Secondary redox mediators

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

Figure 6.5 Three generations of amperometric enzyme electrodes based on the use of natural secondary substrate (a), artificial redox mediators (b), or direct electron transfer between the enzyme and the electrode (c).

Medox/raredox mediator Silfifilssssssj Electrocatalyst Reduced secondary... 

Biofuel cells — Figure. Schematic illustration of identified electron transfer mechanisms in microbial fuel cells. Electron transfer via (a) cell membrane-bound cytochromes, (b) electrically conductive pili (nanowires), (c) microbial redox mediators, and (d) via oxidation of reduced secondary metabolites [v]... 

As has been discussed above, one of the particularly important reactions involving peroxidase-derived radicals is redox mediation, or mediation by electron transfer [10], In this mechanism, radicals (R ), which have been generated by a peroxidase-catalyzed reaction, Eq. (1), may act as diffusible oxidants to oxidize secondary substrates (SH) ... 

Furthermore, these t)qjes of reactions, Eqs. (17) and (18), may have several connotations when investigating the capability of peroxidases for synthesizing bioactive plant products. Foremost is the fact that compounds which cannot be regarded as good peroxidase substrates [e.g. SH in Eq. (17) or S in Eq. (18)] may be oxidized through this sequence of reactions in the presence of appropriate substrates [RH in Eq. (17) or R in Eq. (18)]. This implies that the rate of secondary substrate radical generation [S in Eq. (17) or S in Eq. (18)] may be dramatically stimulated in the presence of the redox mediator [R in Eq. (17) or R in Eq. (18)], in which RH (or R) acts as primary substrate. Under such circumstances, the presence of the primary substrate, RH (or R), may stimulate the oxidation of SH (or S). 

In 1970, Dan Reed, Tom Chaney and this author used a cytochrome-free, reaction-center complex from Rb. sphaeroides R-26 and tried to reconstitute it with mammalian cytochrome c in an attempt to mimic in vivo electron transfer. Although P870 can undergo rapid oxidation by a light flash, its rereduction is very slow, as expected, in the absence of efficient secondary donors, as shown in Fig. 12, upper row. However, P870 may be reduced very rapidly by an externally added redox mediator such as reduced PMS. For example, in the presence of 0.1 mM reduced PMS, P870 can be re-reduced in 36 /js. 

Indirect EET involves the use of so-called electron shuttles which physically transfer electrons from the cell to the electrode [80]. Commonly applied mediators include humic substances such as anthraquinone 2,6-disulfonate (AQDS) [104]. Furthermore, Thrash and Coates reviewed the electron shuttles used in BESs, and reported that the addition of a chemical shuttle can be expensive, toxic, and prone to wash-out of the system [81]. In addition to artificial redox mediators, some microorganisms are able to produce their own mediators such as secondary metabolites like phenazines [46, 92] and flavins [77]. Finally, primary metabolites such as sulfur species [105] and hydrogen gas [106] are also able to convey electrons toward electrodes. 

In 1991, the group of Willetts [13] published one of the first smart combinations of two redox enzymes for the oxidation of a secondary alcohol mediated by an alcohol dehydrogenase (ADH) from Thermoanaerobium brockii and the subsequent Baeyer-Villiger oxidation mediated by the cyclohexanone monooxygenase from Acinetobacter calcoaceticus NCIMB 9871) of the intermediate ketone... 

Elirect evidence of redox mediation between sulfonated PANI and dimercaptane combined in an electrode initially investigated for possible use in secondary batteries [510] was observed with in situ UV-vis spectroscopy [511]. 

PMAS (half ox)/Au NPs also catalyze the dehydrogenative oxidation of cyclic secondary amines such as 2-substituted indoline derivatives in aqueous solution undo molecular oxygen (Scheme 3.35), where the redox mediating effect is demonstrated by... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

The electron transfer between NADH and the anode may be accelerated by the use of a mediator. Synthetic applications have been described for the oxidation of primary and secondary alcohols to aldehydes and ketones catalyzed by yeast alcohol dehydrogenase (YADH) and the alcohol dehydrogenase from Thermoanaerobium brockii (TBADH) with indirect electrochemical regeneration of NAD+ and NADP+, respectively, using the tris(3,4,7,8-tetramethyl-l,10-phenanthroline) iron(II/III) complex as redox catalyst [59],... 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

Figure 5 Hemozoin-mediated lipid peroxidation. Redox cycling of iron in complexes like HZ can initiate lipid peroxidation of fatty acids like arachidonic acid (LH). Abstraction of the bisallylic hydrogen leaves an unpaired electron on the methylene carbon that can rearrange to form a reactive alkyl radical (L ). Oxidation of this radical leads to a peroxyl radical (LOO ), and on reduction, forms a lipid peroxide (LOOM) that can undergo additional reactions to yield a variety of secondary oxidation products.

One particularly appealing route for effecting controlled redox reactions involves an array of surface-mediated reactions initiated by ultraviolet irradiation of suspended semiconductor particles [3-13]. Such reactions involve band-gap excitation of the semiconductor, interfacial electron transfer, and secondary dark chemical reactions of singly oxidized and reduced adsorbates. Because the semiconductor surface is restored to its original structure and oxidation level after these transformations, these photoreactions are often called photocatalytic, leaving the light-responsive photocatalyst ready to act as initiator for another cycle. The use of such photocatalysts also obviates the need to acquire expensive electrochemical equipment. 

In the last few years the term generation has mainly been used to differentiate between the modes of signal transfer between a redox enzyme and an electrode, i.e., via the natural secondary substrates and products of the enzyme catalysed reaction first generation), artificial electron mediators instead of the natural cosubstrates second generation) or in direct electronic... 

Reduction of V, R = Me, in dimethylformamide at mercury leads to conversion of the ketone function to secondary alcohol with essentially no cyclization process observed. However, in the presence of a mediator the course of reaction changes. Addition of N,N-dimethylpyrrolidinium fluoroborate causes formation of the cyclized tertiary alcohol. The pyrrolidinium salt is reduced at —2.7V (vs. SCE) at mercury to yield a complex DMP(Hg5) which is thought to act as a single-electron-transfer mediator [42]. Cyclization also occurs in dimethylformamide at a mercury cathode using a homogeneous redox catalyst such as phenanthrene or 2-methoxybiphenol with a redox potential in the range —2.4 to —2.7 (vs. SCE) [43]. Because during these reactions one electron is delivered to the carbonyl compound in solution by the reduced mediator, cyclization of the reduced carbonyl compound can occur before a second electron donor is encountered. 

A composite catalytic system will consist of a redox transition metal catalyst and an electron mediator. The redox catalyst is selected from metal ions such as Ti ", Mn, Fe etc based on their ability to form the oxo species. The electron mediator is selected from Ru, Rh and Pd based on their well established ability to remove and transport electrons from hydrogen and secondary alcohols. 

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