Oxygen evolution stoichiometry

The electron used for restoring (i.e., reducing) P680 ultimately comes from a water molecule, via a cluster of Mn atoms and a secondary electron-donor molecule called Yz. Stoichiometry requires four, one-electron transfer steps to evolve one molecule of oxygen from two molecules of water. The states generated by this four-step process are called the S-states. Oxygen evolution promoted by electron-transport through photosystem II may be written as ... 

After the initial establishment of the proton release pattern in oxygen evolution, it became known that the stoichiometry is often nonintegral and it also depends on the type of thylakoid samples and the pH (See review by Lavergne and Junge .) Renger first proposed in 1987 that a portion of the protons re-... 

Fig. 3. Model for the organization of manganese and effects of inhibitors on the manganese atoms associated with photosynthetic oxygen evolution. Figure source Yocum, Yerkes, Blankenship, Sharp and Babcock (1981) Stoichiometry, inhibitor sensitivity, and organization of manganese associated with photosynthetic oxygen evolution. Proc Nat Acad Sci,USA78 7511.

Enami I, Kaneko M, Kitamuta N et al. Total immobilization of the extrinsic 33 kDa protein in spinach Photosystem II preparations. Protein stoichiometry and stabilization of oxygen evolution. Biochim Biophys Acta 1991 1060 224-232. 

Yocum CF, Yerkes CT, Blankenship RE et al. Stoichiometry, inhibitor sensitivity and organization of manganese associated with photosynthedc oxygen evolution. Ptoc Natl Acad Sd USA 1981 78 7507-7511. 

In the absence of DMSO the conversion of diphenylmethyl hydroperoxide to benzophenone apparently does follow Reaction 6, at least in alcohol-containing solvents. The stoichiometry becomes nearly one mole of oxygen per mole of diphenylmethane, and the carbinol is eliminated as an intermediate. Table I lists the observed stoichiometries and initial rates of oxidation of diphenylmethane. In pyridine-, DMF-, or HMPA-containing solvents, a high yield of benzophenone was isolated upon hydrolysis after oxygen absorption had ceased. In pure HMPA there was considerable evolution of oxygen upon hydrolysis. 

Early work on peroxo compounds of molybdenum has been reviewed.286 The stoichiometri-cally simplest Mo peroxo complex is the red-brown Mo(02)4 ion formed by the reaction of MoO2- with H202. Although the complex is not stable in solution and decomposes slowly with the evolution of 02, the anion can be crystallized as the Zn(NH3)4+ salt whose structure has been determined. In a sense, Mo(02)4 is an intermediate in the thermodynamically favored decomposition of hydrogen peroxide to water and oxygen. 

A method for the preparation of thin films of Fe4[Ru(CN)6]3 ( ruthenium purple ) involving electrochemical reduction of K3[Ru(CN)6] in a solution of Fe2(S04)3 has been developed.28 This ruthenium purple modified electrode is claimed to be one of the best catalysts for evolution of oxygen and chlorine. Electrochemical studies on polyammonium macrocyclic complexes of [Ru(CN)6]4 indicate a 1 1 stoichiometry with a monoelectronic, reversible, oxidation for these complexes this illustrates the control of redox potential of anions by complexation with appropriate receptor molecules.29 The kinetics of oxidation of [Ru(CN)6]4 by [Mn04] in HC104 have been investigated by stopped-flow techniques. It is found that [Ru(CN)6]4" is quantitatively oxidized to [Ru(CN)6]3 in accordance with equation (1) and that two protonated intermediates [RuH(CN)6]3 and [RuH2(CN)6]3 are involved in the oxidation process.30... 

Equally intriguing is the decomposition of 4. Dioxygen was detected using an oxygen electrode quantitative GC analysis showed less than 2.2% production of methane, based on starting rhenium. The rate of 02 evolution varied inversely with [H202], whereas the final yield of 02 varied linearly with peroxide. Thus the stoichiometry in Eq. (5) was arrived at ... 

However, an alternative mechanism similar to that described in scheme 2, that considers the oxidative addition of aniline to the Rh° finely dispersed on the support, cannot be completely excluded. The evolution of carbamoyl intermediate to DPU should occur still via iodoformamide. The last mechanism could be also operative in the reductive carbonylation of nitrobenzene, when aniline is necessary for its conversion. In this case, the reaction could be better considered as an oxidative carbonylation process in which the nitrobenzene is playing the role of the oxidant in place of the oxygen. It has been ascertained that under these conditions the carbonylation occurs with the stoichiometry of reaction (11) [14], different from the one reported in reaction (4). 

A graphical representation of the standard free energy often proves useful. Figure 2.2 shows the evolution of the free energy of reaction as a function of temperature for the formation of a number of oxides. For the sake of comparison, the stoichiometries of the equations are expressed relative to a single mole of oxygen. Thus, for the formation of the monovalent (M2O), bivalent (MO) and trivalent oxides (M2O3) ... 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

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