Quinone semiquinone radicals

Quinone Semiquinone radical formation/ oxidative stress/thiol trapping... 

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... 

Semiquinone radicals disproportionate with the formation of hydroquinone and quinone [2],... 

It is seen that the substitution of the part of amine or phenol by quinone prolongs the induction period by two or three times. The mechanism of synergistic action of quinone is the same as in the case of nitroxyl radicals. Quinone reacts with InH with production of semiquinone radicals. The latter rapidly reacts with peroxyl radicals and provokes the additional rapid chain termination [47],... 

The reactions of quinones with phenols and amines are endothermic due to low BDE of the formed Q—H bond. Hence, the synergism of quinones should be noticeable at elevated temperatures. The values of BDE in a few semiquinone radicals are given below ... 

Broad-band irradiation from a xenon arc lamp by Felix and Sealy produced semiquinone radicals from catecholamines. They were detected by ESR spectra of their metal complexes. Semiquinones gave rise to o-quinones and... 

Hydroquinone oxidation to quinone occurs via two linked one-electron transfer stages. The first step (typically metal catalysed) yields the semiquinone radical intermediate, which is resonance stabilised. The second step involves electron transfer to molecular oxygen to generate superoxide and quinone [70]. This reaction mechanism is common to all hydroquinones, catechols, resorcinols, and so forth. 

As you read through excerpt 5C, answer the following questions. To guide your reading, we include structures for quinone, the semiquinone radical, and hydroquinone ... 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

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. ... 

It is evident from Table 16.3 that quinones react rapidly with the hydroperoxyl radicals. Calculation shows that the change in the enthalpy of this reaction is relatively small. The dissociation energy of the —H bond in the semiquinone radical 4- 40 is 228.1 kJ mol 1 and the enthalpy of the reaction Q + H02 is A// 220.0 228.1 8.1 kJ mol. ... 

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... 

Hydroquinones, for example, can form semiquinone radicals and then quinones in a two-step oxidation reaction. The semiquinone radical is potentially reactive but so also is the quinone, which can easily form an electrophile. Furthermore, quinones can also take part in redox cycling by accepting electrons and so generate reactive oxygen species. 

The structure of doxorubicin includes a quinone moiety therefore, it can easily accept an electron and undergo redox cycling (Fig. 7.47). Because it accumulates in the mitochondria, it can accept electrons from the electron transport chain and divert them away from complex I. It becomes reduced to the semiquinone radical in the process. This will then reduce oxygen to superoxide and return to the quinone form (Fig. 7.47). This could lead to oxidation of GSH and mtDNA. The subsequent damage may lead to the opening of the mitochondrial permeability transition pore. Consequently, mitochondrial ATP production will be compromised, and ATP levels will decline. 

The pheophytin radical now passes its electron to a tightly bound molecule of quinone (Qa), converting it to a semiquinone radical, which immediately donates its... 

Quinones undergo much interesting photochemistry. Triplet an-thraquinone lies 62.4 kcal above the ground state and readily abstracts hydrogen atoms from alcoholic solvents to yield semiquinone radicals.443... 

The formation of relatively stable semiquinone radicals by electrolytic reduction of quinones has been established by a variety of methods. Some semiquinone radicals undergo reversible dimerization reactions to form peroxides. 

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. 

The photochemistry of the methoxyhydroquinone A / methoxyquinone D couple in solution was extensively studied in our laboratories [14]. It involves, as for the reactivity of other simple quinones [12, 15], the formation of a neutral semiquinone radical which is able, in the low viscosity medium, such as fluid... 

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