Semiquinone radicals

Figure 3.2 Structures of (a) orellanine, (b) orellinine, (c) orilline, and (d) the radical semiquinone of orellanine, suggested as the toxic species.

Both the presence of methyl substituents in the tocopherols and their chromanol structures increase the ability of these compounds to form relatively stable radicals.498 499 This ability is doubtless probably important also in the function of ubiquinones and plastoquinones. Ubiquinone radicals (semiquinones) are probably intermediates in mitochondrial electron transport (Chapter 18) and radicals amounting to as much as 40% of the total ubiquinone in the NADH-ubiquinone reductase of heart mito-... 

Figure 9-6. A sequential mechanism for the oxidation of benzene- 1,2-diols to benzo-l,2-quinones by dioxygen and copper salts. Note the involvement of the radical semiquinone forms.

Mollisol spectra which indicate presence of stabilized free radicals (semiquinones) and resemble those of natural HAs measurement of 02 uptake 1983a)... 

It was noted [229] that magnetic exchange between radical semiquinone ligands results in the near diamagnetism of M(DBSQ)2 (M = Pd, Pt). 

The isoalloxazine nucleus of the flavins [3-(R or H)-7,8-dimethyl-lO-R -isoalloxazines] may exist in the fully reduced (1,5-di-hydro-), the radical (semiquinone), and the fully oxidized (quinone) states. Because of acid-base equilibria, each of these oxidation states... 

The ability to form a stable one-electron-reduced radical (semiquinone) allows flavin cofactors to sit at the crossroads of two-electron and one-electron transfer chains. That is, they can be reduced by organic substrates two electrons at a time and be reoxidized by either obligate one-electron acceptors such as cytochromes (e.g., yeast cytochrome b2 or cytochrome b5 reductase/cytochrome b5) and iron-sulfur cluster proteins (adrenodoxin reductase/adrenodoxin) or by facultative one-electron acceptors such as benzoquinones (coenzyme... 

This has been one of the most controversial areas of bioenergetics and is concerned with the role of coenzyme Q. The simplest view of the role of this coenzyme is that it acts as a mobile (2H+ + 2e ) carrier, linking complexes I and II with complex III. However, coenzyme Q may be involved in (H+ + e ) transfer within complex III. One model for this is the proton-motive Q cycle (Fig. 14-6), developed by Mitchell in 1975. This model satisfies prediction (2) of Example 14.10, in that coenzyme Q acts as an (H+ +e ) carrier in two loops. In this model, reduced coenzyme Q (QH2) is linked to oxidized coenzyme Q (Q) via the free-radical semiquinone (QH-) This model provides an explanation for the H+/e stoichiometry. 

Like the free radicals semiquinone is again paramagnetic because of the presence of an unpaired electron. 

The species one has to deal with in the oxidation of phenothiazine are the radical forms (free radicals, semiquinones, ion radicals), the oxygenated and nonoxygenated phenazathionium cations, and the nonionic oxygenated forms, like the 5-oxide, 3-hydroxyphenothiazine, and phenothiazone. These species are interrelated by redox, proto-l3rtic, and hydrolytic processes, as shown in Scheme 1. 

A cellulose graft copolymer containing 55% PAN was shown to contain 5.4 grafted chains per 1000 cellulose units, every other chain terminating in a quinone fragment as a result of the interaction between a macroradical and a semiquinone radical. Semiquinone also decreases the initiation efficiency by reaction with cellulose macroradicals grafting of every PAN chain is accompanied by the decomposition of about 5 cellulose macroradicals. 

Anthracyclines have the ability inherent in their qui-none structure to form free-radical semiquinones which result in very reactive oxygen species, causing peroxidation of the lipid membranes of the heart. However, this reaction has not been demonstrated with mitoxan-trone, and the mechanism of its cardiotoxicity is unknown. 

Free-radical semiquinone (QH ) intermediates have been detected by EPR spectroscopy in some electron transfers. Coenzyme Q, also called ubiquinone because it occurs in virtually all cells, contains a long isoprenoid tail that enables it to diffuse through membranes rapidly. This quinone derivative, which occurs in both free and protein-bound forms, is called ubiquinol when reduced (Figure... 

FIGURE 8.20 Coenzyme Q can exist in three oxidation states the fully reduced ubiquinol form (C0QH2), the radical semiquinone intermediate (CoQH ), and the fully oxidized ubiquinone form (CoQ). 

Q-cyde a cycle devised by P. Mitchell [FEBS Lett. 56 (1975) 1-6 S9 (1975) 137-139] to overcome the requirement of the redox loop mechanism (see Che-miosmotic hypothesis) for a H electron carrier in the cytochrome hq-containing Complex III of the mitochondrial electron transport chain, the Q.c. proposed that ubiquinone (coenzyme Q), the only mobile, hydrophobic redox component of the chain, participates in electron transfer from cytochrome b to cytochrome c, within Complex III by one-electron steps involving the fully reduced quinol-form (QH2), a stabilized free-radical semiquinone-form (QH ) and the fully oxidized quinone-form (Q). It also made use of the observation that cytochrome b appears to be a dimer composed of b- (b and b (b, which is buried deeply in the membrane with probably on the cytosolic side and by. on the matrix side. In the Hg., outlining the proposed mechanism, it can be seen that two protons are pumped across the membrane (steps 1 9 for uptake from the matrix and steps 3... 

Scheme 3, we postulate that the reactivity with dioxygen is via interaction with the low concentration of radical semiquinone within the polymer. Most significant is the observed increase in hydroxyl radicals generated firom melanin redox-cycling after treatment with divalent Cu or Zn salts. 

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