Two-electron transfer reaction

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

As a rule, high quantum yields for two-electron transfer reactions are expected when the mechanism is one-electron/two-hole or two-electron/one-hole. In the cases of twQ-electron/two-hole or one-electron/one-hole efficient back reactions of the intermediates on the colloidal particles or in solution, respectively, will lead to a low yield of the final products. 

In the monolayer j the two members of the redox couple undergo the electron transfer reactions depicted in Scheme 6.5 with the molecules located in the j + 1 and j — 1 layers. The rates of the two electron transfer reactions between two adjacent sites may be written as... 

Reductive dechlorination or reductive hydrogenolysis is a common transformation of 1- and 2-carbon chlorinated aliphatics under methanogenic conditions [373,374]. 1,1,1-Trichloroethane (l,l,l-TCA),for example,is converted to 1,1-dichloroethane (1,1-DCA) [375], and Perchloroethylene (PCE) is successively converted to TCE, cDCE, VC, and ethane [274]. Each reductive dechlorination is a two-electron transfer reaction. 

Dihaloelimination is a two-electron transfer reaction. Thompson et al. [377] reported reductive dichloroelimination of 1,1,2-TCA and TeCA by hepatic micro-somes from rat Ever, with VC and both tDCE and cDCE as metabolites. Reductive dichloroelimination from hexa- and pentachloroethane by microsomal cytochrome P450 was studied by Nastainczyk et al. [378]. The main products of the in vitro metabolism of hexa- and pentachloroethane were PCE (99.5%) and TCE (96%), respectively, with minor amounts of pentachloroethane (0.5%) and TeCA (4%), respectively, via reductive dechlorination. 

The cation-radicals ArH+ were detected, but they originated from the fast reaction of a one-electron transfer, which does not affect kinetic constants of the oxidation. The rate constant depends linearly on Brown s a constants of substituents (Dessau et al. 1970). All these data are in agreement with the formation of the strong polar dication of an aromatic hydrocarbon as an intermediate. Because PF salts (in particular the diacetate) are not reductants, the two-electron transfer reaction proceeds irreversibly. 

The result can in fact be generalized. Suppose any two electron-transfer reactions taking place at separated interfaces in a cell are considered (Fig. 7.179)... 

Since the initial observation of flavin radical species by Michaelis and coworkers the involvement of flavins in one-electron oxidation-reduction processes in biological systems has occupied the attention of workers in the field of redox enzymology up to the present time. Flavin coenzymes occupy a unique role in biological oxidations in that they are capable of functioning in either one-electron or two-electron transfer reactions. Due to this amphibolic reactivity, they have been termed in a recent review to be at the crossroads of biological redox processes. 

In terms of these redox potentials the free energy change for the net two electron transfer reaction is given as... 

Another flavoprotein that makes use of both one-and two-electron transfer reactions is ferredoxin-NADP+ oxidoreductase (Eq. 15-28). Its bound FAD accepts electrons one at a time from each of the two... 

It is now generally admitted that this reaction involves both one-electron and two-electron transfer reactions. Carbonyl compounds are directly produced from the two-electron oxidation of alcohols by both Crvl- and Crv-oxo species, respectively transformed into CrIV and Crm species. Chromium(IV) species generate radicals by one-electron oxidation of alcohols and are responsible for the formation of cleavage by-products, e.g. benzyl alcohol and benzaldehyde from the oxidation of 1,2-diphenyl ethanol.294,295 The key step for carbonyl compound formation is the decomposition of the chromate ester resulting from the reaction of the alcohol with the Crvl-oxo reagent (equation 97).296... 

There are a number of molecules for which two- or more electron transfers can be detected (i.e., stepwise processes). For molecules capable of giving two-electron transfer reactions (EE mechanism see reaction scheme (3.II)), the addition of the second electron occurs, in the more typical case, with greater difficulty than the first, so the single pulse voltammogram presents two well-separated waves because of the difference between the two formal potentials defined as... 

As in the case of a two-electron transfer reaction (EE mechanism), there exists the possibility of the following disproportionation reaction at the electrode surface ... 

In this section, surface two-electron transfer reactions in line with the following reaction scheme ... 

Galactose oxidase is an extracellular enzyme secreted by the fungus Dactylium den-droides. It is monomeric (M = 68000), contains a single copper site and catalyses the oxidation of a wide range of primary alcohols to the corresponding aldehydes. The two-electron transfer reaction RCH2OH - RCHO + 2H+ + 2e does not utilise a Cu(III)/Cu(I) couple, but a second redox site, involving a tyrosine radical which mediates the transfer of the second electron. 

The most interesting for chemists probably is the ECE mechanism, which involves a chemical step between two electron-transfer reactions ... 

At first, a two-electron transfer reaction took place between gold and the glucose oxidase. However, as time (in terms of minutes) went on, the electrodic response (Fig. 

Mondal MS, Fuller HA, Armstrong FA (1996) Direct measurement of the reduction potential of catalytically active cytochrome c peroxidase Compound I voltammetric detection of a reversible, cooperative two-electron transfer reaction. J Am Chem Soc 118 263-264... 

The reduction of Cuni(H 3G4) has been examined with a number of substrates including I", Fe(CN)64", (tert-Bu)2NO, ascorbic acid, cysteine, hydroquinone, and S032". All of the reductions are rapid and appear to proceed by one-electron steps. Two-electron transfer reactions... 

A comparison of Eq. 54 and Scheme 36 reveals that the nickel center performs two functions. First, the nickel center acts as mediator of the two electron-transfer reactions during which S—C bonds are cleaved and formed (36 —> 37 and 39 —> 40). Second, it facilitates the formation of an acyl group from an alkyl group and CO (38 —> 39). This reaction is expected to be favored when the nickel center has a coordination number lower than 5. A low coordination number of nickel also facilitates the final release of the thioester in the consecutive reaction of 39 with excess CO, because intermediates such as complex 42 can readily form. 

A comparison of Eq. 54 and Scheme 36 reveals that the nickel center performs two functions. First, the nickel center acts as mediator of the two electron-transfer reactions during which S C bonds are cleaved and formed (36 —> 37 and 39 > 40). Second, it facilitates the formation of an acyl group... 

The initial step of the two-electron-transferring reactions is the removal of a proton from the substrate, followed by the intermediate formation of an adduct between the substrate and prosthetic group at N-5 of the flavin. This undergoes cleavage to yield dUiydroflavin and the oxidized product, which is commonly a carbon-carbon double bond. The reduced flavin is then reoxidized by reaction with an electron-transferring flavoprotein, as discussed above, or in some cases by reaction with nicotinamide nucleotide coenzymes. 

Hence, the main difference between these two electron transfer reactions would be in the pre-exponential factor Z (Eq. 65). Following the treatment [24], they differ by a factor equal to the thickness of the inner layer d, i.e., Z/Z ° d. By using a rigorous procedure, Marcus [179] derived the expression for Z, which in the case of the sharp liquid-liquid boundary reads... 

The macromolecular silyl chloride reacts with sodium in a two-electron-transfer reaction to form macromolecular silyl anion. The two-electron-trans-fer process consists of two (or three) discrete steps formation of radical anion, precipitation of sodium chloride and generation of the macromolecular silyl radical (whose presence was proved by trapping experiments), and the very rapid second electron transfer, that is, reduction to the macromolecular silyl anion. Some preliminary kinetic results indicate that the monomer is consumed with an internal first-order-reaction rate. This result supports the theory that a monomer participates in the rate-limiting step. Thus, the slowest step should be a nucleophilic displacement at a monomer by macromolecular silyl anion. This anion will react faster with the more electrophilic dichlorosilane than with a macromolecular silyl chloride. Therefore, polymerization would resemble a chain growth process with a slow initiation step and a rapid multistep propagation (the first and rate-limiting step is the reaction of an anion with degree of polymerization n[DP ] to form macromolecular silyl chloride [DP +J, and the chloride is reduced subsequently to the anion). 

NAD tends to be an electron acceptor in catabolic reactions involving the degradation of carbohydrates, fatty acids, ketone bodies, amino acids, and alcohol. NAD is used in energy-producing reactions. NADP, which is cytosolic, tends to be involved in biosynthetic reactions. Reduced NADP is generated by the pentose phosphate pathway (cytosolic) and used by cytosolic pathways, such as fatty acid biosynthesis and cholesterol synthesis, and by ribonucleotide reductase. The niacin coenzymes are used for two-electron transfer reactions. The oxidized form of NAD is NAD". There is a positive charge on the cofactor because the aromatic amino group is a quaternary amine. A quaternary amine participates in four... 

An example of a voltammogram for two consecutive one-electron reductions is shown in Fig. 4. When the two electron transfer reactions are as well separated as those shown in Fig. 4, the difference between the peak potentials for the first and the second electron transfer is to a good approximation equal to the difference in standard potentials E — E . When the two electron transfer reactions are closely spaced, the difference in standard potentials may be determined from the half-peak width of the overlapping waves [56]. 

---