Water four-electron transfer

Electron transfer oxidases catalyze reduction of oxygen to hydrc n peroxide (two-electron transfer) or to water (four-electron transfer). These oxidases are oxygen-obligative if they reduce oxygen only, but if other electron acceptors can serve as substrates they are oxygen-facuUative. 

The WOC is oxidized stepwise by a nearby tyrosine residue (Tyrz), which is itself oxidized by the chlorophyll cation radical P680+ (formed by light-induced charge separation). The electrons are eventually used by PSII for the reduction of plastoqui-none. After the WOC has lost four electrons, the accumulated oxidizing power drives the formation of molecular oxygen from two substrate water molecules, and the catalytic system is reset. The sequence of the four electron-transfer steps is summarized in the Kok cycle [32] of Figure 4.5.3, where the most probable spectroscopically derived oxidation states of the Mn ions [33] are shown for each of the five redox state intermediates S (n - 0-4). 

The reduction of dioxygen to water (concomitant with the oxidation of other species) is a 4e process, and it is virtually impossible to have the four electrons transferred simultaneously without the formation of intermediates (like O - and H2O2). 

Photosystem 11 (PS II) within higher plants represents a solar-energy-driven process that removes hydrogen atoms from water to form molecular oxygen (O2, dioxygen) through an overall four-electron transfer reaction (equation 20), ... 

Cytochrome c oxidase (COX) is the terminal enzyme in the respiratory system of most aerobic organisms and catalyzes the four electron transfer from c-type cytochromes to dioxygen (115, 116). The A-type COX enzyme has three different redox-active metal centers A mixed-valence copper pair forming the so-called Cua center, a low-spin heme-a site, and a binuclear center formed by heme-fl3 and Cub. The Cua functions as the primary electron acceptor, from which electrons are transferred via heme-a to the heme-fl3/CuB center, where O2 is reduced to water. In the B-type COX heme-u is replaced by a heme-fo center. The intramolecular electron-transfer reactions are coupled to proton translocation across the membrane in which the enzyme resides (117-123) by a mechanism that is under active investigation (119, 124—126). The resulting electrochemical proton gradient is used by ATP synthase to generate ATP. 

Most of the O2 consumed by aerobic organisms is used to produce energy in a process referred to as oxidative phosphorylation, a series of reactions in which electron transport is coupled to the synthesis of ATP and in which the driving force for the reaction is provided by the four-electron oxidizing power of O2 (Reaction 5.1). (This subject is described in any standard text on biochemistry and will not be discussed in detail here.) The next to the last step in the electron-transport chain produces reduced cytochrome c, a water-soluble electron-transfer protein. Cytochrome c then transfers electrons to cytochrome c oxidase, where they are ultimately transferred to O2. (Electron-transfer reactions are discussed in Chapter 6.)... 

We point out that the reverse reaction, the reduction of to water and proton pumping, catalysed by the cytochrome-oxidase is very similar. Cytochrome c donates four electrons to O bound to the bimetallic haem a Fe-Cu centre. As shown recently by Wiktrom[i5], only two of the four electron transfers are linked to the translocation of 2H . In the evolving complex, the binuclear Mn replaces Fe-Cu, with the same stoichiometry of 2K released in only two of the four electron transfers. 

Normally, ORR catalyzed by Pt catalyst occurs predominately through a four-electron transfer pathway to water (or to... 

Some studies have shown that certain modification procedures can be used to transform two-electron reduction metalloN4-macrocyclic complexes into hybrid materials with the capability to reduce oxygen to water, either via the direct four-electron transfer pathway or in the series two-electron transfer pathway. Carbon nanomaterials, carbon nanotubes in particular [58-65], have been reported to significantly increase the catalytic oxygen reduction current, with a substantial reduction of the overpotential for ORR reported in some cases, as shown by the examples in Table 7.4. 

V vs. SHE in 0.1 mol dm H2SO4 at room temperature [74], The effect of the addition of zirconium was also investigated to enhance the ORR activity [76], The Ba-Nb-Zr-O-N/CB showed higher ORR activity with the ORR onset potential of ca. 0.93 V. The ORR proceeded primarily via a four-electron transfer reaction to water, and the maximum proportion of the hydrogen peroxide formation was less than 12 %. The incorporatiOTi of Ba and Nb into Zr" " matrix may have affected the surface structure and/or state of the catalyst, possibly causing the high ORR activity. 

The authors proposed a potential mechanism, shown in Scheme 62, involving the coordination of 2-phenethylamine and chelation of o-nitroan-iline to the iron—sulfur cluster, followed by an intramolecular two-electron transfer, which proceeds through complex 131, accompanied with the loss of ammonia and a molecule of water, and a four-electron transfer and dehydration, yielding 2-phenylquinoxaline (132). 

The related electron transfer mechanism for the reduction to hydrogen peroxide is shown in Fig. 16.12. The authors concluded that there are no evidences for a direct reduction to water, but that the integration of transition metals might enhance the catalytic interaction to enable the four-electron transfer process [135],... 

Four different pathways are considered to describe the complete reaction. In acidic media, a direct four-electron transfer step leads to the formation of water on the catalyst surface ... 

It is apparent that hypotheses proposed for the mechanism of action of the phenolase complex now comprise the reasonable permitted alternatives. Whichever is correct, one atom of the oxygen molecule consumed during hydroxylation of monophenols by the phenolase complex appears in the resulting o-diphenol and the other atom is reduced. The cresolase activity of this complex is accordingly a mixed function oxidase. The catecholase function, on the other hand, results in the reduction of both atoms of the oxygen molecule to water. It is therefore a four-electron transfer oxidase. 

The reduction of molecular nitrogen to ammonium and water oxidation to molecular oxygen causes six- and four-electron transfer to occur eventually in these reactions, respectively. Such processes obviously cannot occur in a single step. Analysis of toe thermodynamics of plausible intermediates rules out one-and two-electron transfers for both reactions and only four-electron mechanisms are energetically allowed.. Evidently, toe direct transport of four electrons fix>m (or to) a mononuclear or even binuclear transition metal complex appears to be ruled out Practically toe only possible variant of toe four-electron mechanism is toe conversion in toe coordination here of a transition metal polynuclear complex. 

The water oxidation half-reaction, introduced by Equation (1.1), is the most challenging obstacle for solar hydrogen production, since it requires a four-electron transfer process coupled to the removal of four protons from water molecules to form the oxygen oxygen bond. In Nature, this process is driven by solar light captured by chlorophyll pigments embedded in the protein antennas... 

The focus of this chapter is on water oxidation reaction in water-splitting process. Water oxidation catalyst ensures efficient and sustainable conversion of solar-to-fuel energy cycle. The water oxidation reaction includes four-electron transfer process. Before studying various water oxidation catalysts, it is important to understand the role of photocatalyst in overall water-splitting process and reactions associated with it. 

---