Quinone radicals

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

In the original rapid-freezing work on xanthine oxidase (53) it was found that in experiments employing about 1 mole of xanthine per mole of enzyme and an excess of oxygen, the time sequence of appearance of the various EPR signals was molybdenum (V), followed by flavin semi-quinone radical (FADH), followed by iron. This suggested that the electron transfer sequence might be ... 

ENDOR spectrum exhibited chlorine and nitrogen splittings indicating a carotenoid-quinone radical adduct formation. 

S. Sinnecker, E. Reijerse, F. Neese and W. Lubitz, Hydrogen bond geometries from paramagnetic resonance and electron-nuclear double resonance parameters Density functional study of quinone radical anion-solvent interactions, J. Am. Chem. Soc., 2004, 126, 3280. 

Substitution products of the carbon-bonded hydrogen were obtained. A synthesis of coenzyme Q, was achieved in this way (example 6, Table IV). The site of attack in quinones is highly specific and corresponds to the noncarbonyl ring site of highest spin density of the quinone radical anion (lOg, 127). 

Equilibrium studies under anaerobic conditions confirmed that [Cu(HA)]+ is the major species in the Cu(II)-ascorbic acid system. However, the existence of minor polymeric, presumably dimeric, species could also be proven. This lends support to the above kinetic model. Provided that the catalytically active complex is the dimer produced in reaction (26), the chain reaction is initiated by the formation and subsequent decomposition of [Cu2(HA)2(02)]2+ into [CuA(02H)] and A -. The chain carrier is the semi-quinone radical which is consumed and regenerated in the propagation steps, Eqs. (29) and (30). The chain is terminated in Eq. (31). Applying the steady-state approximation to the concentrations of the radicals, yields a rate law which is fully consistent with the experimental observations ... 

The intense blue color of the reaction mixture was assigned to the paramagnetic [Fe(HDMG)2(MeIM)(DTBSQ )]+ complex which is characterized by a broad spectral band at imax 680 nm and a distinct doublet with g = 2.00425 and cqn = 3.135 G in the visible and ESR spectra, respectively. This iron(II) species is not involved in a direct redox step and acts only as a reservoir for the semi-quinone radical. 

The sole product of the reaction is DTBQ with the dimethylglyoximato complexes. The high chemoselectivity was rationalized by considering that the cleavage of the substrate may occur when the semi-quinone radical is directly attacked by 02 (presumably this is a slow reaction... 

Spectroscopic flash photolysis allows the absorption wavelengths for the transient species to be found. The triplet state, 3Q, is found to have an absorption at 490nm, and the quinone radical, QH , has an absorption at 410nm. 

Quinone Reduction This is a reversible, one-electron transfer reaction to the semi-quinone radical, followed by a second, reversible electron transfer that results in the formation of hydroquinone, as shown in Fig. 13.2. 

Copper-containing amine oxidases (non-blue copper proteins) catalyze the oxidative deamination of primary amines to the corresponding aldehydes with the release of ammonia and concomitant reduction of oxygen to hydrogen peroxide. They typically use a quinone redox cofactor [topaquinone (TPQ)], which is bound covalently in the active site, and are thought to form a Cu(I)-TPQ semi-quinone radical intermediate during the redox reaction [13]. 

The -tensor anisotropy of the quinone radical-anions is much larger than that of P+ or BChl a +. It can thus often be fully resolved at W-band or even at Q-band frequencies when deuterated samples are employed. An overview of the measured 0-tensor components has already been given by Weber45, and an interpretation of 0-tensors and their usefulness can be found in ref. 131. Significant progress in the calculation of g values of quinone radical-anions has been made, see references 134-139. 

Based on first ESEEM experiments by Bosch et al.m and Lendzian et al.269 to detect 14N hfcs of the surrounding of the quinone radical-anion A, - and A, - in bRC, similar experiments were also performed on QA- in PS II. Different treatments were used to remove the Fe2+, replace it by Zn2+ or convert it to a... 

In summary, it would appear that the oxidation of a catecholamine probably first involves the formation of a semi-quinone radical (this can be brought about by an one-electron transfer, e.g. from Cu++ ions,14 or by photoactivation 1) which rapidly undergoes further oxidation (e.g. with atmospheric oxygen) to an intermediate open-chain quinone (such as adrenaline-quinone) and then cyclizes by an oxidative nucleophilic intramolecular substitution to the amino-chrome molecule. Whilst the initial formation of a leucoaminochrome by non-oxidative cyclization of the intermediate open-chain quinone in some cases cannot be entirely excluded at the moment (cf. Raper s original scheme for aminochrome formation72), the... 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

A concerted electron transfer mechanism, with formation of an alkyl radical and quinone radical anion, has been proposed to account for the products of reaction of benzophenone with alkyllithium or Grignard reagents 92 the ratio of addition to reduction products is dependent on the alkyl group and not on the metal. 

In a previous study we have found that, at low temperature, PS-I electron transfer is largely blocked away from A, and that the state (P-700+, A, ) decays with a half-time of 130us. Analysis of the absorption spectrum of that state showed that A, is presumably a quinone radical anion (Brettel et al, 1986). Chemical analysis, following separation by HPLC, has shown that phylloquinone (a naphthoquinone also named vitamin Kj) is the only quinone present in PS-I. We have found 2 moles of phylloquinone per PS-I. Extraction with dry hexane does not change the electron transfer reactions this treatment only extracts only one phylloquinone per PS-I (Biggins and Mathis, 1987). 

Besides potential pharmacological applications, a number of other uses have been described for phenoxazine derivatives. The semi-quinone radical obtained on oxidation of phenoxazine with various oxidants, which has been shown to give a characteristic absorption at about 530 mp., was used by Sawicki et al.110 in a new method for the spectrophotometric determination of nitrite the aqueous nitrite sample is treated with a 0.1% solution of phenoxazine in glacial acetic acid and the absorbance is read at 530 mp. 

If the original semiquinone radical QH is polarized, the semi-quinone radical anion derived from eq. 52 can be expected to retain much of the initial polarization. Thus in the CIDEP studies of the photoreduct ion of quinones in triethylamine solution, the primary photochemical process was thought to involve the possible exciplexes (42) ... 

Trimethylsilylphenyltelluride could also be used to efficiently bis-silylate quinones to the corresponding bis-protected hydroquinones (Scheme 62) [ 173]. The reaction required two equivalents of the silyltelluride and diphenylditel-luride was also isolated. The proposed mechanism is slightly different from above, featuring an initial single electron-transfer to form the quinone radical-anion, which was presumably silylated to form phenoxyl radical 192. Subsequent reaction with trimethylsilylphenyltelluride delivered 193 and diphenylditelluride. 

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). 

At this point, Q resides in the Q site. A second molecule of QH2 binds to the site and reacts in the same way as the first. One of its electrons is transferred through the Rieske center and cytochrome c j to reduce a second molecule of cytochrome c. The other electron goes through cytochromes b l and Z) to Q bound in the Q site. On the addition of the second electron, this quinone radical anion takes up two protons from the matrix side to form QH2. The removal of these two protons from the matrix contributes to the formation of the proton gradient. At the end of the Q cycle, two molecules of QH2 are oxidized to form two molecules of Q, and one molecule of Q is reduced to QH2, two molecules of cytochrome c are reduced, four protons are released on the cytoplasmic side, and two protons are removed from the mitochondrial matrix. 

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