Reduction potentials electron-transfer oxidation

FIGURE 12.3 Schematic depicting the electron flow in an enzyme-catalyzed mediated electron transfer oxidation of substrate. The relative magnitudes of the standard reduction potentials of each element for efficient mediation are shown beneath the scheme. 

The feasibility of electron transfer oxidation is dictated by the thermodynamic potential , of the substrate RH and requires an anode potential or an oxidant to match the value of El. It is essential to choose an oxidant with an one-electron reduction potential sufficient for the desired oxidation and a two-electron reduction potential insufficient for further oxidation of the radical cation. The suitable oxidant may be a metal ion, a stable radical cation, or a typical PET-acceptor in its excited state. The advantage of electrochemically performed oxidation is obvious. 

Fig. 10. Expected relationship, according to Eq. (28), between the rate constant of the electron transfer steps and the reduction potential of the oxidant for a cross reaction of the type of reaction (24). The reorganizational parameter is given by Eq. (29)...

The overall result is a 1-electron reduction of the metal oxidant with concomitant formation of the substrate radical (R ) and is the same in both processes. The ease of electron transfer oxidation of hydrocarbons by a particular oxidant is related to their ionization potentials (Table VII). However, the ease of elec-... 

The most widely used photosensitizer is the Ru(bpy)32+ complex (bpy =2,2 -bipyridine) (Kalyanasundaram, 1982 Watts, 1983 Juris et. al., in press). It exhibits an absorption band at 450 nm with =14000, the efficiency of conversion from the excited state originally populated by excitation and the reactive state ( isc) is unity, the lifetime of the reactive excited state is in the ps range, the excited state is oxidized at -0.86 V, and the reduction potential of the oxidized form of the complex is +1.26 V. The excited state energy is 2.12 eV, the self exchange rate for electron transfer is larger than 106 M 1 s. ... 

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. 

By convention, the electrode potentials for redox are referred to only in terms of reduction (E n ). For electron-transfer oxidation, however, the values of are employed for RH donors here with the understanding that = -E i of the radical cation. 

The most of chemical reactions accompanied by electron transfer from an atom of one reagent (reducer) to an atom of another reagent (oxidizer). Each element can have some oxidation states. The standard oxidation-reduction potential between two oxidation states of element is bonded with standard thermodynamic free energy of the transition from one state to another by the following equation ... 

Within aerobic biology, the harvesting of the oxidative energy of O2 is fundamental to oxidative metabohsm. Because this is accomplished via the electron-transfer oxidation of four cytochrome c(Fe ) molecules ( pH , +0.3 V vs. NHE) per O2, the challenge is to facilitate the reduction of O2 via four one-electron steps, each with a potential greater than +0.4 V versus NHE at pH 7 (equation 125). The thermodynamics for the uncatalyzed electron-transfer reduction of O2 does not meet this criterion (equation 126) especially for the initial pH-independent electron transfer to O2. 

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. 

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. What, we may ask, is the redox potential In the sense that they involve group transfer, redox reactions (more correctly oxidation—reduction reactions) are like other types of chemical reactions. Whereas, for example, in hydrolytic reactions a functional group is transferred to water, in oxidation-reduction reactions, electrons are transferred from electron donors (reductants) to electron acceptors (oxidants). Thus, in the reaction... 

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 ... 

In a redox reaction, electrons move spontaneously toward atoms or molecules having more positive reduction potentials. In other words, a compound having a more negative reduction potential can transfer electrons to (i.e., reduce) a compound with a more positive reduction potential. In this type of reaction, the change in electric potential A is the sum of the reduction and oxidation potentials for the two half-reactions. The AE for a redox reaction is related to the change in free energy AG by the following expression ... 

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