, Gewirth AA. 2005. Oxygen electroreduction through a superoxide intermediate on Bi-modified Au surfaces. J Am Chem Soc 127 5252-5260. 

KIEs in excess of 1.010 for the rate-determining formation of an t]1-superoxide species. A significant decrease from the 180 EIEs determined for such structures (i.e., 1.005-1.015 in Table 9.1) is actually expected due to variations in the transition state structure, which arise from changes in the reaction driving force.47 The possibility that an rj -superoxide intermediate is formed reversibly, as a... 

The reactions of iron-containing enzymes with O2 often involve high oxidation states of the metal. Generally, the initial reaction of dioxygen with both heme and mononuclear non-heme ferrous enzymes results in the formation of Fe -superoxide intermediates. Highly reactive Fe =0 intermediates often are employed often for C-H activation. The mechanism of substrate oxidation by binuclear non-heme enzymes involves high valent, oxo-bridged species, with Fe in the -i-3 or +4 oxidation state. 

Both of these steps are energetically favorable and could satisfactorily account for the SO2 product. The mechanism is probably more complicated than indicated by (50) and (51) and may involve the aforementioned sulfur superoxide intermediate. 

The scientific programme comprised four main themes photo-oxygen evolution superoxides, intermediates between oxygen and water biological oxygen transport and oxygen-utilising enzymes. 

The mechanism of action of galactose oxidase involves oxidation of the alcohol to the aldehyde by a copper(iii) form of the enzyme which is reduced to copper(i). Reoxidation involves a copper(ii)-superoxide intermediate. ... 

The same oxidation product is detected starting from Cu(II) and KO2, but the use of H2O2 with NEts gives different products. This suggests the formation of a Cu(II) superoxide intermediate [Cu(I) + 02 Cu(II)02], that could abstract H" or H-leading to an alkylhydroperoxide Cu(I) intermediate evolving toward a keto product with H2O release (Eigure 34). 

The ORR mechanism on Ag in alkaline media shows many similarities with that on Pt, with similar Tafel behavior and reaction order in O2, with both two-electron and four-electron pathways occurring, and with the reaction most likely proceeding via a superoxide intermediate [2]. On clean Ag surfaces, the four-electron ORR pathway predominates, but this selectivity is sensitive to surface contamination. On iodine-poisoned Ag surfaces, the two-electron ORR pathway predominates. 

The subsequent treatment focuses on the associative mechanism of the ORR. In this mechanism, O2 first adsorbs on the metal surface, followed immediately by the first proton-electron transfer to form an adsorbed superoxide intermediate. 

Earlier EPR studies were also extended to the nitrosyl adducts of deoxyhe-merythrin since the NO adduct is believed to be a reasonable mimic of the superoxide intermediate expected to be formed upon reaction of hemerythrin wifli O2 [92,93,95,526] The spin-Hamiltonian parameters for the Fe - FeNO unit of NO-deoxyhmerythrin are consistent with antiferromagnetic coupling between the two iron centers, and the coordination of NO to the five-coordinate Fe2, as expected for dioxygen binding. 

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