Formation Of Semiquinone Radicals Research Articles

Electrochemistry and in-situ spectroelectrochemistry properties of N-, S-substituted-1,4-naphthoquinone compounds (NQ’s)

Quinones are ubiquitous in nature and form one of the largest classes of antitumor agents approved for clinical use. Naphthoquinones and their derivatives are important in many fields such as pharmaceutical, medical, and environmental applications. It is also known that such compounds show strong biological activity against various bacteria, viruses, fungi, and cancer cells. They are known to be efficient in inhibiting cancer cell growth. Under physiological conditions, they can undergo non-enzymatic one-electron reduction to give the moderately toxic species of semiquinone radical-anion. Thus, the electrochemical study of quinones might provide a basic knowledge of semi-quinone radical formation in both in vivo and in vitro under different media.In this study, firstly, novel N-, S- and/or halo(Br/Cl)-substituted-1,4-naphthoquinone derivatives (2a-d, 3e, 4, 6a-f, 7, 8 and 9) were synthesized. The structures of all compounds were elucidated by using spectroscopic methods [FT-IR, NMR (1H/13C), MS, and microanalysis]. Electrochemical characterization indicated that all compounds underwent two well-resolved reduction reactions assigned to NQ/NQ.− and NQ.−/NQ2− processes respectively. Additionally, an all-defined irreversible oxidation wave was also recorded for all compounds. The substituents on the NQ ring altered the peak position and reversibility of the electron transfer reactions. Redox processes considerably influenced the spectra of the compounds which showed different colors and spectra for different electrogenerated anionic and cationic forms of the compounds.

Sulfurization alters phenol-formaldehyde resin microplastics redox property and their efficiency in mediating arsenite oxidation

Microplastics weathering by various types of oxidants in the oxic environment and their interaction with environmental contaminants have drawn numerous scientific attention. However, the environmental fate of microplastics under a reducing environment has been largely unresolved. Herein, the change of physicochemical and redox properties of microplastics during the weathering under a sulfate-reducing environment and the interaction with arsenite were addressed. The sulfurization of phenol-formaldehyde resin microplastics under a sulfate-reducing environment generated smooth and porous particles with the induction of organic S species. Multiple spectroscopic results demonstrated thioether and thiophene groups formed by the substitute removal of O-containing functional groups. Moreover, the sulfurization process induced the reduction of carbonyl groups and oxidation of phenolic hydroxyl groups and resulted in the formation of semiquinone radicals. The O-containing functional groups contributed to microplastics redox property and As(III) oxidation while S-containing functional groups showed no obvious effect. The sulfurized microplastics had lower efficiency in mediating arsenite oxidation than the unsulfurized counterparts due to the decreased electron donating capacity. Producing hydrogen peroxides by electron-donating phenol groups and semiquinone radicals and the direct semiquinone radicals oxidation could mediate arsenite oxidation. The findings of this study help us understand the fate of microplastics in redox fluctuation interfaces.

Ascorbate-and iron-driven redox activity of Dp44mT and Emodin facilitates peroxidation of micelles and bicelles

BackgroundIron (Fe)-induced oxidative stress leads to reactive oxygen species that damage biomembranes, with this mechanism being involved in the activity of some anti-cancer chemotherapeutics. MethodsHerein, we compared the effect of the ligand, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), or the potential ligand, Emodin, on Fe-catalyzed lipid peroxidation in cell membrane models (micelles and bicelles). These studies were performed in the presence of hydrogen peroxide (H2O2) and the absence or presence of ascorbate. ResultsIn the absence of ascorbate, Fe(II)/Emodin mixtures incubated with H2O2 demonstrated slight pro-oxidant properties on micelles versus Fe(II) alone, while the Fe(III)-Dp44mT complex exhibited marked antioxidant properties. Examining more physiologically relevant phospholipid-containing bicelles, the Fe(II)- and Fe(III)-Dp44mT complexes demonstrated antioxidant activity without ascorbate. Upon adding ascorbate, there was a significant increase in the peroxidation of micelles and bicelles in the presence of unchelated Fe(II) and H2O2. The addition of ascorbate to Fe(III)-Dp44mT substantially increased the peroxidation of micelles and bicelles, with the Fe(III)-Dp44mT complex being reduced by ascorbate to the Fe(II) state, explaining the increased reactivity. Electron paramagnetic resonance spectroscopy demonstrated ascorbyl radical anion generation after mixing ascorbate and Emodin, with signal intensity being enhanced by H2O2. This finding suggested Emodin semiquinone radical formation that could play a role in its reactivity via ascorbate-driven redox cycling. Examining cultured melanoma cells in vitro, ascorbate at pharmacological levels enhanced the anti-proliferative activity of Dp44mT and Emodin. Conclusions and general significanceAscorbate-driven redox cycling of Dp44mT and Emodin promotes their anti-proliferative activity.

Redox Activity as a Powerful Strategy to Tune Magnetic and/or Conducting Properties in Benzoquinone-Based Metal-Organic Frameworks

Multifunctional molecular materials have attracted material scientists for several years as they are promising materials for the future generation of electronic devices. Careful selection of their molecular building blocks allows for the combination and/or even interplay of different physical properties in the same crystal lattice. Incorporation of redox activity in these networks is one of the most appealing and recent synthetic strategies used to enhance magnetic and/or conducting and/or optical properties. Quinone derivatives are excellent redox-active linkers, widely used for various applications such as electrode materials, flow batteries, pseudo-capacitors, etc. Quinones undergo a reversible two-electron redox reaction to form hydroquinone dianions via intermediate semiquinone radical formation. Moreover, the possibility to functionalize the six-membered ring of the quinone by various substituents/functional groups make them excellent molecular building blocks for the construction of multifunctional tunable metal-organic frameworks (MOFs). An overview of the recent advances on benzoquinone-based MOFs, with a particular focus on key examples where magnetic and/or conducting properties are tuned/switched, even simultaneously, by playing with redox activity, is herein envisioned.

Ruthenium organometallics of chloro-substituted 2′-hydroxychalcones – A story of catecholase biomimetics beyond copper

Four new organoruthenium(II) compounds of 2’-hydroxychalcones, differing in the position of the chloro substituent at chalcone moiety, were prepared by solution synthesis and characterized by chemical analysis, infrared and electron spectroscopy, mass spectrometry and 1D and 2D NMR spectroscopy, as biomimetic functional models of catechol oxidase. The molecular and crystal structures of three compounds were determined by single crystal X-ray diffraction. The organometallics are neutral species having piano-stool pseudo octahedral geometry with ruthenium(II) coordinated by η6-cymene, bidentate monobasic 2’-hydroxychalcone ligand, and chloride ion. The catecholase activity of new organometallics was investigated by electron spectroscopy in three solvents at three temperatures. The kinetic measurements were done under pseudo first order conditions and vmax, kcat, KM, TON, TOF, and Ea values are determined. Compounds (1) – (4) have solvent-dependent catecholase activity with activity decreasing in order ethanol > methanol ≫ acetonitrile. The highest kcat value and the lowest activation energy were found for the reaction catalyzed by organometallic (4), having a chloro substituent at the B ring of the chalcone ligand in the para position. The kcat values of 104 h−1 and 105 h−1 order at 297 K and 307 K, respectively classify these organometallics among the best artificial functional models of catechol oxidase. The results indicate that the catechol oxidation mediated by organometallics (1) – (4) proceeds via semiquinone radical formation.

Goethite enhanced the Cr(VI) reduction by gallic acid through the stabilisation of free radicals: spectroscopic evidences and implication for transformation of Cr(VI) in an Oxisol

ABSTRACT Produced from root exudation activities and the decomposition of organic matter, gallic acid (GA) has been shown to be widely involved in the electron transfer reactions in soils as well as the transformation of redox-sensitive pollutants, while which was subject to the mediation of solid surfaces with the unknown mechanisms. This study aimed to investigate the catalytic effect of goethite on the Cr(VI) reduction by GA and explore the implication for the transformation of Cr(VI) in an oxisol. The results have shown that the adsorption of GA by goethite followed pseudo-second-order model. The GA was very stable at pH 4.0 in solution, whereas its oxidation by O2wasaccelerated in the presence of goethite as evidenced by the rapid appearance of IR peak of quinones. Similarly, the reduction of Cr(VI) by GA proceeded slowly with semi-quinonic species as the intermediate oxidation products of GA, while which would be significantly enhanced with the addition of goethite or oxisol. The mechanism was ascribed to that surface adsorption promoted the dissociation of –OH groups of GA and the subsequent formation of semiquinone radicals. Most importantly, these free radicals could be stabilised on the goethite surfaces, which was believed to be favourable for the reduction of Cr(VI). The catalysis of aqueous Fe(III) was excluded in this study due to its low concentration in the reaction systems. Furthermore, the majority of produced Cr(III) was adsorbed on the surfaces as the form of octahedron. The findings above indicated the environmental risk of Cr(VI)-polluted acidicsoilswould be alleviated when GA was present.

Effects of acetylacetone on the thermal and photochemical conversion of benzoquinone in aqueous solution

Quinones are components of electron transport chains in photosynthesis and respiration. Acetylacetone (AA), structurally similar to benzoquinone (BQ) for the presence of two identical carbonyl groups, has been reported as a quinone-like electron shuttle. Both BQ and AA are important chemicals in the aquatic environment. However, little information is known about their interactions if co-existed. We found here that AA significantly enhanced the conversion of BQ. By analyzing the evolution of chemical concentration, solution pH, dissolved oxygen, and the final products, the interactions between AA and BQ were elucidated. The reactions between BQ and AA generated oxygen but ultimately led to the reduction of solution pH and dissolved oxygen. The reactions proceeded faster under indoor lighting condition than in the dark. The formation of semiquinone radicals is believed as the primary step. The secondary AA-derived radicals might be strongly oxidative or reductive, depending on the concentration of dissolved oxygen. Insoluble humus was generated in the mixture of BQ and AA. These results suggest that the presence of AA might interfere with photosynthesis and respiration through the interactions with quinones.

Interaction of Quinone-Related Electron Acceptors with Hydropersulfide Na2S2: Evidence for One-Electron Reduction Reaction.

We previously reported that 9,10-phenanthraquinone (9,10-PQ), an atmospheric electron acceptor, undergoes redox cycling with dithiols as electron donors, resulting in the formation of semiquinone radicals and monothiyl radicals; however, monothiols have little reactivity. Because persulfide and polysulfide species are highly reducing, we speculate that 9,10-PQ might undergo one-electron reduction with these reactive sulfides. In the present study, we explored the redox cycling capability of a variety of quinone-related electron acceptors, including 9,10-PQ, during interactions with the hydropersulfide Na2S2 and its related polysulfides. No reaction occurred when 9,10-PQ was incubated with Na2S; however, when 5 μM 9,10-PQ was incubated with either 250 μM Na2S2 or Na2S4, we detected extensive consumption of dissolved oxygen (84 μM). Under these conditions, both the semiquinone radicals of 9,10-PQ and their thiyl radical species were also detected using ESR, suggesting that a redox cycle reaction occurred utilizing one-electron reduction processes. Notably, the perthiyl radicals remained stable even under aerobic conditions. Similar phenomenon has also been observed with other electron acceptors, such as pyrroloquinoline quinone, vitamin K3, and coenzyme Q10. Our experiments with N-methoxycarbonyl penicillamine persulfide (MCPSSH), a precursor for endogenous cysteine persulfide, suggested the possibility of a redox coupling reaction with 9,10-PQ inside cells. Our study indicates that hydropersulfide and its related polysulfides are efficient electron donors that interact with quinones. Redox coupling reactions between quinoid electron acceptors and such highly reactive thiols might occur in biological systems.

Graphene Quantum Dots Protect against Copper Redox-Mediated Free Radical Generation and Cardiac Cell Injury.

In this work, we investigated the effects of graphene quantum dots (GQDs) on copper redox-mediated free radical generation and cell injury. Using electron paramagnetic resonance (EPR) spectrometry in conjunction with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap, we found that GQDs at a concentration as low as 1 μg/ml significantly inhibited Cu(II)/H2O2-mediated hydroxyl radical formation. GQDs also blocked Cu(II)-catalyzed nucleophilic addition of H2O to DMPO to form a DMPO-OH adduct in the absence of H2O2, suggesting a potential for GQDs to inhibit copper redox activity. Indeed, we observed that the presence of GQDs prevented H2O2-mediated reduction of Cu(II) to Cu(I) though GQDs themselves also caused the reduction of Cu(II) to Cu(I). To further investigate the effects of GQDs on copper redox activity, we employed the Cu(II)/hydroquinone system in which copper redox activity plays an essential role in the oxidation of hydroquinone to semiquinone radicals with consequent oxygen consumption. Using oxygen polarography as well as EPR spectrometry, we demonstrated that the presence of GQDs drastically blocked the oxygen consumption and semiquinone radical formation resulting from the reaction of Cu(II) and hydroquinone. These results suggested that GQDs suppressed free radical formation via inhibiting copper redox activity. Lastly, using cultured human cardiomyocytes, we demonstrated that the presence of GQDs also protected against Cu(II)/H2O2-mediated cardiac cell injury as indicated by morphological changes (e.g., cell shrinkage and degeneration). In conclusion, our work shows, for the first time, the potential for using GQDs to counteract copper redox-mediated biological damage.

Effect of Redox "Non-Innocent" Linker on the Catalytic Activity of Copper-Catecholate-Decorated Metal-Organic Frameworks.

Two new UiO-68 type of Zr-MOFs featuring redox non-innocent catechol-based linkers of different redox activities have been synthesized through a de novo mixed-linker strategy. Metalation of the MOFs with Cu(II) precursors triggers the reduction of Cu(II) by the phenyl-catechol groups to Cu(I) with the concomitant formation of semiquinone radicals as evidenced by EPR and XPS characterization. The MOF-supported catalysts are selective toward the allylic oxidation of cyclohexene and it is found that the presence of in situ-generated Cu(I) species exhibits enhanced catalytic activity as compared to a similar MOF with Cu(II) metalated naphthalenyl-dihydroxy groups. This work unveils the importance of metal-support redox interactions in the catalytic activity of MOF-supported catalysts which are not easily accessible in traditional metal oxide supports.

The oxidative stress in allelopathy: Participation of prenyllipid antioxidants in the response to juglone in Chlamydomonas reinhardtii

Allelopathy is a phenomenon, where one species releases compounds able to inhibit the growth of other species. Juglone, 5-hydroxy-1,4-naphtoquinone, is an allelochemical produced by walnut trees. The main mode of juglone toxicity is the formation of semiquinone radicals, able to reduce O2 to superoxide. Prenyllipid antioxidants such as tocopherol and plastoquinone are important for antioxidant defense in photosynthetic organisms. Here we assess their participation in the response to juglone. The impact of 20 μM juglone on the content of photosynthetic pigments and prenyllipid antioxidants in green microalga Chlamydomonas reinhardtii was measured over an incubation period of 7.5 h in low light and over 40 min under high light or in darkness. The decrease in pigment and prenyllipid content, accompanied by an increase in lipid hydroperoxides was observed over a longer incubation period with juglone. Simultaneous exposure to high light and juglone led to a pronounced decrease in carotenoids and prenyllipids, while there was no decrease in high light alone and no decrease or only a slight decrease in the series with juglone alone. The fact that semiquinone radicals are generated in juglone-exposed cells was confirmed using EPR spectroscopy. This article also shows that C. reinhardtii may be a suitable model for studies on some modes of phytotoxic action of allelochemicals.

Oxidation mechanism of As(III) in the presence of polyphenols: New insights into the reactive oxygen species

The interaction between humic substances (HS) and arsenic (As(III)) is an important process affecting the fate and transformation of arsenic in the environment. In the present study, batch experiments were conducted to investigate the interactions between As(III) and polyphenols, as simple model compounds of HS in aqueous solutions. The results indicated that As(III) could be oxidized to As(V) in the presence of gallic acid (GA), caffeic acid (CA) and protocatechuic acid (PA) at neutral and alkaline conditions. By using electron paramagnetic resonance (EPR) technique, it was found that the formation of semiquinone radical (SQ−) in polyphenol solutions induced the generation of reactive oxygen species (ROS) including hydrogen peroxide (H2O2) and hydroxyl radical (OH). Both SQ− and ROS were responsible for the oxidation of As(III) to As(V). Free radical quenching studies showed that semiquinone and H2O2 accounted for about 70%, while OH accounted for 30% of As(III) oxidation in GA/As(III) system at pH 9.0. Moreover, the As(III) oxidation kinetics process well followed the pseudo-first-order reaction. The increase of pH and dissolved oxygen concentration favored the formation of ROS, and thus promoted the As(III) oxidation. The findings of this study provided some new insights into the mechanism of the interactions between HS and arsenic, and contributed to the better understanding of arsenic geochemical cycles.

Reducing capacities and redox potentials of humic substances extracted from sewage sludge

Humic substances (HS) are redox active organic materials that can be extracted from sewage sludge generated in wastewater treatment processes. Due to the poor understanding of reducing capacity, redox potentials and redox active functional groups of HS in sewage sludge, the potential contribution of sludge HS in transformation of wastewater contaminants is unclear. In the present study, the number of electrons donated or accepted by sewage sludge HS were quantified before and after reduction by iron compounds that possess different redox potentials and defined as the reducing capacity of the sewage sludge. In contrast to previous studies of soil and commercial humic acids (HA), reduced sludge HA showed a lower reducing capacity than that of native HA, which implies formation of semiquinone radicals since the semiquinone radical/hydroquinone pair has a much higher redox potential than the quinone/hydroquinone pair. It is novel that reducing capacities of sludge HA were determined in the redox potential range from -314 to 430 mV. The formation of semiquinone radicals formed during the reduction of quinone moieties in sludge HA is shown by three-dimensional excitation/emission matrix fluorescence spectroscopies information, increasing fluorescence intensities and blue-shifting of the excitation/emission peak of reduced sludge HA. Knowledge of sludge HS redox potentials and corresponding reducing capacities makes it possible to predict the transformation of redox active pollutants and facilitate manipulation and optimization of sludge loading wastewater treatment processes.

Selected hydroxycoumarins as antioxidants in cells: physicochemical and reactive oxygen species scavenging studies

A set of seven hydroxycoumarin derivatives (1–7) were tested to evaluate their antioxidant properties. These molecules were selected on the basis of the number of hydroxyl groups and the presence of electron‐donating and electron‐withdrawing groups in their scaffolds. Electrochemical studies showed that oxidation process occurs at low potentials, especially with the catechol‐containing coumarins. Electron spin resonance (ESR) experiments corroborated the formation of semiquinone radicals in all compounds. The antioxidant properties against peroxyl and hydroxyl radicals were evaluated using two different assays, namely oxygen radical antioxidant capacity (ORAC) and non‐catalytic Fenton system by ESR measurements. Our results indicated that monohydroxycoumarin derivatives (1 and 2) displayed better ORAC values relative to the standard molecule Trolox. These values are comparable with known flavonoids such as quercetin and catechin. On the other hand, compounds bearing catechol and resorcinol groups, including derivative 4, gave high hydroxyl radical‐scavenging properties. Antioxidant properties were further rationalized using density functional theory calculations. The theoretical results showed good trends among the obtained experimental data, calculated bonding dissociation enthalpy, highest occupied molecular orbital values, free energies of oxidation (ΔGox) and spin density distribution. Finally, in vitro studies revealed low toxicity for most of the coumarins studied. The cytoprotection assays in bovine aortic endothelial cells indicated that compounds 1 and 2 showed better protective properties against peroxynitrite and hydrogen peroxide‐induced cell death than dihydroxycoumarin derivatives. Using confocal microscopy, we have observed intracellular compartmentalization. These results confirm that 1 or 2 would be a good target for future in vivo studies against oxidative stress. Copyright © 2013 John Wiley & Sons, Ltd.

Oxidative stress and innate immunity status in chickens exposed to high dose of ascorbic acid

The effects of high dose ascorbic acid (10 000 mg·kg(-1) in the diet) and the transition metal on the presence of oxidative stress in the internal organs of growing chicks, as well as on the innate immune system status, were investigated. Supplementation with a high dose of ascorbic acid had pro-inflammatory effects on the intestinal mucosa, and lysozyme levels were decreased significantly in all organs studied. High-dose ascorbic acid caused an imbalance between prooxidative and antioxidative activities and was associated with the generation of semiquinone radicals. We observed that ascorbic acid increased iron and cadmium absorption. When a high dose of ascorbic acid was applied, elevated kidney and intestinal mucosa iron concentrations were observed. The amount of free malondialdehyde in the above organs has increased as well. These data have important implications for the mechanism of the oxidative stress development under the influence of high dose of ascorbic acid, indicating the importance of the side reactions of the mitochondrial electron transport chain with the formation of semiquinone radicals and the role of transition metals in this process.

Dietary catechol causes increased oxidative DNA damage in the livers of mice treated with acetaminophen

We have shown that direct reaction of catechol with nitric oxide (NO) results in generation of reactive oxygen and nitrogen species (RNS) through semiquinone radical formation, leading to oxidative DNA damage in rat forestomach. In the present study, we investigated whether dietary catechol systemically exerts the same effects under NO-rich circumstances, when given before and during induction of inflammatory lesions. Male ICR mice were treated with or without 0.8% catechol in the diet for 2 weeks followed by acetaminophen (APAP) administration at a dose of 300 mg/kg by single i.p. injection. Along with several indicators of APAP-induced hepatitis, 8-hydroxydeoxyguanosine (8-OHdG) levels and immunohistochemistry for 3-nitrotyrosine (NO 2Tyr) in the livers were examined at 1.5, 4 and 24 h after APAP injection. 8-OHdG was significantly increased at 24 h in the co-treatment group, but not with either catechol or APAP alone. Elevation of serum ALT and AST activities, decrease of reduced glutathione levels and histopathological liver changes were observed to the same extents in both APAP-treated groups. In view of the finding of positive hepatocytes for NO 2Tyr prior to generation of 8-OHdG, the process of oxidative DNA damage might involve RNS formation. Precise quantitative analysis of NO 2Tyr by means of liquid chromatography with tandem mass spectrometry (LC-MS/MS) in an additional study with the same experimental protocol confirmed increase of RNS due to the reaction of catechol with NO produced after APAP-induced hepatitis. The overall data imply that antioxidants with a catechol structure can cause oxidative DNA damage under inflammatory conditions.

Characterization of a soluble oxidoreductase from the thermophilic bacterium Carboxydothermus ferrireducens

An NAD(P)H-dependent oxidoreductase has been purified approximately 40-fold from the soluble protein fraction of the dissimilatory iron-reducing, anaerobic, thermophilic bacterium Carboxydothermus ferrireducens. The enzyme, a flavoprotein, has broad-substrate specificity-reducing Fe(3+), Cr(6+), and AQDS with rates of 0.31, 0.33, and 3.3 U mg(-1) protein and calculated NADH oxidation turnover numbers of 0.25, 0.25, and 2.5 s(-1), respectively. Numerous quinones are reduced via a two-electron transfer from NAD(P)H to quinone, thus participating in managing oxidative stress by avoiding the formation of semiquinone radicals.

Paramagnetic centers in oak and onion extracts

We have used EPR to study oak bark, bulb onion peel, and their aqueous extracts. We have established that their paramagnetic properties are determined by at least four types of paramagnetic centers: semiquinone radicals, Fe 3+ ions, and two types of Mn 2+ ions. We have observed that metal ions are extracted by water from the plant raw material without any change in their original coordination environment. The aqueous extracts were subjected to oxidative processes: mild oxidation in the dried state, strong oxidation when stored as solutions. We have shown that when the solutions are stored, the oxidation reaction is catalyzed by the metal ions and is accompanied by formation of semiquinone radicals.

Formation of semiquinone radical in the reaction of embelin (2,5-dihydroxy-3-undecyl-1,4-benzoquinone) with reductants as well as oxidants. Characterization by pulse radiolysis and structure investigation by quantum chemical study

Embelin, 2,5-dihydroxy-3-undecyl-1,4-benzoquinone, has two intra-molecular hydrogen bonds between the quinone and hydroquinone groups present on the same ring. Therefore, it is expected to produce semiquinone radical in the oxidation as well reduction reactions. To investigate this aspect, kinetics and mechanism of reduction reactions of embelin have been studied using nanosecond electron pulse radiolysis technique and compared with the oxidation process. Two distinct transients observed in the reduction ( λ max = 450 nm) and the oxidation ( λ max = 410 nm) processes with various free radicals have been characterized and assigned to two different embelin semiquinone radicals on the basis of pulse radiolysis studies. The structures of these transients have been further ascertained using density functional theory with B3LYP functional and 6-31+G(d,p) basis set. Reduction reaction of embelin has been found to produce delocalized semiquinone anion radical. On the other hand, oxidation of embelin is expected to produce several transient species. However, the calculations suggest that oxidation of hydroquinone group is the first step, which is followed by transformation into carbon-centered radical.

Characterization of the quinone reductase activity of the ferric reductase B protein from Paracoccus denitrificans

The ferric reductase B (FerB) protein of Paracoccus denitrificans exhibits activity of an NAD(P)H: Fe(III) chelate, chromate and quinone oxidoreductase. Sequence analysis places FerB in a family of soluble flavin-containing quinone reductases. The enzyme reduces a range of quinone substrates, including derivatives of 1,4-benzoquinone and 1,2- and 1,4-naphthoquinone, via a ping-pong kinetic mechanism. Dicoumarol and Cibacron Blue 3GA are competitive inhibitors of NADH oxidation. In the case of benzoquinones, FerB apparently acts through a two-electron transfer process, whereas in the case of naphthoquinones, one-electron reduction takes place resulting in the formation of semiquinone radicals. A ferB mutant strain exhibited an increased resistance to 1,4-naphthoquinone, attributable to the absence of the FerB-mediated redox cycling. The ferB promoter displayed a high basal activity throughout the growth of P. denitrificans, which could not be further enhanced by addition of different types of naphthoquinones. This indicates that the ferB gene is expressed constitutively.