Mediator redox

FIGURE 6-6 Chemical stmcture of some common redox mediators (a) dimethyl ferrocene (b) tetrathiafidvalene (c) tetracyanoquinodimethane (cl) Meldola Blue. 

NADH. Immobilized redox mediators, such as the phenoxazine Meldola Blue or phenothiazine compoimds, have been particularly useful for this purpose (20) (see also Figure 4-12). Such mediation should be useful for many other dehydrogenase-based biosensors. High sensitivity and speed are indicated from the flow-injection response of Figure 3-21. The challenges of NADH detection and the development of dehydrogenase biosensors have been reviewed (21). Alcohol biosensing can also be accomplished in the presence of alcohol oxidase, based on measurements of the liberated peroxide product. 

If the redox mediator is dissolved in the electrolyte solution together with the substrate it is a homogeneous mediator as opposed to the surface bound species at modified electrodes. Two basic cases of homogeneous mediation have been classified... 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

A general theory based on the quantitative treatment of the reaction layer profile exists for pure redox catalysis where the crucial function of the redox mediator is solely electron transfer and where the catalytic activity largely depends only on the redox potential and not on the structure of the catalyst This theory is consistent... 

Controlled potential electrolysis of the substrates, at —1.4V vs. SCE, at a carbon felt electrode on which only 3.6x10" mol Bjj were immobilized (approx. 1 X 10 ° mol cm" ) resulted in the production of 76pmol of valeronitrile 16, corresponding to a turnover number of 2100 This example shows that the combination of inner sphere redox mediators and high surface electrodes is promising. 

MAYER M p, NiEVELSTEiN V and BEYER p (1992) Purification and characterisation of aNADPH-dependent oxidorednctase from chloroplasts of Narcissus-a redox mediator possibly involved in carotene desaturation . Plant Physiol Biochem, 30, 389-98. 

A new crosslinkable polymer was synthesized by the SBP-catalyzed polymerization of cardanol. When HRP was used as catalyst for the cardanol polymerization, the reaction took place in the presence of a redox mediator (phe-nothiazine derivative) to give the polymer. Fe-salen efficiently catalyzed the polymerization of cardanol in organic solvents (Scheme 29). " The polymerization proceeded in 1,4-dioxane to give the soluble polymer with molecular weight of several thousands in good yields. The curing of the polymer took place in the presence of cobalt naphthenate catalyst at room temperature or thermal treatment (150°C for 30 min) to form yellowish transparent films ( artificial urushi ... 

Cell suspensions of Geobacter sulfurreducens can conple the oxidation of hydrogen to the reduction of Tc(VII) to insolnble Tc(IV). An indirect mechanism involving Fe(II) was also observed, and was snbstantially increased in the presence of the redox mediator AQDS (Lloyd et al. 2000). 

In a wider context, extracellular redox mediators have been implicated in a number of reductions. The specific role of reductive dehalogenation by porphyrins and corrins has been discussed in Chapter 1. 

Keck A, J Ran, T Reemtsma, R Mattes, A Stolz, 1 Klein (2002) Identification of quinoide redox mediators that are formed during the degradation of naphthalene-2-sulfonate by Sphingomonas xenophaga BN6. Appl Environ Microbiol 68 4341-4349. 

Ran J, H-J Knackmuss, A Stolz (2002) Effects of different quinoid redox mediators on the anaerobic reduction of azo dyes by bacteria. Environ Sci Technol 36 1497-1504. 

POSSIBLE DESCRIPTION OF ORR AT A R/R-Ox CATALYST SURFACE AS A REDOX-MEDIATED PROCESS... 

In such cases of redox mediation, it seems clear why step (a) would maintain a steady state population of reduced sites according to the value of urface redox couple whereas the rate of step (b) will be enhanced with E — cathode over-... 

This analogy to a surface redox mediated process is significant. In a way very similar to the reaction sequence (1.14), the standard potential of the redox surface system Pt(H20)/Pt-0Hads (0.80 V with respect to RHE) determines the active (reduced) site population at any cathode potential E, and consequently is the critical parameter in determining the ignition potential for the ORR process. 

Figure 17.4 Cartoon representation of strategies for studying and exploiting enzymes on electrodes that have been used in electrocatalysis for fuel cells, (a) Attachment or physisorption of an enzyme on an electrode such that redox centers in the protein are in direct electronic contact with the surface, (b) Specific attachment of an enzyme to an electrode modified with a substrate, cofactor, or analog that contacts the protein close to a redox center. Examples include attachment of the modifier via a conductive linker, (c) Entrapment of an enzyme within a polymer containing redox mediator molecules that transfer electrons to/from centers in the protein, (d) Attachment of an enzyme onto carbon nanotubes prepared on an electrode, giving a large surface area conducting network with direct electron transfer to each enzyme molecule.

FIG. 25 (a) Schematic representation for a photocatalytic mechanism based on shuttle photosensitizers at liquid-liquid interfaces. (Reprinted with permission from Ref. 182. Cop5right 1999 American Chemical Society.) (b) This mechanism is compared to the photo-oxidation of 1-octanol by the heterodimer ZnTPPS-ZnTMPyP in the presence of the redox mediator ZnTPP. (From Ref. 185.)... 

In a conventional feedback experiment the UME tip is placed in a solution containing some redox species. A redox mediator is reduced (or oxidized) at the tip electrode ... 

Utley et al. were able to perform Diels-Alder reactions in aqueous solution via electrogenerated or//zo-quinodimcthancs.34 They cathod-ically generated the or// o-quinodimethanes in aqueous electrolyte in the presence of /V-mcthylmaleimide, which is both the redox mediator and the dienophile. Competition from the electrohydrodimerization of /V-mcthy Irrialci midc is suppressed, allowing for the efficient formation of the endo-adduct (Scheme 12.1). 

Figure 4.12 Schematic representation of the proposed reaction mechanism for overall photocatalytic water splitting using 03 - redox mediator and a mixture of Pt-Ti02-anatase and Ti02-rutile photocatalysts. Adapted from [161] (2001) with permission from Elsevier.

Abe, R., Sayama, K., Domen, K., and Arakawa, H. (2001) A new type of water splitting system composed of two different Ti02 photocatalysts (anatase, rutile) and a IOj"/r shuttle redox mediator. Chemical Physics Letters, 344 (3-4), 339-344. 

Husain, M. and Husain, Q. (2008) Applications of redox mediators in the treatment of organic pollutants by using oxidoreductive enzymes a review. Critical Reviews in Environmental Science and Technology, 38, 1-42. 

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. 

---