Semiquinone electron donor

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

A large number of other sensitizers has been investigated for use in photolytic de-diazoniation. The excited states of these compounds (S ) react either by direct electron transfer (Scheme 10-97), as for pyrene, or by reaction with an electron donor with formation of a sensitizer anion radical which then attacks the diazonium ion (Scheme 10-98). An example of the second mechanism is the sensitization of arenedi-azonium ions by semiquinone, formed photolytically from 1,4-benzoquinone (Jir-kovsky et al., 1981). 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

Simple Quinones. - Greci and colleagues have detected semiquinones arising from the photochemical (300-400 nm) reduction of 1,4-benzoquinone, Q0 and Qiq (section 6.1). In many cases, radicals derived from the electron donor were also detected. Analysis using Marcus theory led to the proposal of a mechanism involving H-atom transfer to the triplet quinone.150... 

A reduction and activation of HjOj by other one-electron donors, like semiquinones, has also to be considered. This follows from a study of the ethylene production from methionine in the presence of pyridoxal phosphate, a reaction characteristic for OH radicals or for Fenton-type oxidants. The ethylene production in the presence of dioxygen, anthraquinone-2-sulfonate, and an NADPH-generating system in phosphate buffer pH 7.6 was inhibited by SOD and by catalase, but stimulated by scavengers of OH radicals, like 0.1 mM mannitol, a-tocopherol, and formiate... 

The covalent 8a-N(3)-histidyl FAD of mitochondrial succinate dehydrogenase functions as a two-electron acceptor in the oxidation of succinate to fumarate and as a one-electron donor in the reduction of the iron-sulfur centers of the enzyme. Recent ESR spectroscopic data have shown the covalent flavin semiquinone... 

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

According to recent data, the property of dithionite as an electron donor for nitrogenase is different from that of the natural donor flavodoxin (Burgess and Lowe, 1996). Flavodoxin from Azotobacter vinelandii has the redox potential equal to -0.515 V for the reversible transition between the semiquinone and hydroquinone forms of flavodoxin. Unlike dithionite, flavodoxin can reversibly reduce the [Fe4S4]+l cluster Av2 by one electron to the [Fe4S4]° state in which all iron ions exist in the ferrous form. It is assumed that, under natural conditions, two electrons can transfer from Av2 to Avl. Flavodoxin reduces both Av2 bound to Avl and free Av2 in a solution. The apparent rate constants of these reactions are 400 s 1 and > 1000 s"1, respectively (Duyvis et al. 1998). 

Ceo is known to undergo reduction to Ceo by addition of a MeO solution. This reduction is, nevertheless, accompanied by the formation of the adduct anions, C6o(OMe) (n = 1, 3, 5, 7) [174]. Electron transfer from MeO to Ceo is thought to result in the formation of Ceo and the corresponding adduct anions [174]. p-Benzoquinone is also known to be reduced to semiquinone radical anion in a reaction with OH in acetonitrile [175, 176]. The hydroxide ion is a much stronger electron donor in aprotic solvents such as acetonitrile than in water, since the solvation energy for OH is less in aprotic solvents than in water [175]. However, no oxidized products of OH were found in the reaction of p-benzoquinone with OH [176]. The only oxidized product detected evolved from p-benzoquinone itself, namely, the rhodiz-onate dianion that is the ten-electron oxidized product of p-benzoquinone [176]. Thus, the one-electron reduction of ten equivalents of / -benzoquinone is accompanied by the ten electron oxidation of one equivalent of / -benzoquinone. In this case... 

OH- is not the electron donor, but instead 7-benzoquinone itself acts as the electron donor in the presence of OH. The addition of OH" to p-benzoquinone initiates an electron transfer from the OH adduct of p-benzoquinone to p-benzoquinone leading to the noble disproportionation. This yields ten equivalents of semiquinone radical anion and rhodizonate dianion [176],... 

Reactions of /)-benzoquinone and its derivatives with alkoxide ions (RO R = H, Me, Et, i-Pr, PhCH2) in MeCN also result in formation of the corresponding semiquinone radical anions accompanied by the formation of RO-substituted p-benzoquinones, which are the oxidized products of p-benzoquinones [360], Detailed product and kinetic analyses of the reactions indicate that RO-adduct anion of /)-benzoquinone is an actual electron donor and that RO is acting as a very strong base or nucleophile rather than a one-electron reductant in an aprotic solvent, such as MeCN [360], Similarly, the reaction of C o with methoxide anion (MeO ) in benzonitrile (PhCN) results in the disproportionation of Cgo to yield both C(,o and the methoxy adduct [360]. Spectroscopic and kinetic studies also indicate that a methoxy adduct anion of Ceo is a real electron donor and that MeO is acting as a very strong base or nucleophile rather than an electron donor in PhCN [360],... 

Radical 80 has been prepared as its perchlorate salt by anodic oxidation in ethyl acetate in the presence of hthium perchlorate. The reactivity toward nucleophiles of material so prepared was investigated nitrite and nitrate ions give 2-nitrodibenzo[l,4]dioxin although the mechanisms of the reactions are not clear. Pyridine gives 7V-(2-dibenzo[l,4]dioxinyl)pyridinium ion (84). Other nucleophiles acted as electron donors and largely reduced 80 back to the parent heterocycle they included amines, cyanide ion and water. In an earlier study, the reaction of 80 with water had been examined and the ultimate formation of catechol via dibenzo[l,4]dioxin-2,3-dione was inferred. The cation-radical (80) has been found to accelerate the anisylation of thianthrene cation-radical (Section lII,C,4,b) it has been found to participate in an electrochemiluminescence system with benzo-phenone involving phosphorescence of the latter in a fluid system, and it has been used in a study of relative diffusion coefficients of aromatic cations which shows that it is justified to equate voltammetric potentials for these species with formal thermodynamic redox potentials. The dibenzo[l,4]dioxin semiquinone 85 has been found to result from the alkaline autoxidation of catechol the same species may well be in-... 

A study of the Raman spectra of other p-substituted (CH3, F, Cl, Br, OCH3) phenoxyl radicals indicated a progression from the phenoxyl to the semiquinone character as the substituent becomes a stronger electron donor . 

Treatment of the blue form of the lyase with dithionite or irradiation at wavelengths greater than 520 nm in the presence of DTT produces fully reduced flavin cofactor (FADH2) and results in a dramatic increase in activity and quantum yield (146, 153, 154). This suggests that the catalytically active oxidation state of the cofactor may be the reduced flavin, and that it is this form of the cofactor that functions as an electron donor in catalysis (155). Subsequent studies by Sancar and co-workers indicate that E. coli DNA-PL does not contain the flavin semiquinone radical in vivo, and that the blue, radical enzyme does not catalyze dimer cleavage (156). 

Flavins are unique coenzymes that are able to catalyze both one- and two-electron transfers. Because of this, many flavoproteins are involved in transferring electrons between other proteins. Often, flavoproteins are reduced by two-electron donors, such as pyridine nucleotides, and then pass those electrons one at a time to a single-electron acceptor, such as an iron-sulfur cluster in another protein. Conversely, some enzymes accept single electrons from reduced enzymes. In either case, the flavoenzymes are transferring single electrons thus, flavin semiquinone is frequently stabilized and observed during turnover. 

The semiquinone is also an electron donor. The second monoelectronic potential, (Q-Q ) = -0.105 Vnhe, is even more negative than that of the first monoelectronic couple with hydroquinone. Once formed through reaction 12, the semiquinone is thus able, a fortiori, to also transfer electrons to Ag > c by reaction 13. 

Mechanism of cytochrome P450 reductase from the house fly Evidence for an FMN semiquinone as electron donor. FEBS Lett. 453, 201-204. 

O2 ( h202.2) to the positive side and of the first one-equivalent oxidation of electron donors to the negative side, as shown in Figure 1. The Em of these molecules in the bound state on the oxidase will be the same as in the free state provided the aflSnities of the oxidase for the reduced and oxidized forms are equal. The one-order-of-magnitude difference between these affinities will make 0.03 volt difference in the normal potentials (Em) between the free and bound state. It may be said that hydrogen donors and O2 are activated by the oxidase when it stabilizes the intermediates and increases the semiquinone formation constant, K. An increase in Ks results in a rise of Eo and a drop of Ei. 

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