Electron transfer oxidation inhibition mechanism

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. 

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 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... 

The by far most widespread mechanism by which an N-NDR is hidden is the adsorption of a species that inhibits the main electron-transfer process. The species might be dissolved in the electrolyte, e.g., it might be the anion of the supporting electrolyte, or it is formed in a side reaction path, as it is the case in nearly all oxidation reactions of small organic molecules. Before we introduce specific examples of this type of HN-NDR oscillators, it is useful to study the dynamics of a prototype model. This will then help us to identify the essential mechanistic steps in real systems whose quantitative description requires more variables such that the basic feedback loops are not as obvious. 

In order to account for such a mechanism, photochemical excitation of a semiconductor surface might induce the promotion of an electron from the valence band to the conduction band. Since relaxation of the high-energy electron is inhibited by the absence of intra-states, if the lifetime of this photo generated electron-hole pair is sufficiently long to allow the interfacial electron transfer from an accumulation site to an electron acceptor, as well as the interfacial electron transfer from an adsorbed organic donor to the valence-band hole, the irradiated semiconductor can simultaneously catalyze both oxidation and reduction reactions in a fashion similar to multifunctional enzymes reactions [232]. 

NQOl is a homodimer with a flavodoxin fold (5). This enzyme does not stabilize the semiquinone state. The obligate two-electron transfer mechanism prevents the generation of quinone radicals and redox cycling, which would result in oxidative stress. The NADPH and quinone substrates occupy the same site, consistent with the observed ping-pong bi-bi mechanism. NQOl is inhibited by many (poly)aromatic compounds including the anticoagulant dicoumarol and the phytoalexin resveratrol (5). 

Monoamine oxidase, which exists in two distinct forms, referred to as MAO A and MAO B, is one of the enzymes responsible for the degradation of biologically important amines. Compounds that block the catalytic action of MAO A, which is selective for the degradation of norepinephrine and serotinin, have antidepressant effects whereas compounds that inhibit MAO B, which degrades dopamine in the brain, are useful for treating Parkinson s disease [190, 191]. Both MAO A and MAO B contain flavin co-enzyme attached at the 8-a-position to an enzyme-active cysteine residue (54). A one-electron transfer mechanism (Scheme 15) for the oxidations catalyzed by MAO was first proposed by Silverman [192] and Krantz [193,194]. 

The observations that these reactions are inhibited by nitrobenzene (a free radical inhibitor), no hydrogenated by-products are formed and that CF BrCl gives only a-CFjCl carbonyl compounds, led the authors to propose a radical chain mechanism for these reactions (Scheme 1). The chain initiation step is the formation of XFiC radical and enamine radical cation by electron transfer from the enamine to BrCFjX. The addition of this perhaloalkyl radical to the enamine generates a RjNC R R" type radical which is known to have an unusually low oxidation potential with 1/2 in the range of — 1 V (sce). An electron transfer from this radical to another molecule of perhaloalkane then takes place to form the iminium salt and another perhaloalkyl radical which continues the chain. A similar mechanism operates in the case of Rp. ... 

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