Quinone oxidations

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

Consequently, in an inert atmosphere/= 2(1 + k(lls/krcc) > 2. When phenoxyl radicals react only with peroxyl radicals, /= 2 and there is no regeneration. At low dioxygen pressures, phenoxyl radicals react with both peroxyl and alkyl radicals / ranges between 2 and 2(1 +kdis/krec) and increases with decreasing p02- In addition to this, the product of phenol oxidation, quinone, becomes the efficient alkyl radical acceptor at low dioxygen pressure (see earlier). 

The intracellular nucleophile glutathione (GSH y-Glu-Cys-Gly) acts as a protective mechanism against electrophilic insults and may be present at concentrations of up to 10 mM [26]. The reaction of glutathione with a non-polar compound bearing an electrophilic carbon, nitrogen or sulfur atom may be mediated enzymatically by glutathione-S-transferase (GST), with typical substrates being species such as arene oxides, quinones and a,P-unsaturated carbonyl compounds. 

The Quinone Acceptor. - In the bRC two quinones act in sequence in the electron-transfer process. They are coupled to a high spin Fe2+ (S — 2). QA accepts only one electron whereas QB can be doubly reduced and protonated. QbH2 leaves the RC and releases its electron and protons to neighboring membrane complexes. It is replaced by an oxidized quinone from the pool in the membrane. The strikingly different physical properties of QA and QB in the bRC can only be explained by a different protein surrounding. 

The isoalloxazine nucleus of the flavins [3-(R or H)-7,8-dimethyl-lO-R -isoalloxazines] may exist in the fully reduced (1,5-di-hydro-), the radical (semiquinone), and the fully oxidized (quinone) states. Because of acid-base equilibria, each of these oxidation states... 

Reduction of plastoquinone Qb by QA- and protonation at the acceptor side of PSII. The Qa is tightly bound to the protein, acting as a one electron acceptor. It passes electrons to a second plastoquinone, Qb, which can accept two electrons and two protons and acts as a mobile electron carrier connecting PSII to the next complex of the photosynthetic apparatus (i.e. the cytochrome b(f complex). After two electron-reductions and two protonation events, QbH2 leaves the reaction center and is replaced by an oxidized quinone from the pool in the membrane. 

The Indulines are destroyed on oxidation, quinone being formed reducing agents produce easily oxidisable leuco-compounds. 

The major by-product of these syntheses is the hydroquinone of the starting quinone, which can be recovered and oxidized. Quinones with several unsubstituted positions are alkylated nonspecifically, but only products of monoalkylation are observed. 

One of the most characteristic functional properties of melanins is their ability to exchange electrons with reducing and oxidizing agents this accounts for their existence in both the oxidized quinone and reduced... 

Phenols are oxidized by NaBiO3 to polyphenylene oxides, quinones, or cyclohexa-2,4-dienone derivatives, depending on the substituents and the reaction conditions [263]. For example, 2,6-xylenol is oxidized in AcOH to afford a mixture of cyclohexa-dienone and diphenoquinone derivatives (Scheme 14.123) [264] and is oxidatively polymerized in benzene under reflux to give poly(2,6-dimethyl-l,4-phenylene) ether (Scheme 14.124) [265]. Substituted anilines and a poly(phenylene oxide) are oxidatively depolymerized by NaBiO, to afford the corresponding anils [266]. Nal iO, oxidizes olefins to vicinal hydroxy acetates or diacetates in low to moderate yield [267]. Polycyclic aromatic hydrocarbons bearing a benzylic methylene group are converted to aromatic ketones in AcOH under reflux (Scheme 14.125) [268]. 

The classical inhibitor for the Q, site is another antibiotic, antimycin, that is produced by various species of Streptomyces. Antimycin blocks electron transfer from reduced Cyt 6(HP) to the oxidized quinone at the Qr site [see Pig. 12 (B, b)]. Binding of antimycin shifts the a-band of reduced Cyt ( (HP) from 562 to 564 nm in the Cyt bc complex. It has been demonstrated that antimycin also prevents the binding of the semiquinone radical in the Cyt 6(LP) domain. When both of these inhibitors are present, all electron transfers to and from either ofthe 6-cytochromes are blocked, a situation that has often been called a double kill of cytochrome 6 [see Fig. 12 (B, c)]. It should be noted, however, that even though myxothiazol and antimycin are very effective inhibitors for the be complexes, these compounds have virtually no inhibitory action on the A /complex, suggesting that some important structural differences must exist among these complexes. However, antimycin at a high concentration does inhibit cyclic electron flow around photosystem II, presumably by acting on a different protein. 

Although Fig. 15 only shows reactions at the Q site, a complete Q-cycle requires the oxidation of two QH2 molecules in two successive series of turnovers. As Fig. 15 (d) shows, the electron on Cyt / (LP) moves up to Cyt / (HP). This electron from the / -heme chain is used to reduce an oxidized quinone bound at the Qf (or Qj) site. Thus the two catalytic sites in Cyt b are involved in the oxidation or reduction of quinone. The integration of the oxidation and reduciton reactions with the release or uptake of protons in the aqueous phases results in the complex translocating protons across the membrane and generating a proton gradient, as already discussed in Section UFA. and Fig. 11. 

Qq - 2,3-dimethoxy-5-methyl-l,4-benzoquinone - primary quinone acceptor Qg - secondary quinone acceptor Q Fe - quinone-iron complex QHj - dihydroquinone or quinol Q - oxidizing quinone binding site also called Q, - reducing quinone binding site also called Q, Q - electronic transition moments... 

Coenzyme Q (ubiquinone) A two electron accepting quinone that can accept and transfer one electron at a time allowing it to exist in a semi-quinone state as well as the fully oxidized quinone or fully reduced dihydroxy state. It is bound to multiple isoprenoid units (ubiquinone has ten units), allowing it to bind to the membrane. 

The formation of alternating copolymers through the polymerization of pairs of monomers, one of which is the donor and the other the acceptor of an electron, is well known. We shall mention only a few studies out of a great number of those recently published. First, those dealing with the nature of active centers in such systems will be examined. When radical initiators are used, e.g., benzoyl peroxide as in17), and the reaction is inhibited with different radical polymerization inhibitors, such as stable radicals like 2,2,6,6-tetramethylpiperidine 1-oxide, quinones, fluorene etc., questions concerning the nature of active centers can be regarded as solved. 

The aim of this work is to extend the kinetics study of electron transfer to monitor donors of more positive redox potential than previously studied, toward silver clusters, Ag ", as acceptors and thus to approach the domain where clusters get metal-hke properties (13). The selected donor is the naphta-zarin hydroquinone, with properties similar to those of the hydroquinone used as a developer in photography. Its redox potential depends on pH, so that different monitor potentials are available through control of pH. Moreover, the reactivity of the donor may be followed by variation of absorbance when naphtazarin hydroquinone, almost transparent in the visible, is replaced by oxidized quinone with an intense absorption band. 

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