O-semiquinones

Transition metal complexes of o-benzoquinone, o-semiquinone and catecholate ligands. C. G. Pier-pontand R. M. Buchanan, Coord. Chem. Rev., 1981, 38, 45-87 (72). 

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. 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

In the reaction of Mes2Ge = NS02PhMe 137 with 3,5-di-tert-butyl-o-benzoquinone, a single electron transfer process was evidenced by an ESR study, as well as by the characterization of isobutene. The initially formed o-semiquinonic species could afford the dimesitylgermadioxolane 148 according to two different pathways590 (Scheme 27). 

K3[V(catecholate)3]-1.5H20 has been synthesized from VC13 and catechol in ammonia solution, on adding a solution of KOH. The vanadium is surrounded by six oxygen atoms of catecholate ions the average V—O bond length is 2.013(9) A.256 A tris(o-semiquinone)vanadium(III) complex257 from V(CO)6 and quinone was better formulated as bis(catecholate)(o-semiquinone)vanadium(V).256... 

In relation to elemental metals, o-quinones are extremely reactive ligands [11,12,14,133,192-208], especially those containing t-butyl substituents. Complexes of o-benzoquinones, o-semiquinones, and catechols of this type have been obtained by direct interaction between metal powders and the corresponding ligands and reviewed [14]. It was established that thermal decomposition of such complexes of copper in solution leads to the formation of metallic copper and the initial o-quinone. 

Stable paramagnetic binuclear complexes [Ru(bipy)2]2(p-L) 3+ with L being N,0 N, 0 -coordinating 4,7-phenantroline-5,6-semidione and P,0 P, 0 -coordi-nating 2,5-6z,s(diphenylphosphino)-/ -benzosemiquinone were reported [232], o-Semiquinone phdo - complex is formed as follows (5.41) ... 

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.

FIGURE 9. Marcus analysis of electron transfer reactions between MADH and amicyanin. Values of kn Twere determined for the reactions of different redox forms of MADH with amicyanin shown in Figure 8 0-quinol (A), O-semiquinone ( ), N-semiquinone ( ). Rate constants were also obtained for the reverse reactions of the O-quinol (A) and O-semiquinone ( ). The solid line represent fits to Eqs. 4 and 5, which are superimposible. 

Paramagnetic coordination compounds where the spin resides predominantly on one or more of the organic (or inorganic) ligands are quite common, especially complexes of radical anions [17] such as o-semiquinones 1 or other negatively charged chelates such as tetrapyrrole macrocycles 2 [18],... 

In special circumstances the metal and a ligand can compete for the spin in a paramagnetic complex. As this alternative involves oxidation-state changes, it is referred to as valence tautomerization or redox isomerization [78]. Such behavior is observed for o-semiquinone complexes of cobalt and manganese [78] recently, a copper(I)-semiquinone-copper(II)-catecholate equilibrium system (7) of biochemical interest has been analyzed by temperature-dependent ESR [79]. 

The electron-transfer reactions which form 212 when the corresponding heterocycle is oxidized by various metal complexes in perchloric acid have been studied by a stopped-flow technique and the results interpreted in terms of Marcus theory. The reaction forming the corresponding o-semiquinone was also studied. 

The Raman spectra of [Mnn(SQ)3]2, where SQ = o-semiquinone of L-dopa or dopamine, show that the ligands are present in o-semiquinone radical anion form.396... 

Absorption spectra of phenoxyl radicals derived from biologically important molecules were recorded in numerous cases. The tyrosyl radical was studied by many investigators and its spectrum was used to detect tyrosine oxidation in a protein and to follow intramolecular electron transfer from tyrosine to the tryptophan radical in dipeptides and polypeptides . A number of catecholamines, such as adrenaline and dopa, were also studied by kinetic spectrophotometric pulse radiolysis " ". The absorption spectra of most of these substituted o-semiquinone anion were similar to those of the unsubstituted... 

Catecholamines (-i-traces of transition metals) o-Semiquinones/Quinones-i-Oy... 

The free radicals found in dopa polymers are felt to be o-semiquinones (11), whereas those in cysteinyldopa polymers seem to be o-semiquinonimines (12) [164]. Character-... 

Paramagnetic complexes of semiquinones with Tl" (81), have been prepared by the reaction of Et2TlCl with the sodium salt of the o-semiquinones. Their e.s.r. spectra were reported, and hyperfine coupling constants determined. ... 

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