Oxidized electron transfer mediator

The oxidized electron transfer mediator (ETMox). namely the peroxo complexes of methyltrioxorhenium (MTO) and vanadyl acetylacetonate [VO(acac)2] and flavin hydroperoxide, generated from its reduced form (Figure 1.1) and H2O2, recycles the N-methylmorpholine (NMM) to N-methylmorpholine N-oxide (NMO), which in turn reoxidizes the Os(VI) to Os(VIII). While the use of hydrogen peroxide as oxidant without any electron transfer mediators is inefficient and nonselective, various alkenes were oxidized to diols in good to excellent yields employing this mild triple catalytic system (Scheme 1.2). 

The oxidation of N ADH has been mediated with chemically modified electrodes whose surface contains synthetic electron transfer mediators. The reduced form of the mediator is detected as it is recycled electrochemically. Systems based on quinones 173-175) dopamine chloranil 3-P-napthoyl-Nile Blue phenazine metho-sulphatemeldola blue and similar phenoxazineshave been described. Conducting salt electrodes consisting of the radical salt of 7,7,8,8-trtra-cyanoquinodimethane and the N-methylphenazium ion have been reported to show catalytic effects The main drawback to this approach is the limited stability... 

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

Electron-Transfer in Simple Binuclear Complexes. In trying to understand the electron transfer mediation effects of peptide bonds and amino acid side chains on rates of electron transfer in simple systems that are amenable to detailed investigation, we have designed and synthesized a series of complexes which contain within a single molecule two different oxidizing agents —both of which are inert to substitution. The series of complexes we have synthesized is represented schematically by the general structure 1. 

V(IV) is a biologically active oxidation state, acting as an electron-transfer mediator in enzymes such as amavadine, which catalyzes the formation of disulfides [65]. Amavadin is a V(IV) complex found in mushrooms of... 

Fig. 23. Oxidation of a Run(edta) site by -, assisted by intramolecular electron-transfer mediated by the Ru(NH3)5 moiety. A similar mechanism operated for Fen(CN)g as donor.

Nafion sample incorporating both DCA and DPB (DPB-DCA in Nafion mode) only resulted in the electron-transfer-mediated products, 1-4. No singlet-oxygen product, 6, was detected (Fig. 18). Material balance was near 100%. Similarly, the photosensitized oxidation of TS in TS-DCA in Nafion mode only produced the electron-transfer-mediated products 1 and 7-9 (Fig. 19). 

The photosensitized oxidation of DPB in vesicles in the first mode gave 1 and 2 as the unique products (Fig. 22). We believe that these products are derived from the singlet-oxygen pathway. In contrast, the photosensitized oxidation of DPB in vesicles in the second mode only produced the electron-transfer-mediated products 1,2,3, and 5 (Fig. 22). No singlet-oxygen products were detected. These observations demonstrate, once again, that one can control the selectivity in pho-... 

In contrast, when hydroxide ion HO- is present, it is more easily oxidized than the amine substrates. In MeCN, in the absence of substrate, HO- is oxidized at +0.7-0.9 V versus SCE. However, with hydrazines and amines present, as in the case of H2NOH [Eq. (11.25)], the N—H bonds are homo-lytically cleaved by the HO product of HO- oxidation. The latter s oxidation potential is shifted by the difference in the HO—H and RN—H bond energies (—AGbf). Thus, the oxidation of PhNHNHPh is shifted by —1.7V when HO-becomes the electron-transfer mediator with PhNH2 the shift is —1.1 V ... 

The covalent attachment of electron transfer mediators to siloxane or ethylene oxide polymers produces highly efficient relay systems for use in amperometric sensors based on flavin-containing oxidases. It is clear from the response curves that the biosensors can be optimized through systematic changes in the polymeric backbone. The results discussed above, as well as those described previously (25-32), show that the mediating ability of these flexible polymers is quite general and that it is possible to systematically tailor these systems in order to enhance this mediating ability. 

Hale et al. reported the use of an enzyme-modified carbon paste for the determination of acetylcholine [21], The sensor was constructed from a carbon paste electrode containing acetylcholineesterase and choline oxidase, and the electron transfer mediator tetrathiafulvalene. The electrode was used for the cyclic voltammetric determination of acetylcholine in 0.1 M phosphate buffer at +200 mV versus saturated calomel electrode. Tetrathiafulvalene efficiently re-oxidized the reduced flavin adenine dinucleotide centers of choline oxidase. The calibration graph was linear up to 400 pM acetylcholine, and the detection limit was 0.5 pM. 

Another efficient method is the electrochemical oxidation of NADH at 0.585 V vs Ag/AgCl by means of ABTS2- (2,2,-azinobis(3-ethylbenzothiazoline-6-sulfonate)) as an electron transfer mediator [96]. Due to the unusual stability of the radical cation ABTS, the pair ABTS2 /ABTS is a useful mediator for application in large-scale synthesis even under basic conditions. Basic conditions are favorable for dehydrogenase catalyzed reactions. This electrochemical system for the oxidation of NADH using ABTS2 as mediator was successfully coupled with HLADH to catalyze the oxidation of a meso-diol (ws >-3,4-dihydroxymethylcyclohex-l-ene) to a chiral lactone ((3aA, 7aS )-3a,4,7,7a-tetrahydro-3//-isobenzofurane- l-one) with a yield of 93.5% and ee >99.5% (Fig. 18). 

Several findings in the above results are not consistent with earlier reports (Yoshikawa et al., 1995 Van Gelder, 1966 Tiesjema et al., 1973 Schroedl and Hartzell, 1977 Babcock et al., 1978 Blair et al., 1986 Steffens et al., 1993). It has been widely accepted that four electron equivalents are sufficient for complete reduction of the fuUy oxidized enzyme as prepared. However, most of the previous titrations were performed in the presence of electron transfer mediators. In the presence of electron transfer mediators, such as phenazine methosulfate (PMS) under anaerobic conditions, the bovine heart enzyme purified with crystallization also showed a four-electron reduction without the initial lag phase as observed in Fig. 9. A catalytic amount of PMS induced a small spectral change corresponding to the initial lag phase. These results suggest that electron transfer mediators in other titration experiments also induce autoreductions to provide the enzyme form that receives four electrons for the complete reduction. 

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

FIGURE 55.5. An electrochemical oxidation of thiocholine through electron transfer mediator. Med (red) and Med (ox) represent electrode transfer mediator in reduced and oxidized forms. (Reproduced from Anzai, 2006, with permission). 

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