Semiquinone-type free radical

With tyrosinase, on the contrary, a two-electron oxidation occurs, as no EPR signal was detected in the catechol oxidation at pH 5.3 Melanins are polymerization products of tyrosine, whereby tyrosinase catalyses the first steps the formation of dopa (3,4-dihydroxyphenylalanine) and of dopaquinone, leading to an indolequi-none polymer The peroxidase mechanism for the conversion of tyrosine into dopa in melanogenesis was not substantiated In natural and synthetic melanins free radicals of a semiquinone type were detected by EPR 4-10 x 10 spins g of a hydrated suspension (the material was modified on drying and the number of free spins increased). The fairly symmetrical EPR signal had a g-value of 2.004 and a line-width of 4-10 G The melanins seem to be natural radical scavengers. 

Chloroplast lamellae contain different types of quinone. A quinone becomes a semiquinone when one hydrogen atom (H+ + e ) is added (a semiquinone is a free radical because of the presence of an unpaired electron) and a hydroquinone when two are added (a hydroquinone is also called a hydroquinol, or simply a quinol) ... 

The free radicals in humic substances appear to be remarkably stable with respect to time and chemical attack (Steelink and Tollin, 1962 Steelink et al., 1963 Steelink, 1964). This stability has led some workers to suggest that humic substances comprise a free radical, or mixture of free radicals, of the semiquinone type (Steelink and Tollin, 1962 Steelink et al., 1963 Atherton et al., 1967). The increase in spin concentration upon converting humic acids to the sodium salts led Steelink (1964) to propose that the radical species in humic acid is a semiquinone co-existent with a quinhydrone species. Steelink (1964) also suggests that a free radical is an integral part of the humic macromolecule. The stability of this radical has been attributed to the delocalization of the unpaired electron over an aromatic system (Theng and Posner, 1967), or a shielding effect on the free radical due to the macromo-lecular network (Steelink, 1964). 

The populations and role of semiquinone states assumed to be responsible for the characteristic ESR signal have been extensively studied by ESR spectrometry for all types of melanins (75). The increase in the free radical content after reduction of melanins in nonaqueous media may indicate an increased population of semiquinones (153) and/or quin-hydrone-type complexes. In such a case a maximum intensity signal should be observed with half-oxidized-half-reduced melanin. 

In 2007, Dellinger et al. (27A27) conducted and reported on the formation and stability of resonance stabilized free radicals of the type hypothesized by Pryor and his associates in the particulate phase of MSS. They concluded that the commonly observed free radicals in the particulate phase of MSS were not a surface associated semiquinone and were more likely an intrinsic, polymeric radical with a delocalized electron. The EPR signal observed by Pryor in the alcohol extract of the particulate phase of MSS may be from an extracted and autooxidized hydroquinone, not a particulate-phase-associated semiquinone radical. The semiquinone radical was observed in the particulate-phase MSS collected below 400°C and has a five-line spectrum with g 2.006. Semiquinone radicals were formed in the particulate phase of MSS only after aging. 

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. ... 

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. 

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