Radical anions quinone

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). 

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... 

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). 

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. 

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. 

The C +-P-Q state in triads such as 35 eventually recombines to the groimd state, unless it is harvested by subsequent reactions. The simplest possible recombination pathway involves electron transfer from the quinone radical anion directly to the carotenoid radical cation. However, this pathway can be very slow, even if thermodynamics are very favorable, because of the weak electronic coupling between the radical ions. In some cases, charge recombination has been found to fol-... 

Reaction 1 represents formation of the geminate reaction products [the chlorophyll r-cation radical ( Chl+) and quinone radical anion ( Q )] within the bilayer by electron-transfer quenching of the photoexcited chlorophyll triplet state reaction... 

An anthraquinone linked to the 5 -phosphate of an ODN containing 4 separate GG steps has been used to serve as a trap for a migrating radical cation. Irradiation of the quinone leads to electron transfer from the DNA to the quinone, generating a quinone radical anion and a base radical cation. The radical cation migrates along the DNA and causes strand breaks at the GG steps. 

Figure 3. Resonance Raman spectrum (1000-1800 cm1) of tetrafluoro-p-benzosemi-quinone radical anion (kt = 437.8) observed 10 ps after its creation by 1 ps electron pulse radiolysis of a basic solution of tetrafluorohydroquinone (2 10 3 M pH II) in water. The dotted line shows the Raman spectrum of p-benzosemiquinone anion obtained under similar conditions.

Oxidative de-methylation of Z-(a-hydroalkyl- and o -oxoalkyl-)-l,4,4,8-tetramethoxynaphthalene by [Ce(N03)6] produces two isomeric dimethoxynaphthaquinones. The oxidative addition of ortho- and para-quin-ones to four coordinate planar Co(II) complexes has also been studied. For the o-quinones, the reaction produces Co(III) complexes containing o-semi-quinone radical anions, although changes in the structure of the ligands such as to facilitate simple electron transfer results in competition between the oxidative... 

Preceding cyclovoltammetric studies proved that on reduction in aprotic DMF solution (ch+ < 0.1 ppm), its second and reversible half-wave potential is lowered by 0.6 V( ) on addition of the soluble salt Li [B"(C6H5)4] and simultaneously becomes irreversible [13] (Fig. 14 CV). The presumed microscopic reduction pathway in the presence of excess lithium cation, which, owing to its small ionic radius (ru+ = 60 pm), possesses a high effective ionic charge, is supported by independent ESR/ENDOR measurements [13] in THF (Fig. 14 ESR) the solvated radical anion M ", with two equivalent quinone hydrogen atoms, can be detected by its (1 2 1) triplet. In the subsequently formed contact ion-pair radical [M Li ] , the Li countercation docks at the quinone radical anion. Due to reduced symmetry > Cs, a doublet of doublets ESR signal pattern results. 

Moore, Gust, and coworkers synthesized the quinone-porphyrin-carotenoid (Figme 5) triad molecule. Upon excitation of the porphyrin moiety, initial charge separation occurred between porphyrin and quinone. Hole shift from porphyrin to carotenoid formed the final charge-separated state, that is, quinone radical anion and carotene radical cation, with a lifetime of 170 ns. These processes were confirmed by means of the picosecond and nanosecond laser flash photolysis. Their covalent bonding system was extended to tetrad and pentad using similar chromophores. 

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