Reductive activation, of molecular oxygen

It has been hypothesized that activated BLM is a high-valent iron-0X0 complex generated by a reductive activation of molecular oxygen reminiscent to that observed in cytochrome P-450 monooxygenases... 

For example, the lipophilic Mn(III)-tetraphenylporphyrin bearing four cholesteryl units in a-a (3-(3 arrangement can be incorporated into DMPC bilayers, and in the presence of fiavoprotein pyruvate oxydase and aniphiphilic flavine, the reductive activation of molecular oxygen is... 

Devynck et al. showed catalytic oxidation of (f/-tertiarybutylphenol by reductive activation of molecular oxygen by Mn porphyrin fixed on zeolite [329]. Hydroxylation of alkanes by zeolite-encapsulated Fe and Mn porphyrin was also reported by Nakamura et al... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

The biochemical importance of flavin coenzymes ap-pears to be their versatility in mediating a variety of redox processes, including electron transfer and the activation of molecular oxygen for oxygenation reactions. An especially important manifestation of their redox versatility is their ability to serve as the switch point from the two-electron processes, which predominate in cytosolic carbon metabo-lism, to the one-electron transfer processes, which predomi-nate in membrane-associated terminal electron-transfer pathways. In mammalian cells, for example, the end products of the aerobic metabolism of glucose are C02 and NADH (see chapter 13). The terminal electron-transfer pathway is a membrane-bound system of cytochromes, nonheme iron proteins, and copper-heme proteins—all one-electron acceptors that transfer electrons ultimately to 02 to produce H20 and NAD+ with the concomitant production of ATP from ADP and P . The interaction of NADH with this pathway is mediated by NADH dehydrogenase, a flavoprotein that couples the two-electron oxidation of NADH with the one-electron reductive processes of the membrane. 

In the oxidation reactions the problem of oxidant activation is frequently met (in the general case, activation of small molecules [5]). For example, comparatively low reactivity of 02, H202 and 03 under soft conditions requires their prior thermal or catalytic activation, for instance, with the help of transition metal complexes or irradiation. Activation of molecular oxygen by one-electron reduction consumes energy ... 

Lay PA, Levina A. 1998. Activation of molecular oxygen during the reactions of chromium(VI/V/IV) with biological reductants Implications for chromium-induced genotoxicities. J Am Chem Soc 120 6704-6714. 

Superoxide dismutases are a class of oxido-reductase enzymes which contain either Cu, Fe, or Mn at the active site and catalyze the dismutation of superoxide (1), the one-electron reduction product of molecular oxygen (Eqs. 1 and 2, where M" is the metalloenzyme in the reduced state and M" is the enzyme in the oxidized state) to oxygen and hydrogen peroxide. The SOD enzymes have been shown to have efficacy in animal models of disease states proposed to be, in part, mediated by superoxide, such as myocardial ischemia-reperfusion injury, " inflammation, and cerebral ischemia-reperfusion injury. Evidence for superoxide as a mediator... 

We recently reported for the first time on the electroassisted-biomimetic activation of molecular oxygen by a newly prepared electropolymerized polypyrrole-manganese phthalocyanine film " The prepared films and their intervention in the electroassisted-catalytic reduction of molecular oxygen were analyzed by cyclic voltammetry and UV-visible spectrophotometry on optically transparent electrodes. The obtained results demonstrate the probable existence of the... 

Table 8.1 recapitulates the most significant examples of use of electropoly-merized metallophthalopcyanines and metalloporphyrins for the activation of molecular oxygen. The reported fields of application are mainly devoted to electrosynthesis involving O2, mechanistic electroanalysis of O2 reduction and assessment of the feasibility of the O2 activation. 

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