Semi-oxidised quinones

Mukheijee et al [72] have shown that the decay of the one-electron oxidized naphthazarin (NzH and Nz ) proceeds by a complicated mechanism not fully understood in spite of extensive studies. At low dose condition, there was no visible change in the concentration of naphthazarin. The pK of the semi-oxidised naphthazarin was shown as < 4. The most interesting observation was the evidence for the disproportionation (40) proceeding to an equilibrium level in favour of the semi-oxidised quinone, accompanied by reaction (41). The possibility of dimerisation of the one-electron oxidised species was not fully explored. 

Contrary to the naphthazarin case, dihydroxyanthraquinones (QH2) form semi-oxidised quinones which undergo simple bimolecular disproportionation to the diquinone and parent quinone [74]. Both the above reactions have been wrongly given due to printers devil type error m a recent review [12]. The pK for the semi-oxidised anthraquinone derivatives was measured and shown to be around 8. 

Stopped-flow kinetic studies of Amadoriase I using fructosylpropylamine and oxygen as substrates in 10 mM Tris hydrochloride buffer (pH 7.9) at 4 °C pointed to the pyranose form as being the active configuration. The redox potentials were found to be + 48 and -52 mV for the oxidised enzyme/anionic quinone and anionic semi-quinone/reduced enzyme reactions, respectively, at pH 7.0 and 25 °C.620... 

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. 

Although at pH 8 the electron distribution favours the formation of flavin semiquinone and reduced iron-sulfur center, the magnetic moments of the two redox centers do not interact. At pH 10, however, 2-electron-reduced TMADH exhibits the EPR spectrum diagnostic of the spin-mteracting state. In a more detailed analysis using the pH-jump technique, the interconversion of three states of TMADH [state 1, dihy-droflavin-oxidised 4Fe-4S center (formed at pH 6) state 2, flavin semi-quinone-reduced 4Fe-4S center (formed at pH 8) state 3, spin interacting state (formed at pH 10)] were studied in both H2O and D2O (Rohlfs et al., 1995). The kinetics were found to be consistent with a reaction mechanism that involves sequential protonation/deprotonation and electron transfer events (Figure 6). Normal solvent kinetic isotope effects were observed and proton inventory analysis revealed that at least one proton is involved in the reaction between pH 6 and 8 and at least two protons are involved between pH 8 and 10. At least three protonation/... 

On irradiation with ultraviolet light, tyrosine is readily converted to DOPA, which is then oxidised further, probably to dopachrome and then to melanin [29, 85-88]. Synephrine (41) behaves similarly, being oxidised first to adrenaline and then probably to adrenochrome which rearranges to adrenolutin [89]. These oxidations probably involved the initial formation of the semi-quinone followed by oxidation to the open-chain quinone [90]. Ultraviolet irradiation was also found to increase the rate of oxidation of tyrosine by tyrosinase in rat skin. OrrAo-quinones were produced in the reaction and it was concluded that the acceleration was due to the formation of low levels of these compounds from tyrosine [91]. 

The antioxidant activity of phenol is also increased by the presence of additional hydroxyl group in the ortho or para positions. An example of such an antioxidant is TBHQ. The effectiveness of 1,2-dihydroxybenzene derivatives is attributed to a phenoxyl radical stabilised by an intramolecular hydrogen bond (11-8). The activity of 2-methoxyphenol is lower, because the generated radical cannot be stabihsed by a hydrogen bond. The antioxidant activity of 1,2-and 1,4-dihydroxybenzene is partly caused by the fact that the semi-quinone radical can be further oxidised to the corresponding o-quinone orp-quinone, respectively, by reaction with another lipid radical (Figure 11.7) or may disproportionate to the corresponding quinone and hydroquinone. 

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