Reversed electron transfer

It does not seem feasible to us that modulation of kinase activity by regulation of proton flow in membrane-associated electric fields can be the sole determinant of the directionality and flux of metabolic pathways. If, as suggested by others, dehydrogenase-catalyzed reactions were to be poised close to chemical equilibrium, the inevitable consequence would be a pooling of the components of these reactions and proticity would be ineffective as a driving force over the total pathway. It seems reasonable to assume, therefore, that cellular redox reactions are also driven.The electrochemical interpretation of metabolism presented here provides a straightforward mechanism for explaining how this could come about. 

Analogous processes occur in inanimate systems. From elementary chemical principles it is known that when two metallic half-cells are coupled by a salt bridge, the direction and extent of the reaction between them can be determined from knowledge of the concentrations of the various components and the value of their standard electrode potentials. The more negative the standard electrode potential, the greater the reducing power of the half-cell. 

For example, since the standard electrode potential of the Cu/Cu half-cell is +0.34 V and that of Zn/Zn is -0.76 V, it can be predicted that under standard conditions metallic zinc will reduce Cu to Cu, whereas metallic Cu cannot reduce Zn without energy input. However, an applied voltage can be utilized to drive the reduction of Zn by Cu. 

Similar considerations apply to biological systems. The standard electrode potential of the 3-hydroxybutyrate/acetoacetate half-cell is —0.26 V and that of the lactate/pyruvate half-cell -0.19 Hence in the presence of the 

In theory this reaction also could be driven backwards by the input of electrical energy, and this can be shown to occur experimentally under aerobic conditions, t Indeed, the facility to drive redox reactions in a thermodynamically unfavorable direction must be a fundamental requirement for maintenance of the steady-state of living systems. Otherwise, it would not be feasible for rates of reductive syntheses to match those of oxidative degradations. 

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Sodium naphthalene [25398-08-7J and other aromatic radical anions react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions. Although the equihbtium (eq. 10)... 

Photochromism Based on Redox Reactions. Although the exact mechanism of the reversible electron transfer is often not defined, several viologen salts (pyridinium ions) exhibit a photochromic response to uv radiation in the crystalline state or in a polar polymeric matrix, for example,... 

FIGURE 2-6 Cyclic voltammograms for a reversible electron transfer followed by an irreversible step for various ratios of chemical rate constant to scan rate, k/a, where a = nFv/RT. (Reproduced with permission from reference 1.)... 

Reversible electron transfer followed by a reversible chemical reaction, ErCr mechanism ... 

Reversible chemical reaction preceding a reversible electron transfer, C rEr mechanism ... 

FIGURE 3-9 Square-wave voltammograms for reversible electron transfer. Curve A forward current. Curve B reverse current. Curve C net current. (Reproduced with permission from reference 9.)... 

Arenediazonium ions can, of course, bring about electrophilic aromatic substitution giving aromatic azo-compounds. Using PhN=N and PhO , polarized signals have been observed in the N-spectrum (6 MHz) of the coupled product (A, A) and reactant, suggesting that the reaction proceeds, at least in part, by a mechanism involving preliminary reversible electron transfer between the reactants (Bubnov et al., 1972). 

Several of these features remain unexplained but it is clear that here we have an example of a relatively well-behaved reversible electron transfer reaction involving radical intermediates. 

The half-wave potentials of (FTF4)Co2-mediated O2 reduction at pH 0-3 shifts by — 60 mV/pH [Durand et ah, 1983], which indicates that the turnover-determining part of the catalytic cycle contains a reversible electron transfer (ET) and a protonation, or two reversible ETs and two protonation steps. In contrast, if an irreversible ET step were present, the pH gradient would be 60/( + a) mV/pH, where n is the number of electrons transferred in redox equilibria prior to the irreversible ET and a is the transfer coefficient of the irreversible ET. The —60 mV/pH slope is identical to that manifested by simple Ee porphyrins (see Section 18.4.1). The turnover rate of ORR catalysis by (ETE4)Co2 was reported to be proportional to the bulk O2 concentration [Collman et ah, 1994], suggesting that the catalyst is not saturated with O2. 

Only three steps of the proposed mechanism (Fig. 18.20) could not be carried out individually under stoichiometric conditions. At pH 7 and the potential-dependent part of the catalytic wave (>150 mV vs. NHE), the —30 mV/pH dependence of the turnover frequency was observed for both Ee/Cu and Cu-free (Fe-only) forms of catalysts 2, and therefore it requires two reversible electron transfer steps prior to the turnover-determining step (TDS) and one proton transfer step either prior to the TDS or as the TDS. Under these conditions, the resting state of the catalyst was determined to be ferric-aqua/Cu which was in a rapid equilibrium with the fully reduced ferrous-aqua/Cu form (the Fe - and potentials were measured to be within < 20 mV of each other, as they are in cytochrome c oxidase, resulting in a two-electron redox equilibrium). This first redox equilibrium is biased toward the catalytically inactive fully oxidized state at potentials >0.1 V, and therefore it controls the molar fraction of the catalytically active metalloporphyrin. The fully reduced ferrous-aqua/Cu form is also in a rapid equilibrium with the catalytically active 5-coordinate ferrous porphyrin. As a result of these two equilibria, at 150 mV (vs. NHE), only <0.1%... 

Kavamos GJ, Turro NJ (1986) Photosensitization by reversible electron transfer theories, experimental evidence, and examples. Chem Rev 86 401 -49... 

The modulation of the ion-pair dynamics by salt and solvent effects as well as the observation of significant kinetic isotope effects unambiguously establishes that benzylic C—H activation proceeds via a two-step sequence involving reversible electron transfer followed by proton transfer within the contact ion pair, 41c,2°5 (Scheme 18). 

Thermal electron transfer. The high degree of charge-transfer character in [ArH, NO+] complexes is consistent with the fact that a variety of electron-rich aromatic donors undergo reversible electron transfer (in the dark) to form the corresponding cation-radical pair239 (equation 86). 

Much fundamental work yet remains in the study of intramolecular donor-acceptor molecules to find out what structural parameters of the donor, acceptor and particularly the linkage enhance the efficiency of forward electron transfer while at the same time inhibiting the rate of reverse electron transfer. Progress so far is very promising. 

The first step of the proposed mechanism is the reversible electron transfer from C102 to Br2 as in reaction (2) ... 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Electrochemical communication between electrode-bound enzyme and an electrode was confirmed by such electrochemical characterizations as differential pulse voltammetxy. As shown in Fig. 11, reversible electron transfer of molecularly interfaced FDH was confirmed by differential pulse voltammetry. The electrochemical characteristics of the polypyrrole interfaced FDH electrode were compared with those of the FDH electrode. The important difference between the electrochemical activities of these two electrodes is as follows by the employment of a conductive PP interface, the redox potential of FDH shifted slightly as compared to the redox potential of PQQ, which prosthetic group of FDH and the electrode shuttling between the prosthetic group of FDH and the electrode through the PP interface. In addition, the anodic and cathodic peak shapes and peak currents of PP/FDH/Pt electrode were identical, which suggests reversibility of the electron transport process. 

Table IV lists a series of octahedral (phenolato)chromium(III) precursor complexes that contain one or three oxidizable coordinated phenolato pendent arms (146, 154). These complexes display characteristic electrochemistry Each coordinated phenolato ligand can undergo a reversible one-electron oxidation. Thus complexes with one phenolato moiety exhibit in the C V one reversible electron-transfer process, whereas those having three display three closely spaced (AE1/2 250 mV) ligand-centered one-electron transfer processes, Eqs. (7) and (8).

FIGURE 1.25. Successive reversible electron transfers in cyclic voltammetry of attached reactants. Normalized charge (a) and current (b) as a function of the separation between the standard potentials, at 25°C, from right to left A ° — E — E = 0.4, 0.1, 0.0356, —0.2 V. The middle of each curve corresponds to — )/2. (c) Variation of the normalized peak current with AE° in the range where a single wave is observed. 

FIGURE 1.26. a Successive reversible electron transfers in cyclic voltammetry as a function of the separation between the standard potentials, at 25°C, from right to left AE° =... 

The passage from one control to the other is pictured in Figure 2.5 for the cathodic peak potential and the peak width as a function of the scan rate and of the intrinsic parameters of the system. We note that increasing the scan rate tends to move the kinetic control from the follow-up reaction to the electron transfer step. It thus appears that the overall reaction may well be under the kinetic control of electron transfer, even if this is intrinsically fast, provided that the follow-up reaction is irreversible and fast. The reason is that the follow-up reaction prevents the reverse electron transfer from operating, thus making the forward electron transfer the rate-determining step. 

First-order chemical reaction preceding a reversible electron transfer. The process in which a homogeneous chemical reaction precedes a reversible electron transfer is schematized as follows ... 

When K is large (for example, 5= 20), most of Ox will already be present in solution, hence, given the low kinetics of the chemical reaction the response is apparently not disturbed by the latter, i.e. it appears as a simple reversible electron transfer. 

When K is large, once again the response appears as a simple reversible electron transfer, but the measured standard potential, E°, is shifted towards more negative values compared to the standard potential, E01, of the couple Ox/Red by a factor of ... 

Diagnostic criteria to identify a chemical reaction preceding a reversible electron transfer. Granted that when the chemical reaction has a large K value, or its rate is slow, the response looks like an unperturbed reversible process, the simplest and most wished-for opportunity to detect the presence of a preceding equilibrium reaction lies on the possible appearance of an S-shaped curve in the forward scan. Otherwise, other criteria are the following. 

If the rate of the chemical reaction is low, it has little effect on the process, thus reducing it to a simple reversible electron transfer. 

If the rate of the chemical complication is high, the system will always be in equilibrium and the voltammogram will apparently look like a non-complicated reversible electron transfer. However, as a consequence of the continual partial removal of the species Red from the electrode surface, the response (if, as in the present case, one is considering a reduction process) is found at potential values less negative than that of a simple electron transfer by an amount of ... 

Diagnostic criteria to identify a first-order reversible chemical reaction following a reversible electron transfer. To be able to ascertain... 

If kfchemical reaction is very slow compared to the intervention times of cyclic voltammetry) the response is very similar to that of a simple reversible electron transfer and occurs at the formal potential, E°, of the couple Ox/Red. 

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