Potential electron-transfer

Small particles of metals in solution often behave like electrodes although they are not connected to a battery which determines their potential. However, when a chemical reaction occurs in the solution of such particles intermediate free radicals may transfer electrons to them. The particles are thus charged chemically and are able to act as a metal electrode on cathodic potential. Electron transfer reactions become possible at these micro-electrodes which cannot be brought about by the radicals in the absence of the colloidal catalyst. 

The following description of the electron transfer-proton transport scheme is illustrated in Figure 7.26. First, an electron is transferred from doubly reduced dihydroplastoquinone (PQFI2) to a high potential electron transfer chain that consists of the Reiske iron-sulfur protein and the cytochrome protein containing heme f. Rappaport,Lavergne and co-workers have reported a midpoint potential at pH 7.0 of +355 mV for heme f. These two centers reside on the electropositive (lumen or p) side of the membrane, exterior to the membrane. As a result, two protons are transferred to the aqueous lumen phase. A second electron is transferred from PQH2 sequentially to heme bp. 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

As discussed above, the photosynthetic reaction center solves the problem of rapid charge recombination by spatially separating the electron and hole across the lipid bilayer. In order to achieve photoinitiated electron transfer across this large distance, the reaction center uses a multistep sequence of electron transfers through an ensemble of donor and acceptor moieties. The same strategy may be successfully employed in photosynthesis models, and has been since 1983 [42-45]. The basic idea may be illustrated by reference to a triad Dj-D2-A, where D2 represents a pigment whose excited state will act as an electron donor, Di is a secondary donor, and A is an electron acceptor. Excitation of D2 will lead to the following potential electron transfer events. 

Figure 6.23 Schematic illustration of an interfacial supramolecular assembly incorporating a Ru-Ru diad, showing the potential electron transfer pathways in the interfacial supramolecular triad T1O2-Ru-Ru...

Joliot, P. and Joliot, A. 1987. The low potential electron transfer chain in the cytochrome b/f complex. Biochim. Biophys. Acta, in press. 

It is important to note that the description of electron transfer kinetics is different in the case of semiconductor electrodes. For an n-type semiconductor electrode in the dark, the rate of electron transfer depends not only on the concentration of redox species in the solution but also on the potential dependent density of electrons in the semiconductor. Under depletion conditions, most of the potential drop is located in the solid, so that to a good approximation the activation energy for electron transfer is independent of potential. Electron transfer at semiconductor electrodes is therefore characterised in terms of a second order heterogeneous rate constant with units cm4 s-1. 

For macrocyclic and cage ligands the size of the ligand cavity (often referred to as the hole size) and its shape are important features that have been used to rationalize stabilities, redox potentials, electron transfer rates and ligand field spectra. Some of these applications will be studied in the following lessons (see Chapters 9, 10, 11). 

The terms oxidation and reduction with respect to chemical processes in soil-water systems refers to potential electron-transfer processes. Under oxidation, a chemical element or molecular species donates electrons (e ), whereas under reduction a chemical element or molecular species accepts electrons. The potential of an atom of any given element to react depends on the affinity of its nucleus for electrons and the strong tendency of the atom to gain maximum stability by filling its outer electron shell or comply with the octet rule. The octet rule states that to gain maximum stability an atom must have eight electrons in its outer shell or outermost energy level. 

Low-potential electron-transfer mediators such as viologens can substitute natural cofactors (particularly NADH) in some enzymatic reactions [184], The electrochemical reduction of viologens has been studied extensively [185] and they and other reductive electron mediators have been utilized to drive enzyme-catalyzed reactions [186], For instance, the electrochemical reduction of NAD(P)+ to NAD(P)H with a current efficiency of more than 97 % was achieved using alcohol dehydrogenase in the presence of acetophenone as an electron mediator [187], The addition of acetone or acetaldehyde as a substrate to the above bioelectrocatalytic system allowed the reduction of the substrate to the corresponding alcohol at alcohol dehydrogenase accompanied by the oxidation of the resulting NAD(P)H. 

Electrochemical reactions at an electrode snrface differ from normal heterogeneous chemical reactions in that they involve the participation of one or more electrons that are either added to (reduction) or removed from (oxidation) the reactant species. The explicit inclusion of electrons as reactants or products means that the reaction rate depends on the electric potential. Electron transfer processes occur within a small portion of the double layer immediately adjacent to the electrode surface (10 to 50 mn in thickness) where solution-phase electroneutrality does not hold and where very strong electric fields (on the order of 10 V/cm) exist during a charge transfer reaction. We begin the analysis of electrochemical kinetics by defining a generic electrode reaction ... 

G.A. Reid, C. von Wachenfeldt, and S.K. Chapman (2003). Expression, purification and characterisation of a Bacillus subtilis ferredoxin A potential electron transfer donor to cytochrome P450 Biol. J. Inorg. Biochem. 93, 92-99. 

Table II summarizes the sources and key properties of isolated HiPIPs, almost all of which have been isolated from photosynthetic organisms, and there has been extensive speculation on their involvement in respiratory electron transport chains (18, 21, 91-93, 95, 96, 102-105). Evidence in support of such a hypothesis has recently emerged from studies of a partially reconstructed reaction center (RC) complex from Rhodoferax fermentans (93, 95). The kinetics of photo-induced electron transfer from HiPIP to the reaction center suggested the formation of a HiPIP-RC complex with a dissociation constant of 2.5 fx,M. In vivo and in vitro studies by Schoepp et al. (94) similarly have demonstrated that the only high-redox-potential electron transfer component in the soluble fraction of Rhodocyclus gelatinosus TG-9 that could serve as the immediate electron transfer donor to the reaction-center-bound C3d ochrome was a HiPIP. In vitro experiments have shown HiPIP to be an electron donor to the Chromatium reaction center (106). Fukumori and Yamanaka (107) also reported that Chromatium vinosum HiPIP is an efficient electron acceptor for a thiosulfate-oxidizing enzyme isolated from that organism.

Some other mixed dinuclear complexes of potential electron-transfer interest include Cu -Fe, Sn -Cu, Fe -Snii, and V -Cu systems. 

Malca SH, Girhard M, Schuster S, Durre P, Ur-lacher VB (2011) Expression, purification and characterization of two Clostridium acetobutylicum flavodoxins potential electron transfer partners for CYP152A2. Biochim Biophys Acta 1814 257-264... 

This leads to interesting and predictable differences between complexes of the isomeric pentadentate ligands and L. The redox potentials, electron transfer kinetics, and oxidation/oxygenation reactivities of the corresponding high-valent iron complexes as well as the Fe CH202 chemistry have been studied in detail (K. Benzing, P. Comba, A.-M. Rensland, and S. Wunderlich, unpublished data)... 

Immobilized low potential electron-transfer mediators (e.g. viologens) are more promising than diffusional mediators for the practical regeneration of NAD(P)H coupled with further biocat-alytic reactions. The immobiKzation of viologens usually results in significant positive potential shift of their redox potential [269-272], which however, badly affects their efficiency. The potential shift... 

Ensure that improvements observed in peak potentials (electron transfer kinetics) firom using graphene are not simply due to changes in mass-transport, that is, giving rise to thin-layer type behaviour (zone HI - see Chaps. 2 and 3) [16, 17]. 

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