Inhibition mechanism, electron transfer

Concluding Remarks. It has recently been shown that the activation energy of many "aq-solute reactions is 3.4 db 0.6 kcal./mole (4, 8). From this result we have reasoned that the rates of many of the slow reactions are determined principally by their entropies of activation. These findings do not conflict with the mechanisms discussed above. If electron tunneling is a major contributing factor in these electron transfer reactions, the slower rates may be attributed to transmission coefficients smaller than unity. The factors which partially inhibit an electron transfer may thus be considered geometrical in nature, demonstrable by the entropy term of the free energy of activation, or by a decrease in the transmission coefficient as a result of an increase in the width of the potential barrier. 

Since 1946, a series of antibiotic antimycins A (AA, 94-la 94-9) have been isolated from various Streptomyces species [77-80] (Fig. 5). Antimycin A complex, a mixture of derivatives, has been widely used for biochemical studies. As it inhibits the electron transfer of ubiquinol-cytochrome c ox-idoreductase [81], many scientists have investigated their structure-activity relationships and mechanism of action [82-89]. The related deacyl compounds, deisovalerylblastmycin (95a) [90], urauchimycins 95b and 95c [91], and kitamycins 95d and 95e [92] were isolated from Streptomyces species. In addition, the corresponding L-serine derivatives, UK-2A 2D (96a-d) [93] and UK-3A (97) [94], were added to this series. 

The oxidation of carbon fibers can be inhibited to some extent by the use of dopants such as boron. Three mechanisms could be involved in the inhibition process active site blockage resulting from the formation of a boron oxide layer, chemical inhibition by electron transfer, and development of fiber structure/microtexture which is catalyzed by boron. For example, at 700°C, the oxidation rate of the T300 fiber decreases 30 fold when 2000 ppm B are added and the P55 fibers having 5% B never reach 25% burn-off [67]. 

Dinitrophenol is a member of the aromatic family of pesticides, many of which exhibit insecticide and fungicide activity. DNP is considered to be highly toxic to humans, with a lethal oral dose of 14 to 43mg/kg. Environmental exposure to DNP occurs primarily from pesticide runoff to water. DNP is used as a pesticide, wood preservative, and in the manufacture of dyes. DNP is an uncoupler, or has the ability to separate the flow of electrons and the pumping of ions for ATP synthesis. This means that the energy from electron transfer cannot be used for ATP synthesis [75,77]. The mechanism of action of DNP is believed to inhibit the formation of ATP by uncoupling oxidative phosphorylation. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

An inhibition mechanism involving electron transfer between a chain-propagating radical and the antioxidant has frequently been suggested but has rarely been identified with any certainty. This process remains one of the least understood of all inhibition mechanisms. Probably the most clear-cut example of inhibition by one electron transfer (either partial or complete) has come from studies of metal-catalyzed oxidations. Many workers have reported that under certain conditions transition metals may inhibit rather than catalyze oxidations. Cobalt, manganese, and copper are particularly prominent in this respect. 

The radical addition and hydrogen transfer mechanisms of inhibition by chain-breaking antioxidants are now reasonably well understood in both qualitative and quantitative terms. The electron-transfer mechanism of inhibition deserves greater attention. 

The binding of small inorganic ions such as [Fe(CN)6]3 and [Cr(en)3]3+ can be used to explore binding sites on electron-transfer proteins. Thus redox inactive [Cr(en)3]3+ inhibits the oxidation of cytochrome bs with [Co(NH3)6]3+ and other oxidizing agents, and also blocks association with cytochrome c. The reaction of cytochrome b5 with the oxidants occurs by a mechanism involving association of the reactants prior to electron transfer. The reaction may then be inhibited by redox... 

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