Oxygen reduction reaction 2-electron transfer pathway

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

The biochemical importance of flavin coenzymes ap-pears to be their versatility in mediating a variety of redox processes, including electron transfer and the activation of molecular oxygen for oxygenation reactions. An especially important manifestation of their redox versatility is their ability to serve as the switch point from the two-electron processes, which predominate in cytosolic carbon metabo-lism, to the one-electron transfer processes, which predomi-nate in membrane-associated terminal electron-transfer pathways. In mammalian cells, for example, the end products of the aerobic metabolism of glucose are C02 and NADH (see chapter 13). The terminal electron-transfer pathway is a membrane-bound system of cytochromes, nonheme iron proteins, and copper-heme proteins—all one-electron acceptors that transfer electrons ultimately to 02 to produce H20 and NAD+ with the concomitant production of ATP from ADP and P . The interaction of NADH with this pathway is mediated by NADH dehydrogenase, a flavoprotein that couples the two-electron oxidation of NADH with the one-electron reductive processes of the membrane. 

The reductions of H2O2 and O2 by [Ti(EDTA)(H20)], [Ti(H20)6], [Fe(EDTA)] -, [Fe(H20)6], and [Ru(NH3)j2 produce HO radicals with H2O2 as substrate, as detected by spin trapping techniques. The reactions with O2 proceed by both inner- and outer-sphere electron transfer pathways to produce the O J anion. The reduction of O2 by either [Fe(EDTA)] or [Fe2(ttha)] proceeds by an inner-sphere pathway with the subsequent attack of the coordinated O2 either on an adjacent carboxylate moiety or on sacrificial mediators in the solvent cage. [Ru(bpy)3] is quantitatively photooxidized to [Ru(bpy)3] in strongly acidic solutions containing dissolved oxygen. The process is represented by equations(2) and (3). 

The oxygen reduction reaction is covered in considerable detail in a review text by Kinoshita [10]. The charge-transfer reaction itself is quite complicated, and controversy still exists around the details of which of the many possible charge-transfer mechanisms determine electrode performance. The two generalized pathways that are considered are the direct four-electron reaction ... 

The oxygen reduction reaction is a multi-electron process involving numerous steps and intermediate species. As stated above, ORR may proceed via four or two electron transfer in aqueous acidic medium. The most relevant reactions pathways and their thermodynamic electrode potentials in acidic medium are shown below ... 

M. The data do not, however, allow a distinction to be drawn between electron transfer at the bridging oxygen, terminal water, or porphyrin periphery. In the presence of histidine, the major species is [Fe (TMpyP)(his)a], which is reduced by ascorbate in an inner-sphere pathway involving protonation and dissociation of one of the axial histidine ligands. The rate constant for histidine dissociation, 25.2 s, is remarkably similar to the rate of dissociation of methionine-80, 60 s, in the ascorbate reduction of cytochrome c. Autoreduction of [Fe TPP] in the presence of piperidine proceeds by proton abstraction from co-ordinated piperidine as shown by a large isotope effect in the reaction. Electron transfer produces iron(ii) and a nitrogen radical. 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

Studies have revealed that the type of interaction between catalyst and oxygen dictates the oxygen reduction reaction pathway and end products. The direct four-electron pathway requires dissociation of oxygen prior to the transfer of the first electron [25]. Research has demonstrated that both side-on and bridge interactions weaken the 0-0 bond to such an extent that a break is inevitable. Therefore, four-electron reduction will follow and the final product will be water. On the other hand, catalysts that cannot effectively stretch the 0-0 bond will only end up with the two-electron reduction product H2O2, which makes the catalyst unstable and degrades the fiiel cell system [31]. 

Depending on the nature of the central metal, the oxygen reduction reaction takes place either via a 2-electron transfer pathway to peroxide, or a 4-electron transfer pathway to water, or a mixed pathway of 2- and 4-electron transfers. The central metal ion of a macrocycle seems to play a decisive role in the ORR mechanism. Most Fe complexes can reduce oxygen directly to water through a 4-electron... 

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