Water four-electron oxidation

Very recently a new kind of electrocatalyst has been propounded using the dinuclear quinone-containing complex of ruthenium (25).492,493 Controlled-potential electrolysis of the complex at 1.70 V vs. Ag AgCl in H20 + CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of 02 evolution increases up to 33,500 when the electrolysis is carried out in water (pH 4.0) with an indium-tin oxide(ITO) electrode to which the complex is bound. It has been suggested that the four-electron oxidation of water is achieved by redox reactions of not only the two Run/Ruin couples, but also the two semiquinone/quinone couples of the molecule. 

In photosystem II an intermediate tyrosyl radical is formed which then repetitively oxidizes an adjacent manganese cluster leading to a four-electron oxidation of two water molecules to dioxygen. In broad detail, the model compounds" described above were demonstrated to undergo similar reactions on photochemical excitation of the respective ruthenium centers. 

The oxidation of water to dioxygen occurs as the consequence of Photosystem 11-dependent generation of a very strong oxidant. Protons liberated by the water-oxidation reaction then contribute to the thylakoid transmembrane electrochemical gradient that drives ATP synthesis. Brudvig et aL describe how flash-induced proton-release measurements have resolved key steps that provide insights on how the 02-evolving center of PSll mediates this four-electron oxidation of water. 

Although catalytic water oxidation (dark reaction) is the first and important reaction of the electron flow in the photosynthesis represented by Fig. 19.1 whereby water is used as the source of electrons provided to the whole system, its catalyst and reaction mechanism are not yet established.10-13) In the photosynthesis Mn-protein complex works as a catalyst for the difficult four-electron oxidation of two molecules of water to liberate one 02 molecule (Eq. (19.2)). It is inferred that at least four Mn ions are involved in the active center, but its structure is not yet completely elucidated. 

In the four-electron oxidation of water to 02, the polymanganese system acts as (i) an electron reservoir, accumulating charge in an exactly controlled fashion at physiologically high redox potential and (ii) as a non-3Orretaining catalyst. 

By far the most important role of manganese in nature is its direct involvement in the photocatalytic, four-electron oxidation of water to dioxygen in green plant photosynthesis, an essential process for the maintenance of life. Pirson, in 1937, first discovered the requirement of manganese in photosynthesis by showing that plants grown in a Mn-deficient medium lost their water oxidation capacity (184). During the next four decades, several researchers showed that two photosystems, photosystem I (PSI) and photosystem II (PSII), were involved in photosynthesis and that 02 evolution and Mn were localized at PSII (for a review, see Ref. 185). 

A plot of the potential-dependence of the Raman intensities of a Ru binuclear p-oxo water oxidation catalyst showed evidence for one-, two-, and four-electron oxidized species. Oxygen isotopic labeling was used to characterize the catalytically active fully oxidized state.91... 

Cytochrome oxidases occur in all three kingdoms of life [281,282], They are the terminal oxidases of aerobic respiratory chains and, as such, catalyze the four-electron oxidation of oxygen to water, a feat only they and the blue oxidases are capable of. 

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.)... 

While IPNS structurally (Figure 16) and evolutionarily falls within the same enzyme category as the 2-OG-dependent systems, it is unique in that it does not utilize the 2-OG cosubstrate seen in the previously discussed proteins. Rather, it catalyzes the four-electron oxidation of the tripeptide delta-(L-alpha-amino-delta-adipoyl)-L-cysteinyl-D-valine (ACV) to penicillin N via the complete reduction of O2 to water. Although no spectroscopic intermediates in the reaction pathway of IPNS have been characterized in detail, kinetic studies have led to the mechanism proposed in Figure This pathway is based primarily on the results of structural characterizations and... 

Mn clusters for the four electron oxidation of water to oxygen in the mechanism of photosynthesis. 

The exact structure of the Mn cluster is not yet certain. At least four Mn ions are involved in the OEC to realize four electron oxidation of two water molecules wherein a histidine residue is inferred to work also as an oxidation site. A Mn cluster model is proposed as shown in Fig. 2-25. 

Biochemically, manganese is considered an essential trace element, participating in a number of hiomolecules superoxide dismutase (Mn-SOD), catalase, Mn-ribonucleotide reductase, Mn-peroxidase, ligninase, the o>ygen-evolving centre (OEC) of photosystem ii (PS-ii), and Mn-thiosulfate oxidase. The enzymes mechanisms are very diverse and include oxo-atom transfer (four-electron oxidation of water to diojygen in PS-ii, extradiol dioxygenase), electron transfer (SOD, catalase), reduction of ribonucleotides to water and deoxyribonucleotides and oxidation of thiosulfate to sulfate. ... 

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