Quinones semiquinones from

Os(bpy)2LL]2+/1+/0 (L-L are quinone, semiquinone, or catecholato ligands derived from catechol, 3,5-di-ter<-butylcatechol, or tetrachlo-rocatechol) have been characterized by UV/Vis/NIR and EPR spectroscopies. These spectroscopic properties and the crystal structure of [0s(bpy)2(dbcat)]C104 confirm an Os( III (-catecholate ground state for the +1 ion. This contrasts with the ground state of the +1 ions of Ru analogs, which are best described as Ru(II)-semiquinone complexes... 

The equilibria governing semi-quinone formation from quinones are similar to those for the flavin semiquinones which were discussed in Section B,6. Two consecutive one-electron redox steps can be defined. Their redox potentials will vary with pH because of a pfCa for the semiquinone in the pH 4.5 -6.5 region. For ubiquinone this pKa is about 4.9 in water and 6.45 in methanol. A pKa of over 13 in the... 

Dopaminergic neurons contain neuromelanin, a pigment composed of lipofus-cin along with a complex mixture of polymers of the various catecholamines, metal ions, cysteine, and possibly other substances (A4). This waste pigment is presumed to be derived from the oxidation of dopamine and other catecholamines to produce quinones, semiquinones, and quinhydrones, some of which undergo... 

The results are consistent [148] with a mechanism involving one-electron transfer from reduced flavoprotein to quinone. The resulting semiquinones from naphthoquinone and menadione subsequently transfer an electron to oxygen benzosemi-quinone does not. A similar one-electron reduction mechanism apparently operates with cytochrome-hj reductase. 

Coenzyme Q is capable of accepting either one or two electrons to form either a semiquinone or hydroquinone form. Figure 11-4 shows the quinone, semiquinone, and hydroquinone forms of the coenzyme. Coenzyme Q is not bound to a protein instead it is a mobile electron carrier and can float within the inner membrane, where it can transfer electrons from Complex I and Complex II to Complex 111. 

Figure 6 1,2-Dioxolene redox system with catecholate, o-semiquinone, and o-quinone structures (from left to right).

Kalyanaraman B, Premovic PI, Sealy RC (1987) Semiquinone anion radicals from addition of amino acids, peptides, and proteins to quinones derived from oxidation of catechols and catecholamines. An ESR spin stabilization smdy. J Biol Chem 262 11080-11087 Haslam E (1979) Vegetable tannins. Recent Adv Phytochem 12 475-523 Bear JE, Lilley TH, Haslam E (1985) Plant polyphenols - secondary metabolism and chemical defence some observations. Phytochemistry 24 33-38... 

After the second electron transfer from semiquinone to heme 6l (step 4), the interaction between the Rieske cluster and the resulting quinone is weakened so that the reduced Rieske protein can now occupy the preferred ci positional state (E), which allows rapid electron transfer from the Rieske cluster to heme Ci (step 5). 

Although reduction of chromate Cr to Cr has been observed in a number of bacteria, these are not necessarily associated with chromate resistance. For example, reduction of chromate has been observed with cytochrome Cj in Desulfovibrio vulgaris (Lovley and Phillips 1994), soluble chromate reductase has been purified from Pseudomonas putida (Park et al. 2000), and a membrane-bound reductase has been purified from Enterobacter cloacae (Wang et al. 1990). The flavoprotein reductases from Pseudomonas putida (ChrR) and Escherichia coli (YieF) have been purified and can reduce Cr(VI) to Cr(III) (Ackerley et al. 2004). Whereas ChrR generated a semi-quinone and reactive oxygen species, YieR yielded no semiquinone, and is apparently an obligate four-electron reductant. It could therefore present a suitable enzyme for bioremediation. 

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). 

Broad-band irradiation from a xenon arc lamp by Felix and Sealy produced semiquinone radicals from catecholamines. They were detected by ESR spectra of their metal complexes. Semiquinones gave rise to o-quinones and... 

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