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Query: UNIPROT:O14944 (EPR)
 13,097 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Benzoquinone (BQ), deuterobenzoquinone (d4-BQ), and hydroquinone (BQH2) are investigated in ethylene glycol by means of direct detection fast time-resolved EPR spectroscopy after laser flash photolysis. The development of the magnetization as a function of time and magnetic field is obtained and analyzed in terms of the Bloch equations and hyperfine parameters. The signals are attributed to the semiquinones BQH(*) and d4-BQH(*). The presence of 1,2-dihydroxyethyl radicals during the photolysis of BQ and d(4)-BQ is verified. No alkyl radicals are observed in solutions of BQ with excess BQH2. Detailed analysis of the chemically induced dynamic electron polarization spectra with respect to their development in time shows that polarization patterns of the semiquinones can be traced back to a superposition of triplet mechanism and radical pair mechanism, the latter arising from geminate T-pairs. Hence, two independent pathways for polarization are assumed: reaction of triplet benzoquinone with ethylene glycol leads to the semiquinone and dihydroxyethyl radicals with all signals in emission, whereas the reaction of triplet BQ and BQH2 yields two semiquinones exhibiting both net emissive and multiplet emissive/absorptive intensity distributions.
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PMID:A time-resolved EPR study of the electron-spin-polarization pathways of p-benzosemiquinone. 1133 Sep 79

The use of light-induced spin polarization to study the structure and function of type I reaction centres is reviewed. The absorption of light by these systems generates a series of sequential radical pairs, which exhibit spin polarization as a result of the correlation of the unpaired electron spins. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important structural results from such studies on photosystem I (PS I) are summarized. Quinone exchange experiments which demonstrate the influence of protein-cofactor interactions on the polarization patterns are discussed. The results show that there are significant differences between the binding sites of the primary quinone acceptors in PS I and purple bacterial reaction centres and suggest that pi-pi interactions probably play a more important role in PS I. Studies using spin-polarized EPR transients and spectra to investigate the electron transfer pathway and kinetics are also reviewed. The results from PS I, green-sulphur bacteria and Heliobacteria are compared and the controversy surrounding the role of a quinone in the electron transfer in the latter two systems is discussed.
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PMID:Light-induced spin polarization in type I photosynthetic reaction centres. 1168 16

Oxidation of the anticancer anthracyclines doxorubicin (DXR) and daunorubicin (DNR) by lactoperoxidase(LPO)/H(2)O(2) and horseradish peroxidase(HRP)/H(2)O(2) systems in the presence and absence of nitrite (NO(2)(-)) has been investigated using spectrophotometric and EPR techniques. We report that LPO/H(2)O(2)/NO(2)(-) causes rapid and irreversible loss of anthracyclines' absorption bands, suggesting oxidative degradation of their chromophores. Both the initial rate and the extent of oxidation are dependent on both NO(2)(-) concentration and pH. The initial rate decreases when the pH is changed from 7 to 5, and the reaction virtually stops at pH 5. Oxidation of a model hydroquinone compound, 2,5-di-tert-butylhydroquinone, by LPO/H(2)O(2) is also dependent on NO(2)(-); however, in contrast to DNR and DXR, this oxidation is most efficient at pH 5, indicating that LPO/H(2)O(2)/NO(2)(-) is capable of efficiently oxidizing simple hydroquinones even in the neutral form. Oxidation of anthracyclines by HRP/H(2)O(2)/NO(2)(-) is substantially less efficient relative to that by LPO/H(2)O(2)/NO(2)(-) at either pH 5 or pH 7, most likely due to the lower rate of NO(2)(-) metabolism by HRP/H(2)O(2). EPR measurements show that interaction of anthracyclines and 2,5-di-tert-butylhydroquinone with LPO/H(2)O(2)/NO(2)(-) generates the corresponding semiquinone radicals presumably via one-electron oxidation of their hydroquinone moieties. The possible role of the (*)NO(2) radical, a putative LPO metabolite of NO(2)(-), in oxidation of these compounds is discussed. Because in vivo the anthracyclines may co-localize with peroxidases, H(2)O(2), and NO(2)(-) in tissues, their oxidation via the proposed mechanism is likely. These observations reveal a novel, peroxidase- and nitrite-dependent mechanism for the oxidative transformation of the anticancer anthracyclines, which may be pertinent to their biological activities in vivo.
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PMID:Peroxidase- and nitrite-dependent metabolism of the anthracycline anticancer agents daunorubicin and doxorubicin. 1173 18

During turnover of cytochrome bo(3) from Escherichia coli, a semiquinone radical is stabilized in a high-affinity binding site. To identify binding partners of this radical, site-directed mutants have been designed on the basis of a recently modeled quinone binding site (Abramson et al., 2000). The R71H, H98F, D75H, and I102W mutant enzymes were found to show very little or no quinol oxidase activity. The thermodynamic and EPR spectroscopic properties of semiquinone radicals in these mutants were characterized. For the H98F and the R71H mutants, no EPR signal of the semiquinone radical was observed in the redox potential range from -100 to 250 mV. During potentiometric titration of the D75H mutant enzyme, a semiquinone signal was detected in the same potential range as that of the wild-type enzyme. However, the EPR spectrum of the D75H mutant lacks the characteristic hyperfine structure of the semiquinone radical signal observed in the wild-type oxidase, indicating that D75 or the introduced His, interacts with the semiquinone radical. For the I102W mutant, a free radical signal was observed with a redox midpoint potential downshifted by about 200 mV. On the basis of these observations, it is suggested that R71, D75, and H98 residues are involved in the stabilization of the semiquinone state in the high-affinity binding site. Details of the possible binding motif and mechanistic implications are discussed.
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PMID:Identification of the residues involved in stabilization of the semiquinone radical in the high-affinity ubiquinone binding site in cytochrome bo(3) from Escherichia coli by site-directed mutagenesis and EPR spectroscopy. 1218 53

Helianthrones 2-4 are a new class of synthetic photosensitizers, which have a molecular skeleton related to that of hypericin. We established that irradiation of heliantrones with visible light leads to the formation of semiquinone radicals and reactive oxygen species. The structures of the paramagnetic anion species produced by electron transfer were calculated on the density functional level and investigated by cyclovoltammetry, UV/vis, and EPR/ENDOR spectroscopy. As with hypericin, the pi system of the helianthrones was found to be considerably deviated from planarity, and, upon electron transfer, deprotonation in the bay region occurs. The structure of the semiquinone radicals was found to be identical in THF, DMF, and aqueous buffered solutions regardless of the means by which reduction was achieved. Semiquinone radicals can be formed via self-electron transfer between the excited state and the ground state or via electron transfer from an electron donor to the excited state of helianthrone. Therefore, the presence of an electron donor significantly enhanced the photogeneration of semiquinone and superoxide radical. The kinetic studies showed that no significant photochemical destruction of helianthrones occurred upon irradiation. Generation of superoxide and singlet oxygen upon irradiation of helianthrones was established by spin trapping techniques. This shows that both type I and type II mechanisms are of importance for the photodynamic action of these compounds.
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PMID:Chemical and photochemical electron transfer of new helianthrone derivatives: aspects of their photodynamic activity. 1255 41

The membrane-bound pyrroloquinoline quinone (PQQ)-containing quinoprotein glucose dehydrogenase (mGDH) in Escherichia coli functions by catalyzing glucose oxidation in the periplasm and by transferring electrons directly to ubiquinone (UQ) in the respiratory chain. To clarify the intramolecular electron transfer of mGDH, quantitation and identification of UQ were performed, indicating that purified mGDH contains a tightly bound UQ(8) in its molecule. A significant increase in the EPR signal was observed following glucose addition in mGDH reconstituted with PQQ and Mg(2+), suggesting that bound UQ(8) accepts a single electron from PQQH(2) to generate semiquinone radicals. No such increase in the EPR signal was observed in UQ(8)-free mGDH under the same conditions. Moreover, a UQ(2) reductase assay with a UQ-related inhibitor (C49) revealed different inhibition kinetics between the wild-type mGDH and UQ(8)-free mGDH. From these findings, we propose that the native mGDH bears two ubiquinone-binding sites, one (Q(I)) for bound UQ(8) in its molecule and the other (Q(II)) for UQ(8) in the ubiquinone pool, and that the bound UQ(8) in the Q(I) site acts as a single electron mediator in the intramolecular electron transfer in mGDH.
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PMID:Occurrence of a bound ubiquinone and its function in Escherichia coli membrane-bound quinoprotein glucose dehydrogenase. 1461 41

Dietary phenolic antioxidants have been shown to prevent LDL modifications mediated by several physiologic oxidants including peroxynitrite. However, more recent data demonstrated that CO(2) affected the fate of peroxynitrite in biological fluids and significantly reduced peroxynitrite scavenging by polyphenols, raising doubts concerning their antioxidant activity. We found that the oxidation of LDL lipids mediated by peroxynitrite decreased in the presence of bicarbonate, while Trp oxidation and 3-nitroTyr formation increased, suggesting a redirection of peroxynitrite reactivity toward the protein moiety. We therefore evaluated the protective activity of some phenolic antioxidants (quercetin, oleuropein, resveratrol, (+)-catechin, (-)-epicatechin, tyrosol, alpha- and gamma-tocopherol, ascorbate) on peroxynitrite-mediated oxidation of LDL aromatic residues. Some of these phenols protected LDL Trp from oxidation better than ascorbate or alpha-tocopherol, although protection at 100 microM did not exceed 30-40%. However, the same phenolic antioxidants were more active in inhibiting 3-nitroTyr formation and those with a catechin structure provided significant protection (IC(50%) 40-50 microM). Red wine, a polyphenol-rich beverage, showed a protective effect comparable to that of the most active phenolic antioxidants. Direct EPR studies showed that bicarbonate significantly increased the peroxynitrite-dependent formation of O-semiquinone radicals in red wine, supporting the hypothesis that polyphenols are efficient scavengers of radicals formed by peroxynitrite/CO(2). Ascorbate was a poor inhibitor of peroxynitrite/CO(2)-induced LDL tyrosine nitration, but the simultaneous addition to the most active polyphenols halved their IC(50%). In conclusion, although cooperation with other antioxidants can further decrease the IC(50%) of polyphenolics, as demonstrated for ascorbate, their antioxidant activity appears to occur at concentrations at least 1 order of magnitude higher than their bioavailability.
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PMID:Phenolic antioxidants and the protection of low density lipoprotein from peroxynitrite-mediated oxidations at physiologic CO2. 1513 27

Plants synthesize a diversity of tannin structures but little is known about whether these different types have different oxidative activities in herbivores. Oxidative activities of hydrolyzable and condensed tannins were compared at pH 10 with two methods: EPR spectrometry was used to quantify semiquinone radicals in anoxic conditions and a spectrophotometric assay was used to measure the rate of browning of phenolics oxidized in ambient oxygen conditions. A little-studied group of hydrolyzable tannins (ellagitannins) contained the most active tannins examined, forming high concentrations of semiquinone radicals and browning at the highest rates. On average, galloyl glucoses and high-molecular-weight gallotannins had intermediate to low oxidative activities. Condensed tannins generally formed low levels of semiquinone radicals and browned most slowly. The results suggest that ellagitannin-rich plants have active oxidative defenses against herbivores, such as caterpillars, whereas the opposite may hold true for plants that contain predominantly condensed tannins or high-molecular-weight gallotannins.
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PMID:Ellagitannins have greater oxidative activities than condensed tannins and galloyl glucoses at high pH: potential impact on caterpillars. 1701 21

A novel cyclolignanic quinone, 7-acetyl-3',4'-didemethoxy-3',4'-dioxopodophyllotoxin (CLQ), inhibits topoisomerase II (TOPO II) activity. The extent of this inhibition was greater than that produced by the etoposide quinone (EQ) or etoposide. Glutathione (GSH) reduces EQ and CLQ to their corresponding semiquinones under anaerobic conditions. The latter were detected by EPR spectroscopy in the presence of MgCl(2) but not in its absence. Semiquinone EPR spectra change with quinone/GSH mol ratio, suggesting covalent binding of GSH to the quinones. Quinone-GSH covalent adducts were isolated and identified by ESI-MS. These orthoquinones also react with nucleophilic groups from BSA to bind covalently under anaerobic conditions. BSA thiol consumption and covalent binding by these quinones are enhanced by MgCl(2). Complex formation between the parent quinones and Mg(+2) was also observed. Density functional calculations predict the observed blue-shifts in the absorption spectra peaks and large decreases in the partial negative charge of electrophilic carbons at the quinone ring when the quinones are complexed to Mg(+2). These observations suggest a possible role of Mg(+2) chelation by these quinones in increasing TOPO II thiol and/or amino/imino reactivity with these orthoquinones.
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PMID:Thiols oxidation and covalent binding of BSA by cyclolignanic quinones are enhanced by the magnesium cation. 1832 25

Polychlorinated biphenyls (PCBs) comprise a group of persistent organic pollutants that differ significantly in their physicochemical properties, their persistence, and their biological activities. They can be metabolized via hydroxylated and dihydroxylated metabolites to PCB quinone intermediates. We have recently demonstrated that both dihydroxy PCBs and PCB quinones can form semiquinone radicals (SQ(*-)) in vitro. These semiquinone radicals are reactive intermediates that have been implicated in the toxicity of lower chlorinated PCB congeners. Here we describe the synthesis of selected PCB metabolites with differing degrees of chlorination on the oxygenated phenyl ring, e.g., 4,4'-dichloro-biphenyl-2,5-diol, 3,6,4'-trichloro-biphenyl-2,5-diol, 3,4,6,-trichloro-biphenyl-2,5-diol, and their corresponding quinones. In addition, two chlorinated o-hydroquinones were prepared, 6-chloro-biphenyl-3,4-diol and 6,4'-dichloro-biphenyl-3,4-diol. These PCB (hydro-)quinones readily react with oxygen or via comproportionation to yield the corresponding semiquinone free radicals, as detected by electron paramagnetic resonance spectroscopy (EPR alias ESR). The greater the number of chlorines on the (hydro-)quinone (oxygenated) ring, the higher the steady-state level of the resulting semiquinone radical at near neutral pH.
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PMID:Chlorination increases the persistence of semiquinone free radicals derived from polychlorinated biphenyl hydroquinones and quinones. 1883 91


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