Quinone-mediated redox reductions

C.2 for further discussion of electron-mediated reductions) (Schwarzenbach, et al., 1990 Tratnyek and Macalady, 1989). Quinoid-type compounds are thought to be constituents of natural organic matter (Thurman, 1985 see Chapter l.B.3c). It has been hypothesized that some free radicals in humic substances are quinone-hydroquinone redox couples (Tollin et al., 1963 Steelink and Tollin, 1967). 

Autoxidation and/or enzyme-mediated oxidation of pentachlorophenol catechol and hydroquinone to the corresponding semiquinones and quinones followed by subsequent reduction of quinones initiate redox cycling cascades and generate reactive oxygen species, which are believed to be responsible for pentachlorophenol clastogenicity (Naito et al. 1994, Dahlhaus etal. 1996, Wang etal. 1997). 

Using benzylviologen and FeCy as mediators, direct reduction of the quinone can be achieved. Since the mediator is first reduced at a very low potential, and then reoxidized at a potential where the quinone(s) stay reduced, no interfering IR difference bands from the mediator will be observed. Without playing this "redox trick", bands appearing froni the mediator would have to be detected with a potential titration and subtracted using appropriate blanks. 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

In the field of the reductive (bio)transformation of priority pollutants, the reported redox mediator molecules include cytochromes, pyridines, cobalamins, porphyrins, phenazines, flavines, and quinines [12-15]. However, Quinones have been studied as the most appropriate RM for the reductive (bio)transformation of azo dyes [12]. 

Redox mediators, such as flavins or quinones, are usually involved in the azo bond reduction. Therefore, the azo bond cleavage is a chemical, unspecific reaction that can occur inside or outside the cell, relying on the redox potential of the redox mediators and of the azo compounds. Also the reduction of the redox mediators can be both a chemical and an enzymatic process. As a consequence, it is an evidence that environmental conditions can affect the azo dyes degradation process extent both directly, depending on the reductive or oxidative status of the environment, and indirectly, influencing the microbial metabolism. 

Inasmuch as flavins can accommodate two electrons but possess a relatively stable one-electron intermediate, an obvious question which can be asked of any flavin-mediated two electron redox reaction is whether or not the mechanism includes the radical species on a direct line between reactants and products. The mere observation of semiquinones in a reaction mixture is not sufficient evidence for their intermediacy, due to the existence of side reactions such as comproportionation (F -I- FH2 2 FH-) which can generate radicals rapidly. Bruice has discussed this question from a physical-organic point of view and concluded that there must exist a competition between one-electron and two-electron processes and that the actual mechanism should be determined mainly by the free energy of formation of substrate radical and the nucleophilicity of the substrate. Bruice has analyzed a variety of systems which he feels should proceed via one-electron mechanisms among these are quinone and carbonyl group reduction by FH2... 

This explains why superoxide is easily produced from quinones in mammalian cells, through redox cycling. Redox cycling occurs when the one-electron reduction of oxygen is mediated by an electron donor which is rapidly recycled... 

Couples such as hydroquinone/quinone have been hypothesized to dominate the redox properties of humic and fulvic acids, and to act either as electron transfer mediators or as the direct donors of electrons for dechlorination reactions (Schwarzenbach et al., 1990 Dunnivant et al., 1992). For example, it has been shown in sediment-water systems that the rates of alkyl halide reduction increase with organic matter content (Peijnenburg et al., 1992). Further support for this hypothesis was obtained by Svenson et al. (1989), who reported a first-order dependence between rates of hexachloroethane reduction and hydroxyl concentrations. Aside from alkyl halides, structural features of organic matter have been shown to catalyze (Fu et al., 1999) or accelerate (Barkovskii and Adriaens, 1998) the dechlorination of dioxins (Figure 9). 

This method was applied to assemble integrated electrically-contacted NAD(P)-dcpcndcnt enzyme electrodes. The direct electrochemical reduction of NAD(l ) cofactors or the electrochemical oxidation of NAD(P)H cofactors are kineticaUy unfavored. Different diffusional redox mediators such as quinones, phenazine, phenoxazine, ferrocene or Os-complexes were employed as electrocatalysts for the oxidation of NAD(P)H cofactors An effective electrocatalyst for the oxidation of the NAD(P)H is pyrroloquinoline quinone, PQQ, (7), and its immobilization on electrode surfaces led to efficient electrocatalytic interfaces (particularly in the presence of Ca ions) for the oxidation of the NAD(P)H cofactors. This observation led to the organization of integrated electrically contacted enzyme-electrodes as depicted in Fig. 3-20 for the organization of a lactate dehydrogenase electrode. 

Amperometric sensors are based on heterogeneous electron transfer reactions, i.e., the oxidation and reduction of electroactive substances (Fig. 10). Oxygen and H2O2, being the cosubstrate and the product of several enzyme reactions, as well as artificial redox mediators, such as ferricyanide, N-methylphenazinium ion (NMP+), ferrocene, and benzo-quinone may be determined amperometrically. 

For the efficient electrooxidation of NAD(P)H, mediated electrocatalysis is necessary [22, 170, 171], and a wide range of diffusional mediators has been studied [188-193], Organic compounds that undergo two-electron reduction-oxidation processes and also function as proton acceptors-donors upon their redox transformations (such as ortho- and para-derivatives of quinones, phenylenedi-amines and aminophenols) have been found to be ideal for the mediation of NAD(P)H oxidation, although single-electron-transfer mediators (e.g. ferrocene derivatives) are also capable of oxidizing NAD(P)H [190, 191], Some compounds demonstrate very high rates for the mediated oxidation of NAD(P)H in aqueous solutions [188,189,194,195],... 

---