Semi-quinone intermediate

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

Nitrite (N02 ) catalyses mono-etherification of 1,4-hydroquinone (147) by methanol, via oxidation to the semi-quinone intermediate. " The reaction has been extended to other alcohols, and for substituted hydroquinones, reaction occurs exclusively at the less hindered phenol. 

Figure 11-2. Oxidoreduction of isoalloxazine ring in fiavin nucieotides via a semi-quinone (free radicai) intermediate (center).

Copper-containing amine oxidases (non-blue copper proteins) catalyze the oxidative deamination of primary amines to the corresponding aldehydes with the release of ammonia and concomitant reduction of oxygen to hydrogen peroxide. They typically use a quinone redox cofactor [topaquinone (TPQ)], which is bound covalently in the active site, and are thought to form a Cu(I)-TPQ semi-quinone radical intermediate during the redox reaction [13]. 

In summary, it would appear that the oxidation of a catecholamine probably first involves the formation of a semi-quinone radical (this can be brought about by an one-electron transfer, e.g. from Cu++ ions,14 or by photoactivation 1) which rapidly undergoes further oxidation (e.g. with atmospheric oxygen) to an intermediate open-chain quinone (such as adrenaline-quinone) and then cyclizes by an oxidative nucleophilic intramolecular substitution to the amino-chrome molecule. Whilst the initial formation of a leucoaminochrome by non-oxidative cyclization of the intermediate open-chain quinone in some cases cannot be entirely excluded at the moment (cf. Raper s original scheme for aminochrome formation72), the... 

Recent work on the stepwise reduction of phenothiazine dyes brought direct evidence for the existence of the intermediate semi-quinone stage, first considered by Michaelis in his work on the... 

FIGURE 4. Spectral forms of intermediates in the reductive half-reaction of TMADH. Panel A, photodiode array analysis of the reaction of TMADH with TMA. Panel B, Denconvoluted spectra for the intermediates of the reductive half-reaction. Spectrum A, oxidised enzymes spectrum B, reduced enzyme (dihydroflavin) spectrum C, reduced enzyme (flavin semi-quinone/reduced iron-sulfur centre) spectrum D, spin-interacting state. 

Despite lack of sequence homology, the function of the quinone reduction site (Qi site) is similar to that of the secondary quinone-binding site (Qb site) of bacterial reaction centers. Both sites have a conserved histidine residue as quinone ligand and both quinone molecules are reduced to hydroquinone in two consecutive one-electron transfer steps. The midpoint potential for the first step is pH-independent at near neutrality, whereas that for the second reduction varies by 120mV per pH unit (Robertson et al., 1984). This suggests a reaction pathway Q —> Q" QH2, with both protons added concomitantly with the second electron. A stable semi-quinone anion intermediate can be detected by EPR spectroscopy of samples frozen during turnover (Yu et al., 1980 de Vries et al., 1980) or with the redox potential adjusted near the midpoint of ubiquinone (Robertson et al., 1984 Ohnishi and Trumpower, 1980). The semiquinone signal is not observed in the presence of antimycin, which is consistent with the proposal that antimycin inhibits the reaction at the site (Mitchell, 1976 Mitchell, 1975). 

These experiments suggest that the secondary complexes should no longer be regarded as Michaelis-Menten enzyme substrate complexes but as reaction intermediates in the same sense that free radicals and semi-quinones are reaction intermediates, for all three classes of compounds provide a path for stepwise reactions. As a consequence the accepted mechanism for peroxidase action needs revision. 

A final remark concerns the application of poly (acetylene) in electrical storage batteries. It is now quite clear, that poly(acetylene) is an example of yet another redox-polymer - a term first used by H. G. Cassidy many years ago - to describe a class of polymers which can be reversibly oxidized and reduced and may thus be used as exchange resins etc. The prototype polymer of this class is poly(vinylhydroquinone) which can be reversibly oxidized to poly(vinylquinone) the semi-quinone radical being an intermediate. 

As the diphenol antioxidant, TBHQ reacts with hydroperoxyl radicals to form semi-quinone radicals stabilised by resonance. These intermediates yield dimers and the original hydroquinone regenerates by dismutation. The intermediates can also react with other lipid radicals. The reaction is shown in Figure 11.11. AH degradation products of TBHQ exhibit antioxidant activity, 2,3-dihydro-2,2-dimethylbenzofuran-5-ol (11-19) and 2-(2-hydr-oxy-2-methyl-l-propyl)hydroquinone (11-20) have even higher antioxidant activity than TBHQ. 

In his investigations of the mechanism of action of the flavoproteins, Beinert 24) studied the spectral changes induced by enzjrmic or chemical reduction and oxidation of FMN and FAD. Evidence has been obtained for two intermediates, a semi-quinoid free radical, monomeric form, and a dimeric (possibly quinhydrone-hke) form of the free radical, which is formed from the monomer in a relatively slow reaction. The semi-quinones are formed rapidly enough to permit them to function kinetically as intermediates. A reaction scheme has been proposed 24). 

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