Electron transfer rates quinone reduction

Proton-coupled electron transfer is a prominent theme in biological redox systems. There are three basic mechanisms for these processes (Figure 18). In the first mechanism (path A), electron transfer occurs prior to proton transfer. This mechanism is commonly observed for the electrochemical reduction and oxidation of quinones and flavins in protic media [52], In this interfacial environment, proton transfer is manifested as an ECE (E represents an electron transfer at the electrode surface and C represents a homogeneous chemical reaction) two-electron reduction of these systems to their fully reduced states (Figure 19). As electron transfer occurs prior to the proton transfer event, proton transfer does not affect either the redox potential or the electron transfer rate to or from the cofactor. 

The micellar residence time of Br was calculated to be 1.5 x 10 s . In like manner, pulse radiolysis of Ag2S04 in sodium laurylsulphate micelles gives Ag2, and the various decay processes may be analysed. Electron-transfer between quinone radical anions and quinones (the former generated from eaq reduction) may occur at the micelle surface in sodium laurylsulphate solution. The encounter rate constant for radical anion plus anionic micelle is an order of magnitude lower than for neutral molecule-micelle interactions because of charge-charge repulsions. " ... 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

Each of the photosystems ejects an electron from the excited chlorin complex to a quinone within a nanosecond, followed by electron transfer along chains leading out of the charge separation center within 100 ns. The high potential reaction of Tyr and Mn in PSII is quite rapid, beginning in the simulations on the same time scale as the quinone reduction reaction. However, it has been suggested that tyrosine oxidation may not be rate limited by tunneling, but by H+ transfer (Diner et al., 2001). 

Couples such as hydroquinone/quinone have been hypothesized to dominate the redox properties of humic and fulvic acids, and to act either as electron transfer mediators or as the direct donors of electrons for dechlorination reactions (Schwarzenbach et al., 1990 Dunnivant et al., 1992). For example, it has been shown in sediment-water systems that the rates of alkyl halide reduction increase with organic matter content (Peijnenburg et al., 1992). Further support for this hypothesis was obtained by Svenson et al. (1989), who reported a first-order dependence between rates of hexachloroethane reduction and hydroxyl concentrations. Aside from alkyl halides, structural features of organic matter have been shown to catalyze (Fu et al., 1999) or accelerate (Barkovskii and Adriaens, 1998) the dechlorination of dioxins (Figure 9). 

In this case, a plot of log krcl versus h(W) for a series of substituted nitrobenzenes should yield a straight line with a slope of (1/0.059), which as is shown in Figure 9, is actually found for the compounds listed in Table 7. Hence, for the reduction of neutral substituted nitrobenzenes in homogeneous aqueous hydrogen sulfide solution with lawson as electron-transfer mediator, the relative reaction rate of a given compound may be predicted from its one-electron reduction potential. Similar results have been obtained for the same set of compounds with another quinone (i.e., 5-hydroxynaphthoquinone (reaction 10 in Table 5) see Schwarzenbach et al. (in press b). 

The photoinduced reduction of some quinones by zinc porphyrin and also by its tetraphenyl derivative has been studied in micellar systems. The mean time for intramicellar electron transfer has been established as 0.2 ps, and for duroquinone the rates of entry and exit from the micelle have been found to be 5 x 10 m s and6 x 10 m s respectively. Quinones possessing long chains are less mobile and partial charge separation could be achieved. Irradiation of anthraquinone in aqueous sodium dodecyl sulphate leads to anthraquinol and the surfactant-anthrahydroquinone ether as major products via the triplet state of the anthraquinone. ... 

If quinones are used as the oxidant, the reaction of the OH radical adducts of the nucleobases proceeds by outer sphere electron transfer to yield a carbocation but not an adduct [118]. The rate constant for this interaction shows a strong dependence on the one-electron reduction potential of the quinone. This reaction does not lead to ssb induction in poly C in the presence of benzoquinone but to a reduction in the yield of ssb. 

As shown in Table 1, most nitroarenes, and many quinones, have reduction potentials lower than that of oxygen (1 mol dra ), i.e. equilibrium (4) is over to the right. The rate of electron transfer from the radical to oxygen is redox-controlled, as shown in Figure 6 for nitroarenes/heteroarenes. However, it is important to note that quinone radicals are much more reactive towards oxygen as nitro compounds or A/-oxides of the same reduction potential [111,152] (the benzotriazine, tirapazamine behaves kinetically as a nitroarene [16]). 

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