Oxygen reduction intermediate steps

Other reachons involving oxygen are those reachon steps in oxygen reduction which are of importance in their own right, such as the formation of hydrogen peroxide as a relahvely stable intermediate ... 

As with the phase diagrams and Pourbaix diagrams, the theoretical standard hydrogen electrode also allows us to calculate the relative energies of intermediates in electrochemical reactions. As an example, we investigate the oxygen reduction reaction (ORR). We look at the four proton and electron transfer elementary steps ... 

Mitochondria are also involved in the cell s response to oxidative stress. As we have seen, several steps in the path of oxygen reduction in mitochondria have the potential to produce highly reactive free radicals that can damage cells. The passage of electrons from QH2 to cytochrome bL through Complex III, and passage of electrons from Complex I to QH2, involve the radical Q as an intermediate. The Q can, with a low probability, pass an electron to 02 in the reaction... 

Figure 18-11 Possible catalytic cycle of cytochrome c oxidase at the cytochrome a3 - CuB site. The fully oxidized enzyme (O left center) receives four electrons consecutively from the cyt c —> CuA —> cyt a chain. In steps a and b both heme a and CuB, as well as the CuA center and cyt a3/ are reduced to give the fully reduced enzyme (R). In the very fast step c the cyt a3 heme becomes oxygenated and in step d is converted to a peroxide with oxidation of both the Fe and Cu. Intermediate P was formerly thought to be a peroxide but is now thought to contain ferryl iron and an organic radical. This radical is reduced by the third electron in step/ to give the ferryl form F, with Cu2+ participating in the oxidation. The fourth electron reduces CuB again (step g) allowing reduction to the hydroxy form H in step h. Protonation to form H20 (step ) completes the cycle which utilizes 4 e + 4 H+ + 02 to form 2 H20. Not shown is the additional pumping of 4 H+ across the membrane from the matrix to the intermembrane space.

It is anticipated that despite the specially favored environment provided for oxygen reduction by the protein the fundamental principles of chemistry in simple systems will apply to the enzyme. Thus, any proposed mechanism for the enzymic reduction of dioxygen will have to accommodate two electron steps leading sequentially to peroxide and water and provide a means to overcome the characteristic stability of the peroxide intermediate. 

De I squale reported that a series of coordina lively unsaturated nickel complexes, such as Ni(PPli3)2 or Ni(PCy3)2, act as excellent catalysis. A mechanism is proposed consisting of a sequence of oxidative addition, insertion and reductive elimination steps which involve an oxometallocyclobutane intermediate [225], The decisive step is Uie insertion of carbon dioxide mto a metal -oxygen bond. 

Anderson and Albu investigated an outer-sphere oxygen reduction with ab initio calculations, whereby the electrode was assumed to not directly interact with the intermediate species. They proposed four one-electron transfer steps involving proton transfer from the electrolyte as ... 

Calvo and Balbuena examined the structure and reactivity of Pd-Pt nanoclusters with 10 atoms in the oxygen reduction reaction. In contrast with what is expected in a periodic slab calculation, they found that mixed states with randomly distributed Pd atoms in a Pt7Pd3 cluster was more stable than an ordered cluster structure due to more eflective charge transfer in the mixed state. They found that increasing the concentration of Pd in the surface favors formation of the OOH species in the first step of the reaction, but Pt atoms were needed to promote the second stage of the oxygen reduction reaction. They report that due to charge transfer eflhcts the Pd atoms have an intermediate reactivity between pure Pd and Pt, and in the mixed cluster the Pd atoms the Pd atoms act more similarly to Pt than in the ordered cluster. 

The above mechanisms hold for polycrystalline (148,167) as well as singlecrystal platinum [100] and [111] (148). The surface appears to be uniformly active for the oxygen reduction, although reaction intermediates are adsorbed more strongly on steps than on the flat surface (148). Kunz (168) suggested a different rds for porous, high surface area, supported platinum... 

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