Two-electron reaction

Thus, in the latter case, the term ECE has to be abandoned and replaced by disp . (the SET occurs obviously via a disproportionation process). Finally, the strong base R formed after an overall two-electron reaction is protonated by the solvent or by any acidic impurity. Alternative mechanisms could be proposed taking into account that R or Ar may abstract hydrogen atoms from the solvent ... 

Cathodic hydrogen evolntion is a complex two-electron reaction occurring through several consecutive, simpler intermediate steps. Each of these steps is sometimes referred to with the name of the scientist who had suggested that it was rate determining for the overall reaction the steps are ... 

An electrode reaction in which the oxidized form accepts more than one electron usually proceeds as a series of one-electron reaction steps. As will be demonstrated below, if the formal potentials of these partial electrode reactions satisfy certain conditions, then the electrode reaction simulates the transfer of several electrons in one step (Eq. 5.2.5) and obeys Eq. (5.2.24). An example is the two-electron reaction of substance Au converted to substance A3 by the transfer of two electrons, where the reaction occurs through the unstable intermediate A2 ... 

If the single-electron mechanism has not been demonstrated to be the rate-controlling process by an independent method, then, in the publication of the experimental results, it is preferable to replace the assumed quantity ax by the conventional value cm, provided that the charge number of the overall reaction is known (e.g. in an overall two-electron reaction it is preferable to replace = 0.5 by or = 0.25). If the independence of the charge transfer coefficient on the potential has not been demonstrated for the given potential range, then it is useful to determine it for the given potential from the relation for a cathodic electrode reaction (cf. Eq. 5.2.37) ... 

We next focus on the use of fixed-site cofactors and coenzymes. We note that much of this coenzyme chemistry is now linked to very local two-electron chemistry (H, CH3", CH3CO-, -NH2,0 transfer) in enzymes. Additionally, one-electron changes of coenzymes, quinones, flavins and metal ions especially in membranes are used very much in very fast intermediates of twice the one-electron switches over considerable electron transfer distances. At certain points, the chains of catalysis revert to a two-electron reaction (see Figure 5.2), and the whole complex linkage of diffusion and carriers is part of energy transduction (see also proton transfer and Williams in Further Reading). There is a variety of additional coenzymes which are fixed and which we believe came later in evolution, and there are the very important metal ion cofactors which are separately considered below. 

Hydrogen peroxide then can be reduced or decomposed in water and oxygen. Thus, oxygen reduction can proceed by two parallel ways (a) the four-electron reaction of conversion into water and (b) the two-electron reaction of transformation into hydrogen peroxide. The four-electron reaction can proceed, if oxygen adsorbs with a rupture of chemical bonds in 02 ... 

Initially, it was thought that all two-electron reaction pathways were of this general sort, and a so-called scheme of squares was elaborated, which, for the system shown, would take the form ... 

Rate Constants (I mol-1 s-1) for Two-Electron Reaction (8) and One-Electron Reaction (19) of MPO and MPO Compound I with Hydroperoxides... 

The rate constants for two-electron reaction (8) (A+) and one-electron reaction (19) (k 19) are cited in Table 22.3. As seen from Table 22.3, ks values are about 100 1000 times greater than kig values therefore, the production of superoxide (or hydroperoxyl radical) by peroxidases might play an insignificant role compared to the two-electron oxidation of hydroperoxides. 

Ph4Pb, generally thought to be oxidized by a two-electron reaction, is also found to react by a SET under short pulses. However, PI13P is slow to dimerize and an alternative... 

Our calculated reaction path may reasonably well represent the overall reaction of pyrite as it oxidizes, but it does little to illustrate the steps that make up the reaction process. Reaction 31.2, for example, involves the transfer of 16 electrons to oxygen, the electron acceptor in the reaction, from the iron and sulfur in each FeS2 molecule. Elementary reactions (those that proceed on a molecular level), however, seldom transfer more than one or two electrons. Reaction 31.2, therefore, would of necessity represent a composite of elementary reactions. 

Because the oxidation of COad is a two-electron reaction, if every COad... 

Davidson et al. 2000) (Section 7.13), it can be conclnded that the formal valence state of Ni shnttles between 2+ and 3 +. Hence, in the absence of mediators, we have to assume the involvement of one of the Fe-S clnsters in the two-electron reaction of the A. vinosum enzyme with H2 ... 

Several characterized NRPSs utilize alternative methods for chain termination. In some synthetases, the TE domain of the final module is replaced by an NAD(P)H-dependent reductase domain. Reduction of a peptidyl-S-PCP substrate through a two-electron reaction leads to the formation of a transient aldehyde, which is subsequently converted into a cyclic imine or hemiaminal through intramolecular cyclization. This two-electron reaction is utilized in the biosynthesis of nostocyclopeptides, the saframycins, ° and anthramycin. Alternatively, a four-electron reduction to the primary alcohol is observed in the biosynthesis of mycobacterial peptidolipids, linear gramicidin," " the myxalamides, lyngbyatoxin, " and myxochelin A 75,76 alternative four-electron reduction pathway involving aldehyde formation, transamination, and reduction to a primary amine occurs in the biosynthesis of myxochelin B. ... 

The donor types D3, D4, and D6 of Keilin and Nicholls (37) all reduce compound I of Type A enzymes directly to the ferric state in a two-electron process without detectable intermediates. Each of these donors is probably also able to bind in the heme pocket of the free enzyme. Alcohols (type D3) form complexes with free ferric Type A enzymes whose apparent affinities parallel the effectiveness of the same alcohols as compound I donors (39). Formate (type D3) reacts with mammalian ferric enzyme at a rate identical to the rate with which it reduces compound I to free enz5mie (22). Its oxidation by compound I may thus share an initial step analogous to its complex formation with ferric enzyme. Formate also catalyzes the reduction of compound II to ferric enzyme by endogenous donors in the enz5mie (40, 41). Both compound I and compound II may thus share with the free enzyme the ability to ligate formate in the heme pocket. Nitrite, which is oxidized to nitrate by a two-electron reaction with compoimd I (type D4), also forms a characteristic complex with free enzyme (42). In both cases the reaction involves the donor in its protonated (HNO2) form. 

Reduction of allyl bromides and iodides at vitreous carbon in an aptotic solvent generally gives good yields of the dimer. This product arises by rapid substitution by the allyl carbanion, formed in an overall two-electron reaction, onto a second molecule of allyl halide [55, 56]. 

Reduction of Re (terpy)2Cl" is irreversible and re-forms Re (terpy)2 in a two-electron reaction. Other seven-coordinate complexes [e.g. Re (bpy)3 (CNBu ) +] are generated in this manner and exhibit similar reactivity [31]. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

In general, these free radical reactions bear some resemblance to the reactions found for the air-free bleaching of ferric mercaptoacetate (7) and ferric cysteinate (6) A bimolecular reaction occurs between two one-electron oxidants, one of which is an iron (III) ion with two mefcaptide groups coordinated to it in a two-electron reaction each of the oxidants is reduced and the mercaptides are oxidized to give a molecule of disulfide. 

Dialkylaziridinium salts are reducible both in aqueous and aprotic media. In aqueous solution the data are consistent with one two-electron reaction. The products obtained from a preparative electrolysis stem in part from the electrode reaction, but side reactions between primary products and starting material complicate the product mixture.181... 

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