Mediators oxidized form

Medox Electrochemical active mediator (oxidized form)... 

Figure 11.39 summarizes the reactions taking place in this amperometric sensor. FAD is the oxidized form of flavin adenine nucleotide (the active site of the enzyme glucose oxidase), and FAD1T2 is the active site s reduced form. Note that O2 serves as a mediator, carrying electrons to the electrode. Other mediators, such as Fe(CN)6 , can be used in place of O2. 

Several factors affect drug-mediated derepression of ALASl in liver—eg, the administration of glucose can prevent it, as can the administration of hematin (an oxidized form of heme). 

An alternative to the direct anodic oxidation of organic contaminants are the methods of indirect oxidation with the aid of oxidizers formed electrochemically in situ. These oxidizers (or mediators) can be obtained in both anodic and cathodic processes. Anodic agents are the salts of hypochloric acid (hypochlorites), the permanganates, the persulfates, and even ozone. 

It was seen when analyzing the kinetic data for alcohol oxidation reactions that the catalytic action of nickel oxide is due to a mediator mechanism. Higher oxide forms interact with the adsorbed organic species and oxidize them. In the following step the higher oxide forms are regenerated by electrochemical oxidation of lower oxide forms. 

Yamauchi, R. and K. Kato (1998). Products formed by peroxyl radical-mediated oxidation of canthaxanthin in benzene and in methyl linoleate. J. Agric. Food Chem. 46(12) 5066-5071. 

Nitrogen-containing side chains in amino acids can be altered by oxidation forming chloramines or even become deaminated. The result is often the formation of carbonyls (e.g., aldehydes) and hydroxyls. Lee, S et al. (2006) found that Fe-EDTA-mediated oxidation of human... 

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

The mechanism of direct oxidation of cyclohexene to cyclohexanone by N20 mediated oxidation was analyzed by density functional theory (DFT) using B3LYP/6-31G approximation. A two-step reaction mechanism was predicted where the substituted 1,2,3-oxadiazoline ring system 5 forms as the first intermediate in the process before subsequent conversion to the cyclohexanone <1999JOC6710, 2003CC42, 2005MI177>. 

Belkner et al. [32] demonstrated that 15-LOX oxidized preferably LDL cholesterol esters. Even in the presence of free linoleic acid, cholesteryl linoleate continued to be a major LOX substrate. It was also found that the depletion of LDL from a-tocopherol has not prevented the LDL oxidation. This is of a special interest in connection with the role of a-tocopherol in LDL oxidation. As the majority of cholesteryl esters is normally buried in the core of a lipoprotein particle and cannot be directly oxidized by LOX, it has been suggested that LDL oxidation might be initiated by a-tocopheryl radical formed during the oxidation of a-tocopherol [33,34]. Correspondingly, it was concluded that the oxidation of LDL by soybean and recombinant human 15-LOXs may occur by two pathways (a) LDL-free fatty acids are oxidized enzymatically with the formation of a-tocopheryl radical, and (b) the a-tocopheryl-mediated oxidation of cholesteryl esters occurs via a nonenzymatic way. Pro and con proofs related to the prooxidant role of a-tocopherol were considered in Chapter 25 in connection with the study of nonenzymatic lipid oxidation and in Chapter 29 dedicated to antioxidants. It should be stressed that comparison of the possible effects of a-tocopherol and nitric oxide on LDL oxidation does not support importance of a-tocopherol prooxidant activity. It should be mentioned that the above data describing the activity of cholesteryl esters in LDL oxidation are in contradiction with some earlier results. Thus in 1988, Sparrow et al. [35] suggested that the 15-LOX-catalyzed oxidation of LDL is accelerated in the presence of phospholipase A2, i.e., the hydrolysis of cholesterol esters is an important step in LDL oxidation. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

S, substrate R, product P, reduced form of the cosubstrate (mediator) Q, oxidized form of the cosubstrate (mediator) Ei, reduced form of the enzyme E2, oxidized form of the e... 

By applying a potential to the electrode equal to the reduction potential of the catalyst (the redox mediator) the catalyst is reduced, but, upon contact with the oxidized form Ox, a redox reaction takes place in which Ox is reduced to Red and the mediator reoxidized. At this point the continuous cathodic reduction of the catalyst reactivates the whole process and the catalytic cycle is repeated. 

In general, biotransformation reactions are beneficial in that they facilitate the elimination of xenobiotics from pulmonary tissues. Sometimes, however, the enzymes convert a harmless substance into a reactive form. For example, CYP-mediated oxidation often results in the generation of more reactive intermediates. Thus, many compounds that elicit toxic injury to the lung are not intrinsically pneumotoxic but cause damage to target cells following metabolic activation. A classic example of this is the activation of benzo(a)pyrene, which is a constituent of tobacco smoke and combustion products, and is... 

In the direct method substrates are oxidized by electron transfer from the substrate to the anode, while in the indirect method they are oxidized in solution by the oxidized form of a mediator that is generated from the reduced form at the anode. Thus, efficiencies in direct and indirect methods are largely dependent on the oxidation potentials of substrates and mediators, respectively. 

The direct electrochemical oxidation of alcohols is in many cases unselective because of the high oxidation potentials of the alcohols. One possible way to avoid this disadvantage is the use of a mediatory system (an indirect oxidation). Thereby a mediator is converted to its oxidized form at a less positive potential than that required for the direct oxidation of the alcohol, and the oxidized form of the... 

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