Electrochemically-mediated

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

The electron-transfer rate between large redox protein and electrode surface is usually prohibitively slow, which is the major barricade of the electrochemical system. The way to achieve efficient electrical communication between redox protein and electrode has been among the most challenging objects in the field of bioelectrochemistry. In summary, two ways have been proposed. One is based on the so-called electrochemical mediators, both natural enzyme substrates and products, and artificial redox mediators, mostly dye molecules and conducted polymers. The other approach is based on the direct electron transfer of protein. With its inherited simplicity in either theoretical calculations or practical applications, the latter has received far greater interest despite its limited applications at the present stage. 

Hutchins RS, Bachas LG. Nitrate-selective electrode developed by electrochemically mediated imprinting doping of polypyrrole. Anal Chem 1995 67 1654-1660. 

An alternative method is to use electrochemical mediators that are at a higher concentration that O2 and can therefore be shuttled back and forth between the protein and the electrode faster than the enzyme is reduced, so that the arrival of the glucose is always rate-limiting. A typical chemical that works in this way is ferrocene, which is an iron cation between two cyclopentadienyl anions, as shown in Figure 6.47. It exists in neutral and - -1 oxidation state that are readily interconvertible at metal or carbon electrodes. 

For this reason, in the last decade, inorganic electrochemical mediators, which catalyse the oxidation or reduction of H202 have been preferred to HRP and have been used for the assembling of oxidase-based biosensors [18-20]. This results in a decrease of the applied potential and the consequent avoidance of electrochemical interferences. Many electrochemical mediators have been used and many of them have found broad application, especially in glucose biosensors for diabetes control. However, due to the solubility of the mediator, they are generally employed in a single-use sensor and present some problems due to the low operative stability. 

Our research in this field, which is summarised in this chapter, has been directed at obtaining a sensor modified with PB as electrochemical mediator which could avoid electrochemical interferences and could also couple the advantages of the screen-printed electrodes. For this purpose, an in-depth study of the modification procedure for PB deposition on the electrode surface was first conducted and then when an optimised procedure capable of providing an efficient and stable PB layer was obtained, it was applied with screen-printed electrodes in real analytical systems. Thus, our main goal has been not only to obtain a PB modification procedure suitable for a mass production of modified screen-printed electrodes, as already pointed out above, but also to achieve a stable PB layer in terms of operative and storage stability. 

The biochemical-electrochemical pathway used to determine the inhibition consisted of two enzymatic reactions (Eqs. (29.1) and (29.2)) generating a chemical oxidation (Eq. (29.3)) that was determined by cathodic chronoamperometry (Eq. (29.4)). The used iron containing electrochemical mediator was the widely used Prussian blue. 

The inhibitory effect of pirimiphos-methyl on AChE was evaluated comparing the decrease in the current produced by the reduction of the electrochemical mediator Prussian blue. 

Chen X, Matsumoto N, Hu Y, Wilson GS. Electrochemically-mediated electrodeposition/ electropolymerization to yield a glucose microbiosensor with improved characteristics. Analytical Chemistry 2002, 74, 368-372. 

Fig. 49 Electrochemically mediated manganese-catalyzed radical additions...

Katsumata and colleagues disclosed electrochemically mediated vitamin B12-catalyzed tandem radical cyclizations (Fig. 68, entryl6) [329]. Bromoacetaldehyde dienyl acetal 288 reacted in a 5-exo/5-exo cyclization sequence applying electrochemically reduced vitamin B12 247 as the catalyst. Ester 289 was isolated in 65% yield as a mixture of four diastereomers after oxidation of the lactol. 

Fig. 70 Electrochemically mediated cobalt-catalyzed aryl radical 5-exo cyclizations...

Fig. 23 Nickel-catalyzed electrochemically mediated reductive radical addition reactions...

Several electrochemically mediated Ni-catalyzed addition reactions with aryl halides were reported, but their mechanism is not fully clarified. Using 10 mol% of NiBr2 as a catalyst, heteroaryl halides were added to a, p-unsaturated carbonyl compounds affording (1-aryl carbonyl compounds in 15-86% yield [127]. These addition reactions seem to proceed rather by classical Ni(0)-Ni(II) or Ni(I)-Ni(III) catalytic cycles than by a radical catalysis mechanism. 

Fig. 26 Electrochemical mediated nickel-catalyzed cyclizations of esters 109...

Recently, Chaminade and coworkers reported Ni-catalyzed electrochemically mediated radical 5-exo cyclization reactions of /V-allyl-a-bromo amides 116 (Fig. 28) [138]. Among the catalysts tested 98a and 98b were the best in DMF as the solvent, but the cyclized products 117 were obtained in moderate yields of. For /V,/V-diallylamides 116 (R2=Allyl), overreduction of the extra alkenyl unit competed under the reaction conditions. Using ethanol instead gave considerably better results and pyrrolidones 117 were isolated in 49-99% yield. Reduction of the allyl group could, however, not be prevented under these conditions. 

Katsumata and colleagues reported the first electrochemically mediated Ni(I)-catalyzed tandem radical cyclization reactions [139]. A bromoacetaldehyde dienyl acetal or the corresponding enynyl acetal underwent tandem 5-exo/5-exo cyclization reactions in the presence of Ni(cyclam) complex 98a under electrochemical reductive conditions (see Part 2, Sect. 5.3.3, Fig. 68). Bicyclic esters were isolated in 35% and 72% yields as a mixture of two and four diastereomers, respectively. 

Reduction of horse cytochrome C with [Colsepll ", [Co(diAMsar)]2+, and [Co(NOcapten)]2+ cations was reported in Refs. 316-320. The intrinsic reactivity of these complexes with proteins make it possible the use of clathrochelates as potential protein redox titrants, electrochemical mediators, and electrode modifiers. 

Relatively simple syntheses for the majority of macrobicyclic complexes, compared with conventional techniques for the preparation of macrocyclic compounds, have made such complexes attractive not only for research, but also for practical application as electron carriers, catalysts for electro- and photochemical processes, and some other purposes (e.g., protein redox titrants, biological electrochemical mediators, and ionophore and electrode modifiers). 

X. Chen, N. Matsumoto, Y. Hu and G.S. Wilson, Electrochemically mediated electrodeposition/electropolsrmerization to 5deld a glucose microbiosensor with improved characteristics, A aZ. Chem., 74 (2002) 368-372. 

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