Semiquinone radical anion

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 mixture of tetra-chloro-phenoxy phenols and tetra-chlorodihydroxy biphenyls, no polymeric products detected 2,6-dichlorophenoxyl semiquinone radical anion semiquinones rapidly disproportionate to 2,6-dichlorobenzoquinone and 2,6-dichlorohydroquinone 50% degraded in 10 days major products chloranilic acid, yellow compound (C12HO4CI7) identified as 3,4,5-trichloro-6-(2 -hydroxy-3, 4, 5, 6 -tetra chloro-... 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

A proof for the formation of alkyl radicals was found by their addition to the aci-nitromethane anion (CH2=N02 ) and by their reaction with p-benzoquinone to give the optically active nitroalkane radical-anion and the semiquinone radicals, respectively. In the case of di-r-butyl sulfoxide the f-butyl radical was observed directly by its absorption spectra. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

The semiquinone radical dissociates to a proton and semiquinone radical anion in the polar solvent (water). 

Self-reactions of semiquinone radical anions proceed via electron transfer reaction [2]. 

Now, we may consider in detail the mechanism of oxygen radical production by mitochondria. There are definite thermodynamic conditions, which regulate one-electron transfer from the electron carriers of mitochondrial respiratory chain to dioxygen these components must have the one-electron reduction potentials more negative than that of dioxygen Eq( 02 /02]) = —0.16 V. As the reduction potentials of components of respiratory chain are changed from 0.320 to +0.380 V, it is obvious that various sources of superoxide production may exist in mitochondria. As already noted earlier, the two main sources of superoxide are present in Complexes I and III of the respiratory chain in both of them, the role of ubiquinone seems to be dominant. Although superoxide may be formed by the one-electron oxidation of ubisemiquinone radical anion (Reaction (1)) [10,22] or even neutral semiquinone radical [9], the efficiency of these ways of superoxide formation in mitochondria is doubtful. 

The coordination chemistry of phenoxyls in many aspects resembles that of other coordinated radical anions and, in particular, that of semiquinones. We refer... 

The first reported low-spin iron(III) semiquinonate complex contained the 3,5-di-t-butyl-l, 2-benzoquinonate radical anion and a tetraazamacrocycle (tazm). It reacts reversibly with acetonitrile to give [Fe (tazm)(MeCN)2] plus 3,5-di-t-butyl-l,2-benzoquinone. " The related 1,2-iminobenzosemiquinonate (ibsq) complexes [Fe(ibsq)2X] have 5 = 5/2 for X = C1 and 5 =3/2 for X = I the complex with X = Br is a mixed spin (5=5/2, 3/2) species. " ... 

Interrupted oxidations of 9,10-dihydroanthracene or 9,10-dihydro-phenanthrene in DMSO (80% )-terf-butyl alcohol (20%) containing potassium ferf-butoxide produced the 9,10-semiquinone radical anions, apparently as a product of oxidation of the monoanion. 

The characterization of the semiquinone radical anion species of PQQ in aprotic solvents was undertaken to provide information about the electrochemistry of coenzyme PQQ and to give valuable insight into the redox function of this coenzyme in living systems <1998JA7271>. The trimethyl ester of PQQ and its 1-methylated derivative were examined in aprotic organic solvents by cyclic voltammetry, electron spin resonance (ESR), and thin-layer UV-Vis techniques. The polar solvent CH3CN was found to effectively solvate the radical anion species at the quinone moiety, where the spin is more localized, whereas the spin is delocalized into the whole molecule in the nonpolar solvent CH2CI2. 

Wurster in 1879 had already prepared crystalline salts containing radical cation 23 (equation 12). Subsequently, radical cations of many different structural types have been found, especially by E. Weitz and S. Hunig, and recently these include a cyclophane structure 24 containing two radical cations (Figure 3). Leonor Michaelis made extensive studies of oxidations in biological systems, " and reported in 1931 the formation of the radical cation species 25, which he designated as a semiquinone. Michaelis also studied the oxidation of quinones, and demonstrated the formation of semiquinone radical anions such as 26 (equation 13). Dimroth established quantitative linear free energy correlations of the effects of oxidants on the rates of formation of these species. ... 

As mentioned above, in an earlier work, Nohl et al. [9] suggested that neutral ubisemi-quinone reduced dioxygen to superoxide (this suggestion was dropped in subsequent studies of these authors). Although the participation of neutral semiquinone in the reduction of dioxygen is impossible, the observation of these authors might be interpreted as the support of a role of ubihydroquinone in mitochondrial superoxide production. If neutral semiquinone is indeed formed in mitochondria via the protonation of semiquinone radical anion (Reaction... 

Nirofuran compounds are also effective anti-parasitic drugs. Nifurtimox, for example, is used to treat Chagas disease (caused by Trypansoma cruzi) but has side effects. In exploring the use of alternatives to nifurtimox, Olea-Azar et al. have examined radical formation from two analogues. Radical anions were observed upon electrolytic reduction of the compounds and a nitroxide, believed to be the glutathionyl radical-adduct, was detected upon electrolysis in the presence of DMPO and GSH. Radical adducts were also detected upon incubation of one of the analogues with microsomes from T. Cruzi.m A novel endo-peroxide reductase has been isolated from T. Cruzi. Whereas the flavoenzyme was found to reduce quinones to their semiquinones, nifurtimox underwent a direct, two-electron reduction, without the formation of radicals.129... 

A new class of heteroaromatic compound was introduced by the synthesis of a diphosphathienoquinone (20). It can be reduced to a semiquinone radical anion and dianion at lower potentials than phosphaalkenes. The ESR spectrum indicates that the two P atoms are not equivalent213. 2,4>6-Tricyano- 1,3,5-triazine undergoes dimerization to yield 4,4, 6,6 -tetracyano-2,2 -bitriazine214. 

They were found not to react with BESOD, the rate constant was estimated to be < 10 M s , if there was a reaction at all The reaction of BESOD was also investigated with several other radicals generated by pulse radiolysis. With the semiquinone of riboflavin 5 -phosphate no reaction was detected. The semiquinone of 9,10-anthraquinone-2-sulfonate and the radical anion of 4-nitroacetophenone converted the enzyme into an unreactive form... 

The reduction of a quinone requires two electrons, and it is possible that these electrons could be transferred either together or one at a time. The product of a single-electron transfer leads to what appropriately is called a semiquinone, 1, with both a negative charge and an odd electron (a radical anion) ... 

The P870iBPh7 radical pair disappears in about 2 x 10-10 s when an electron jumps from BPh7 to one of the quinones (QA). This leaves the reaction center in the state P870tQA7, where QA is the anionic semiquinone (see fig. 15.12). 

The mechanism for the generation of quinone methide 58 by reductive elimination of 55 has been investigated.106 Single-electron reduction by 55 by pulse radiolysis in water gives the semiquinone radical anion 56, whose decay was monitored by transient absorption spectroscopy. This radical anion partitions between disproportionation to 60 and elimination to form the radical 58. Disproportionation dominates at pH 7, but as the pH is lowered to 3 the competing elimination reaction to form the quinone methide radical 58 is observed for X = -OMe and -OPh. It was proposed that the product yields are controlled by the position of the equilibrium for protonation of 56 and that 56 undergoes mainly disproportionation, while the semiquinone radical 57 - undergoes mainly elimination of HX (Scheme 28). The quinone methide 59 is then formed by the one-electron reduction of 58. 

In summary, these results constitute strong evidence for the two-step reaction sequence. They require that the deprotonation of the aminium radical cation be competitive on the CIDNP timescale i.e. surprisingly fast since it involves a carbon acid. The results delineate the fate of the amine derived intermediates with particular clarity, since they are observed directly for amine derived products. The conclusions based on the above CIDNP results were confirmed by time resolved optical spectroscopy in a variety of systems [179-182]. However, in essentially all these systems the reaction progress is monitored by following the complementary spectra of the acceptor derived radical intermediates, such as ketyl, semiquinone, stilbene, or thioindigo radical anions. 

Electron transfer between oxygen and semiquinone radical anions, duro-quinone and nitro-substituted radical anions, and two identical semiquinone radical anions H20 (22 -0)... 

Hydro quinone transforms in the presence of irradiated nitrite to yield ben-zoquinone and hydroxybenzoquinone [78,79]. At the irradiation wavelength adopted in the cited works (365 nm), hydroquinone direct photolysis should be limited and benzoquinone most likely forms upon reaction between hydroquinone and hydroxyl (reactions 44 and 45 hydroquinone absorbs radiation at A, < 320 nm). Hydroxybenzoquinone is likely to be a product of benzoquinone photolysis. No nitration or nitrosation intermediates of hydroquinone were observed in the presence of nitrite under irradiation, differently from the cases of resorcinol and catechol [78,79]. The reaction between hydroquinone and nitrogen dioxide is, however, quite rapid [106,115], as confirmed by the marked inhibition of phenol nitration upon nitrite photolysis by added hydroquinone [62], The point is that the reaction between hydroquinone and NO2 mainly yields benzoquinone [62], Another interesting feature in the case of hydroquinone is the formation of the fairly stable semiquinone radical anion upon reaction between benzoquinone and depro-tonated hydroquinone. The spectrum of the resulting solution shows the typical absorption bands of the semiquinone at 308, 315, 403, and 430 nm [79]. 

The definition also includes species in which a common radical function such as a ketyl, nitro anion-radical, or semiquinone is conjugated with a heteroaromatic system, e.g., 6-9. Certain major radical series are, however,... 

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) ... 

While the semiquinone radical anion was strongly polarized, the counter radical cation was never observed. Therefore, it remains a good probability that the primary process may be an abstraction... 

---