Semiquinone structure

The isoalloxazine ring system in its various oxidation states and the reaction of FADH2 with GSSG are shown in equation 20 (the semiquinone structure of flavins is... 

The above described rationale of SAR can be exemplified by the bioactivities of the bromotyrosine derivatives, aeroplysinin-1 and dienone, as well as their complicated molecular substrates aerophobin-1 and isofistularin-3, isolated from the Mediterranean sponge, Aplysina aerophoba. Aeroplysinin-1 and dienone exhibit pronounced biological activity whereas aerophobin-1 and isofistularin-3 were always inactive when tested in equimolar concentrations [23]. Aeroplysinin-1 and dienone are antibiotically active against a broad spectrum of marine bacteria such as Vibrio, Micrococcus, and Alteromonas species [24] and are also cytotoxic to Ehrlich ascites tumour cells and HeLa tumour cells in the microculture tetrazolium (MTT) and clonogenic assays [25]. In these assays, it has been demonstrated that the free radical transformation of aeroplysinin-1 and dienone to its semiquinone structures are responsible for the cytotoxicity [25], Aeroplysinin-1 and dienone are the pharmacophores of aerophobin-1 and isofistularin-3. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Peroxides oxidize TPDD to Wurster s blue, a product with a semiquinone diimine structure [1]. Similarly Wurster s blue is also produced from TPDD by reaction with halogen-containing substances produced by the reaction of aromatic amines and triazines with chlorine gas. 

This organism is able to oxidize acetate to CO2 under anaerobic conditions in the presence of Fe(III). A study of the intermediate role of humic and fulvic acids used ESR to detect and quantify free radicals produced from oxidized humic acids by cells of G. metallireducens in the presence of acetate. There was a substantial increase in the radical concentration after incubation with the cells, and it was plausibly suggested that these were semiquinones produced from quinone entities in the humic and fulvic structures (Scott et al. 1998). 

Component B is a monomeric reductase with a molecular weight of 35,000 and contains per mol of enzyme, 1 mol of FMN, 2.1 mol of Fe, and 1.7 mol of labile sulfur. After reduction with NADH, the ESR spectrum showed signals that were attributed to a [2Fe-2S] structure and a flavo-semiquinone radical (Schweizer et al. 1987). The molecular and kinetic properties of the enzyme are broadly similar to the Class IB reductases of benzoate 1,2-dioxygenase and 4-methoxybenzoate monooxygenase-O-demethylase. 

The oxidative polymerization of 5,6-dihydroxyindole (1) and related tyrosine-derived metabolites is a central, most elusive process in the biosynthesis of eumelanins, which are the characteristic pigments responsible for the dark color of human skin, hair, and eyes. Despite the intense experimental research for more than a century,36 the eumelanin structure remains uncharacterized because of the lack of defined physicochemical properties and the low solubility, which often prevents successful investigations by modem spectroscopic techniques. The starting step of the oxidative process is a one-electron oxidation of 5,6-dihydroxyindole generating the semiquinone 1-SQ (Scheme 2.7). 

Most quinone reductions go through an intermediate radical or semiquinone stage, usually revealed by a one-electron step in the redox potential.100 The radical formed by the reduction of compound VI is especially stable, probably because of the additional involvement of the benzoyl group.101 The ordinary semiquinones are more stable in basic solution since some of the resonance structures of the neutral radical involve separation of charges. 

Radicals in which the odd electron is on a nitrogen next to an aromatic ring are stabilized by resonance analogous to that of tri-phenylmethyl. In the case of Wurster s salts, the nitrogen analogs of semiquinones, there are two equivalent resonance structures in the acid form, but in the less stable basic form one of the structures requires separation of charge. Evidence for the unpaired electron has been obtained by measurement of the paramagnetism.144... 

Indeed a limited number of monomeric [Mn(dioxolene)2] complexes have been structurally characterized (M = Ni dioxolene = 3, 6-di-terf-butyl-l,2-benzosemiquinone, n = 0 51a dioxolene = A AT -bis, 3-dihydroxybenzoyl)-l,7-diazaheptane, n = 2 51b dioxolene = 1,2-cate-cholate, n = 2 51c M = Pd, Pt dioxolene = 3,5-di-terf-butyl- 1,2-benzo-semiquinone, n = 052), likely due to the fact that Ni(II) in the presence of... 

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

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