Electron transfer proton coupling

Keywords Excited-state intramolecular proton transfer Fluorescence dye Photoinduced electron transfer Proton coupled electron transfer Relaxation dynamics... 

Metal-oxygen intermediates react with inorganic or organic substrates via various reaction pathways, such as oxygen atom transfer, hydrogen atom transfer, hydride transfer, electron transfer, proton-coupled electron transfer, free radical reactions, and others.14-16 The preferential reactivity pathways depend on the nature and oxidation state of the metal, the nuclearity of the complex, and the coordination mode and protonation state of coordinated oxygen-derived ligand(s). 

NADH-coenzyme Q (CoQ) oxidoreductase, transfers electrons stepwise from NADH, through a flavoprotein (containing FMN as cofactor) to a series of iron-sulfur clusters (which will be discussed in Chapter 13) and ultimately to CoQ, a lipid-soluble quinone, which transfers its electrons to Complex III. A If, for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

Electron transfer is coupled with proton transfer involving the metal-bound solvent molecule. For Eqs (14) and (15) to be thermodynamically favorable, the potential of the Mn / -SOD couple must lie between the values for superoxide oxidation and reduction (02 — O2 + e ,... 

II. Intra-Pair Reactions Back electron transfer Proton, atom, or group transfer Coupling... 

The mechanism of polyamine hydrogenation (Fig. 6.8) is believed to involve successive electron transfer (from polyamine to fullerene) - proton transfer (from polyamine radical cation to fullerene radical anion) steps (Briggs et al. 2005 Kintigh et al. 2007). At or near room temperature, aliphatic amines and polyamines are known to hydroaminate [60]fullerene (Miller 2006), likely also involving preliminary electron transfer - proton transfer steps followed by free radical coupling of C and N based radicals (Fig. 6.8). At elevated temperatures in polyamine solution, however, this latter free radical coupling step becomes uncompetitive with... 

The cis-[RuVI(tet-Me6)(0)2]2+ complex has also been reported to display proton-coupled electron-transfer redox couples in aqueous medium (134,136). The following electrode reactions for the couples have been observed (pH 1.0) ... 

No, it is inconsistent with this hypothesis, which predicts an H+/e value of 1.0. Such stoichiometries may, however, be explained by proton-pump mechanisms, in which electron transfer is coupled to changes in the pKa values of proteins within Complex I. 

Fig. 1. A schematic illustration of the mitochondrial (A) and Escherichia coli (B) respiratory chains. Respiratory enzymes perform a series of oxidation-reduction reactions by transferring electrons (dashed lines) through mobile electron carriers. Electron transfer is coupled to the pumping of protons (thick black arrows) from the A/-side (negative side) to the P-side (positive side) generating a proton gradient that ultimately drives the conversion of ADP to ATP.

Fig. 1.4. Slipping proton pumps. (A) Simulation of the relationship between electron transfer rate (Jg) and AgH for different degrees of coupling ( ) of an electron transferring proton pump. A/Ih expressed in units Z AGq. (B) Actual experimental results (from Ref. 44 with kind permission from the authors and the copywright owner) for different segments of the mitochondrial respiratory chain as proton pump. Both in (A) and in (B) the rate of electron transfer is varied through titration with an electron transfer inhibitor.

A number of studies have been made of the reduction of model enones in buffered aqueous or buffered ethanolic solutions in order to elucidate the sequence of electron transfer, proton transfer, and coupling steps as a function of pH [39-42,91-95]. The experimental methods applied include polarography, CV, LSV, and chronocoulometry. 

The chapters by Winkler and Gray (Chapter 1), and by Wikstrbm (Chapter 2) have testified to high detail and sophistication in the mapping of electron transfer and coupled electron and proton transfer of proteins and protein complexes in homogeneous solution and in protein-membrane... 

HYDROGEN ATOM TRANSFER/PROTON-COUPLED ELECTRON TRANSFER... 

SCHEME 10.8 Scheme for reaction of 4-pyridine thione in which the initial 19 e thione adduct reacts with Cr(CO)3Cp by a coupled electron transfer/proton transfer mechanism. 

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