Semiquinone anions

Oxidation of this UQHg occurs in two steps. First, an electron from UQHg is transferred to the Rieske protein and then to cytochrome C. This releases two to the cytosol and leaves UQ , a semiquinone anion form of UQ, at... 

The second half of the cycle (Figure 21.12b) is similar to the first half, with a second molecule of UQHg oxidized at the Q site, one electron being passed to cytochrome C and the other transferred to heme bj and then to heme bfj. In this latter half of the Q cycle, however, the bn electron is transferred to the semiquinone anion, UQ , at the Q site. With the addition of two from... 

Flowers, L. Ohnishi, T. Penning, T. M. DNA strand scission by polycylic hydrocarbon o-quinone role of reactive oxygen species, Cu(II)/(I) redox cycling, and o-semiquinone anion radicals. Biochemistry 1997, 36, 8640-8648. 

O Malley, P., and S. J. Collins. 1996. Density functional studies of free radicals accurate geometry and hyperfine coupling prediction for semiquinone anions. Chem. Phys. Lett. 259, 296. 

One of the most important properties of quinoid compounds is the two step redox reaction. Quinoid compounds undergo one electron reduction to so-called semiquinone anion radicals, and further one electron reduction of semiquinone anion radicals gives dianions (Scheme 14). 

Qrunones can accept one or two electrons to form the semiquinone anion (Q ") and the hydroquinone dianion (Q ). Single-electron reduction of a quinone is catalyzed by flavoenzymes with relatively low substrate selectivity (Kappus, 1986), for instance NADPH cytochrome P-450 reductase (E.C. 1.6.2.3), NADPH cytochrome b5 reductase (E.C. 1.6.2.2), and NADPH ubiquinone oxidoreductase (E.C. 1.6.5.3). The rate of reduction depends on several interrelated chemical properties of a quinone, including the single-electron reduction potential, as well as the number, position, and chemical characteristics of the substituent(s). The flavoenzyme DT-diphorase (NAD(P)H quinone acceptor oxidoreductase E.C. 1.6.99.2) catalyzes the two-electron reduction of a quinone to a hydroquinone. 

Reactions of cobalt porphyrin with p-fluoranyl or phenyl-A -butylimidazole proceeds analogously—both fluoranyl and imidazole coordinate with the cobalt contained in the porphyrin complex, but fluoranyl was fixed in its semiquinone, anion-radical form (Okamoto and Fukuzumi 2003). 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

FIGURE 2. Zinc(II) porphyrin-o-quinone dyads and two typical reactions (a) photoinduced charge separation, (b) chelation of in situ formed semiquinone anions. PQ = porphyrin quinine M = metal... 

Fig. 1. Absorption spectra of neutral and anionic flavin semiquinones. (-) Anionic riboflavin...

The condition for stabilization of the semiquinone by resonance is that the two structures IV be equivalent. This condition is satisfied for the semiquinone anion, but not for the semiquinone III itself, in which the presence of the hydrogen atom destroys the equivalence of the two structures. We thus expect the semiquinone to be stable only in the form of the anion. This is verified by experiment. Michaelis and his collaborators67 have shown that the semiquinone of phenanthrene-3-sulfonate is stable in alkaline solution as the semiquinone ion,... 

Verdazyls (111) can also transfer an electron to o -quinones to give the verdazylium cation (113) and a semiquinone anion (114) (80IZV2785), or to tetranitromethane to give the cation (113) and the tetranitromethane anion radical (115) (74MI22100). Rate constants and activation parameters for the electron transfer from triphenylverdazyl to tetracyanoethylene have been determined by Soviet chemists (79ZOR2344). 

Intramolecular photoinduced electron transfer reactions of homonaphthoquinones are also made possible by the presence of Mg(C104)2 in MeCN [212]. As shown in Scheme 27, the photoexcitation of 10 in the presence of Mg2 + results in intramolecular electron transfer due to the complexation of Mg2 + with the semiquinone anion moiety, which can accelerate the photoinduced electron transfer and at the same time may retard the back electron transfer [212], No reaction occurred in the absence of Mg2+ or in the dark at ordinary temperature [212], The generated radical ion I undergoes ring... 

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... 

A molecular modeling program to calculate electron paramagnetic resonance hyper-fine couplings in semiquinone anion radicals was offered [173b]. 

In some cases radical cations may undergo cycloadditions with an acceptor derived intermediate without prior proton transfer. This is observed especially for radical cations without sufficiently acidic protons, although it is not limited to such species. For example, the photoreaction of chloranil with 3,3-dimethylindene results in two types of cycloadducts [141]. In the early stages of the reaction a primary adduct is identified, in which the carbonyl oxygen is connected to the p-position of the indene (type B) in the later stages this adduct is consumed and replaced by an adduct of type A, in which the carbonyl oxygen is connected to the a-position. CIDNP effects observed during the photoreaction indicate that the type B adduct is formed from free indene radical cations, which have lost their spin correlation with the semiquinone anions. 

Figure 12.2 Cyclic voltammograms of (a) 2.0 mM 3,5-di-rert-butyl-o-benzoquinone (b) 2.0 mM, 3,5-di-ferf-butyl-o-semiquinone anion (formed by controlled potential electrolysis at —0.80 V (c) solution b plus 2.0 mM tetraethylammonium hydroxide, initial cathodic scan and (d) solution c, initial anodic scan. All solutions were in dimethyl-formamide that contained 0.1 M TEAP at a Pt electrode (surface area 0.23 cm2). Scan rate was 0.1 V s-1.

HjCat represents catechol or hydroquinone, Q quinone, SQ semiquinone anion, HCat- catechol monoanion, and Cat2 catechol dianion DTBQ represents 3,5-di-rerr-butyl-o-quinone, o-Q o-benzoquinone, p-Q p-benzoquinone, TCQ tetrachloro-o-benzoquinone, and TFQ tetrafluoro-o-benzoquinone. 

Benzene metabolites have been shown to bind to DNA and RNA, and have been shown to induce hyperphosphorylation of gene material, suggesting a mechanism for chromosomal damage and carcinogenesis (Dees and Travis 1994 Lutz and Schlatter 1977 Snyder et al. 1978b). Copper-mediated oxidation of hydroquinone to semiquinone anion radicals has been implicated in direct DNA damage (Li et al. 

Fio. 13. Yeast glutathione reductase, semiquinone anion production from the 2-electron reduced form. Curve 1, oxidized enzyme, anaerobic conditions, pH 7.6 curve 2, 1 min after the addition of 1 equivalent of NADPH curve 3, 22 hr later curve 4, 1 hr after the addition of 10 equivalents of NADP curve 5, 235 hr later curve 6, 185 hr after the addition of 5 equivalents of NADPH and ciirve 7, 36 min after opening to air. 

Although the uncharged tris(3,5-di-t-butylcatecholate) complex of iron [Fe(DTBC)3] has been extensively studied, " the proposed bonding in these reports is unclear. The most common formulation is as an ionic salt between iron(lll) and three semiquinone anion radicals, Fe +(DTBSQ 03 However, the magnetic moment is 2.9 BM (consistent with an 5 = 2/2 spin state) and the electrochemistry indicates a ligand-centered reduction. Both of these characteristics are analogous to ferrate dianion. 

The reaction of H0 with 9,10-anthraquinone in MeCN produces an adduct (stable at -20 which reacts further at room temperature to yield the semiquinone anion radical (AQ ). The equilibrium constants for the formation of the adducts and the rate constants for the reaction of the adduct with a second quinone molecule are given in Table 18 (see Scheme 18). 

When the g anisotropy (see below) is very small, as often is the case for radicals, better resolution can be obtained by using spectrometers that operate at higher frequencies (and fields). An example is the two neighboring semiquinone anion radicals (Q and Qb) in the bacterial photosynthetic reaction center recorded at X-, Q- and W-band freqnencies (Figure 1). 

Figure 1 Comparison of the EPR spectra of two semiquinone anion radicals (Q), Qg, and + Qg) from the bacterial photosynthetic reaction center recorded at 9.60, 35.0 and 94.0 GHz. Note that as the spectrometer frequency increases, the overlapping spectra become better resolved. Spectra taken from Ref. 2 (Reprinted with permission from Calvo, Abresch, Bittl, Feher, Hofbauer, Isaacson, Lubitz, Okamura and Paddock. 2000 American Chemical Society)...

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