Tetrachloro-1,4-benzoquinone Research Articles

A new method for hydroxyl radical detection by chemiluminescence of flue-cured tobacco extracts

Chemiluminescence (CL) reactions usually take place in a severely restricted pH regime, thereby confining their application in media at different pH. In this paper, the CL behavior of tobacco-methanol extract (TME) was explored. Surprisingly, TME exhibited CL behavior upon mixing with Fe2+/H2O2, HRP/H2O2 and gold nanoparticles/H2O2 oxidation systems, i.e., in acidic, neutral and alkaline solution respectively. Addition of different reactive oxygen species scavengers verified that the hydroxyl radical (OH) triggers TME CL reactions. Then, the CL behavior of TME was applied to determine OH in tetrachloro-1,4-benzoquinone (TCBQ)/H2O2 system in acidic, neutral and alkaline solutions. CL intensity correlated most strongly (R2 = 0.99) with TCBQ concentration, which was used as a means to indirectly denote OH concentration. This implies that OH could be determined by a TME CL method in a semi-quantitative way regardless of pH value. Therefore, the TME CL method may be a promising method for OH determination in various OH-generating systems.

P-227 - An unexpected double radical homolysis mechanism for the reactions between the pyridinium aldoxime nerve agent antidotes and polyhalogenated quinones under normal physiological conditions

We have recently shown that the pyridinium aldoximes, best-known as therapeutic antidotes for chemical warfare nerve-agents, could markedly detoxify the carcinogenic tetrachloro-1,4-benzoquinone (TCBQ) via an unusual double Beckmann fragmentation mechanism. However, it remains not clear why pralidoxime (2-PAM) cannot provide full protection against TCBQ-induced biological damage even when 2-PAM was in excess. Here we show, unexpectedly, that TCBQ can also activate pralidoxime to generate a reactive iminyl radical intermediate in two-consecutive steps, which was detected and unequivocally characterized by the complementary application of ESR spin-trapping, HPLC/MS and nitrogen-15 2-PAM isotope-labeling studies. The end product of iminyl radical was isolated and identified as its corresponding reactive and toxic aldehyde. We proposed that the reaction of 2-PAM and TCBQ might be via the following two competing mechanisms: double Beckmann fragmentation vs double radical homolysis. This study represents the first detection and identification of reactive iminyl radical intermediates produced under normal physiological conditions, which provides direct experimental evidence to explain the potential side-toxic effects induced by 2-PAM and other pyridinium aldoxime nerve-agent antidotes.

Dynamic Tracking of Highly Toxic Intermediates in Photocatalytic Degradation of Pentachlorophenol by Continuous Flow Chemiluminescence.

Photocatalytic degradation is a powerful technique for the decomposition of pollutants. However, toxic intermediates might be generated which have become a great concern recently. In the present work, a continuous flow chemiluminescence (CFCL) method was developed for dynamic monitoring of toxic intermediates generated in the photocatalytic degradation of pentachlorophenol (PCP). Among the main intermediates, tetrachloro-1,4-benzoquinone (TCBQ) and trichlorohydroxy-1,4-benzoquinone (OH-TrCBQ) showed higher or similar toxicity to PCP. As both TCBQ and OH-TrCBQ can produce chemiluminescence (CL) in the presence of H2O2, a CFCL system was established for the dynamic tracking of the two toxic intermediates. A PCP/TiO2 suspension was irradiated in a photoreactor, pumped continuously into a detection cell, and mixed with H2O2 to produce CL. The time-dependent CL response displayed two distinctive peaks at pH 7, which were attributed to the generation of OH-TrCBQ and TCBQ, respectively, by comparing with their changes measured by high-performance liquid chromatography (HPLC). Furthermore, the CL response curve of PCP/TiO2 suspension showed a pattern very similar to their bacteria inhibition. Therefore, the CFCL could be used as a simple and low-cost method for online monitoring of TCBQ and OH-TrCBQ to ensure complete removal of not only PCP but also highly toxic degradation intermediates.

82 - Nitroxide Radical Production from Hydroxamic Acids and Carcinogenic Halogenated Quinones via an Unexpected Nitrogen-centered Radical Intermediate Pathway

We have found previously that a nitroxide radical could be produced from desferrioxamine (DFO) with tetrachloro-hydroquinone (TCHQ, the major genotoxic metabolite of the widely used wood preservative pentachlorophenol). However, the underlying molecular mechanism remains unclear. In this study, N-methyl acetohydroxamic acid (N-MeAHA) was used as a simple model compound for DFO for further mechanistic study. As expected, the N-MeAHA-nitroxide radical (N-MeAHA-O•) was found to be produced during the reaction between N-MeAHA and TCHQ by direct ESR. Interestingly and unexpectedly, when TCHQ was substituted by tetrachloro-1,4-benzoquinone (TCBQ), N-MeAHA-O• was also observed, but no tetrachlorosemiquinone anion radical (TCSQ•—) or TCHQ was detected, suggesting the formation of N-MeAHA-O• is not a direct redox reaction between N-MeAHA and TCBQ (or TCSQ•—). To detect whether other radical intermediates were formed during the reaction of N-MeAHA and TCBQ, ESR spin-trapping method was used with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as trapping agent. We found, surprisingly, that a new nitrogen-centered radical was produced, which was identified as Ac-(CH3)N• by HPLC-MS and the high resolution FTICR-MS. Taken together, we propose the production of nitroxide radical from N-MeAHA and TCHQ is via an unexpected new pathway via H-abstraction from the N-OH group by the novel nitrogen-centered radical intermediate.

84 - The Critical Role of Quinone-Enoxy Radicals in the Production of the More Mutagenic dIz from DNA by Carcinogenic Halogenated Quinones and Hydroperoxides

Halogenated quinones (XQ) are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. We found recently that XQ could enhance hydroperoxide decomposition and formation of alkoxyl and quinone enoxy/ketoxy radicals metal-independently. However, it is not clear whether XQ/hydroperoxides can oxidize dG to its oxidation products, and if so, what is the underlying molecular mechanism. Here we show that the major oxidation product was dIz when dG was treated with tetrachloro-1,4-benzoquinone (TCBQ) and t-BuOOH, which was markedly inhibited by typical spin-trapping agents. Further ESR and HPLC/MS results showed that the quinone enoxy radicals played a critical role in dIz formation, and O2 was also involved in dIz formation by using oxygen-18-labelled O2. We proposed that the oxidation of dG by TCBQ/t-BuOOH might be through the following mechanism: the quinone enoxy radical may first abstract H from dG, forming dG(-H)• radical, which may combine with O2 to form its corresponding hydroperoxide, resulting in a cascade of decomposition reactions leading to dIz formation. This is the first report that XQ/hydroperoxides can induce potent oxidation of dG to dIz via a quinone-enoxy radical-mediated mechanism, which may partly explain their potential carcinogenicity.

43 - An Unexpected New Molecular Mechanism for the Formation of Nitroxide Radical from Hydroxamic Acids and Polyhalogenated Quinoid Carcinogens

We have found previously that a nitroxide radical could be produced from desferrioxamine (DFO) with tetrachlorohydroquinone (TCHQ, the major genotoxic metabolite of the widely used wood preservative pentachlorophenol). However, the underlying molecualr machanism remains unclear. In this study, N -methyl acetohydroxamic acid ( N -MeAHA) was used as a simple model compound for DFO for further mechanistic study. As expected, the N -MeAHA-nitroxide radical ( N -MeAHA-O ● ) was found to be produced during the reaction between N -MeAHA and TCHQ by direct ESR. Interestingly and unexpectedly, when TCHQ was substituted by tetrachloro-1,4-benzoquinone (TCBQ), N -MeAHA-O ● was also observed, but no tetrachlorosemiquinone anion radical (TCSQ ●– ) or TCHQ was detected, suggesting the formation of N -MeAHA-O ● is not a direct redox reaction between N -MeAHA and TCBQ (or TCSQ ●– ). To detect whether other radical intermediates were formed during the reaction of N -MeAHA and TCBQ, ESR spin-trapping method was used with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as trapping agent. We found, surprisingly, that a new nitrogen-centered radical was produced, which was identified as Ac-(CH 3 )N ● by HPLC-MS and the high resolution FTICR-MS. Taken together, we propose the production of nitroxide radical from N -MeAHA and TCHQ is via an unexpected new pathway via H-abstraction from the N-OH group by the novel nitrogen-centered radical intermediate.

45 - Molecular Mechanism for the Production of the More Mutagenic DIz from DNA by Halogenated Quinoid Carcinogens and Hydroperoxides: The Critical Role of Quinone-Enoxy Radicals

Halogenated quinones (XQ) are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. We found recently that XQ could enhance hydroperoxide decomposition and formation of alkoxyl and quinone enoxy/ketoxy radicals metal-independently. However, it is not clear whether XQ/hydroperoxides can oxidize dG to its oxidation products, and if so, what is the underlying molecular mechanism. Here we show that the major oxidation product was dIz when dG was treated with tetrachloro-1,4-benzoquinone (TCBQ) and t-BuOOH, which was markedly inhibited by typical spin-trapping agents. Further ESR and HPLC/MS results showed that the quinone enoxy radicals played a critical role in dIz formation, and O2 was also involved in dIz formation by using oxygen-18-labelled O2. We proposed that the oxidation of dG by TCBQ/t-BuOOH might be through the following mechanism: the quinone enoxy radical may first abstract H from dG, forming dG(-H)● radical, which may combine with O2 to form its corresponding hydroperoxide, resulting in a cascade of decomposition reactions leading to dIz formation. This is the first report that XQ/hydroperoxides can induce potent oxidation of dG to dIz via a quinone-enoxy radical-mediated mechanism, which may partly explain their potential carcinogenicity.

Tetrachloro-1,4-benzoquinone induces apoptosis of mouse embryonic stem cells

Pentachlorophenol (PCP) is a widespread, persistent environmental contaminant, and it is enzymatically activated to form a reactive metabolite, tetrachloro-1,4-benzoquinone (TCBQ). To our knowledge, there is no information about TCBQ toxicity on embryonic stem cells. Here, we demonstrated that TCBQ induced significantly apoptosis of mouse embryonic stem cells in a concentration-dependent manner. We also showed that TCBQ elevated genomic 5-hydroxymethylcytosine (5hmC) by affecting ten-eleven translocation (Tet) dioxygenases in mouse embryonic stem cells. We further investigated whether Tet dioxygenases were implicated in TCBQ-induced apoptosis. By depleting all three dioxygenases (Tet1-3), we found that Tet dioxygenases slightly inhibited both early and late apoptosis induced by TCBQ at a low concentration (30μmol/L). Meanwhile, treated by TCBQ at higher concentrations (40 and 50μmol/L), the total percentage of apoptotic cells was not affected by Tet dioxygenases. However, Tet dioxygenases tended to arrest mouse ES cells to be at early apoptotic stage and to reduce the cells to enter later apoptotic stage. These results indicate that Tet dioxygenases play a role in shaping TCBQ-induced apoptosis in mouse embryonic stem cells. Our study provides new insights into the toxicology of PCP and its reactive metabolite TCBQ.

Evaluating the Effects of Tetrachloro-1,4-benzoquinone, an Active Metabolite of Pentachlorophenol, on the Growth of Human Breast Cancer Cells.

Tetrachloro-1,4-benzoquinone (TCBQ), an active metabolite of pentachlorophenol (PCP), is genotoxic and potentially carcinogenic. As an electrophilic and oxidative molecule, TCBQ can conjugate with deoxyguanosine in DNA molecules and/or impose oxidative stress in cells. In the current study, we investigated the effects of TCBQ on intracellular ROS production, apoptosis, and cytotoxicity against three different subtypes of human breast cancer cells. Luminal A subtype MCF7 (ER+, PR+, HER2−) cells maintained the highest intracellular ROS level and were subjected to TCBQ-induced ROS reduction, apoptosis, and cytotoxicity. HER2 subtype Sk-Br-3 (ER−, PR−, HER2+) cells possessed the lowest intracellular ROS level. TCBQ promoted ROS production, inhibited apoptosis, and elevated cytotoxicity (due to necrosis) against Sk-Br-3 cells. Triple-negative/basal-like subtype MDA-MB-231 cells were less sensitive towards TCBQ treatment. Therefore, the effect of prolonged exposure to PCP and its active metabolites on cancer growth is highly cancer-cell-type specific.

Hydroxyl radical-dependent two-step chemiluminescence production by polyhalogenated quinones and H<sub>2</sub>O<sub>2</sub>

Tetrachloro-1,4-benzoquinone (TCBQ) is one of the major genotoxic and carcinogenic quinoid metabolites of the widely used wood preservative pentachlorophenol. We found previously that TCBQ and H2O2 could produce hydroxyl radicals (·OH) independent of transition metal ions. However, the underlying molecular mechanism is still unclear. Here we show that an unusual two-step chemiluminescence (CL) can be produced by TCBQ and H2O2. The two-step CL produced by TCBQ/H2O2 was found to be well correlated to and dependent on its two-step production of ·OH: markedly quenched by the classic ·OH scavengers, but enhanced further by classic Fenton reagent, which is known to produce extra ·OH. The initial reaction intermediate of TCBQ/H2O2 were found to be trichlorohydroxy-1,4-benzoquinone, and the major final products were identified as 2,5-dichloro-3,6-dihydroxy-1,4-benzoquonine, and the ring-opening 2,3-dichloromaleic acid and CO2 by complementary applications of multiple analytical methods such as HPLC-MS, GC-MS, ion chromatography (IC) and total organic carbon (TOC). Based on these data, we proposed that ·OH-dependent formation of quinone-dioxetane and electronically excited carbonyl species might be responsible for this unusual two-step CL production by TCBQ and H2O2. These findings may have potential applications in detecting and quantifying · OH and the ubiquitous polyhalogenated aromatic carcinogens, which may have broad biological and environmental implications for future research on these types of important species.

Potent methyl oxidation of 5-methyl-2′-deoxycytidine by halogenated quinoid carcinogens and hydrogen peroxide via a metal-independent mechanism

Halogenated quinones are a class of carcinogenic intermediates and are newly identified chlorination disinfection by-products in drinking water. We found recently that the highly reactive and biologically important hydroxyl radical (•OH) can be produced by halogenated quinones and H2O2 independent of transition metal ions. However, it is not clear whether these quinoid carcinogens and H2O2 can oxidize the nucleoside 5-methyl-2′-deoxycytidine (5mdC) to its methyl oxidation products and, if so, what the underlying molecular mechanism is. Here we show that three methyl oxidation products, 5-(hydroperoxymethyl)-, 5-(hydroxymethyl)-, and 5-formyl-2′-deoxycytidine, could be produced when 5mdC was treated with tetrachloro-1,4-benzoquinone (TCBQ) and H2O2. The formation of the oxidation products was markedly inhibited by typical •OH scavengers and under anaerobic conditions. Analogous effects were observed with other halogenated quinones and the classic Fenton system. Based on these data, we propose that the oxidation of 5mdC by TCBQ/H2O2 might be through the following mechanism: •OH produced by TCBQ/H2O2 may first abstract hydrogen from the methyl group of 5mdC, leading to the formation of 5-(2′-deoxycytidylyl)methyl radical, which may combine with O2 to form the peroxyl radical. The unstable peroxyl radical transforms into the corresponding hydroperoxide 5-(hydroperoxymethyl)-2′-deoxycytidine, which reacts with TCBQ and results in the formation of 5-(hydroxymethyl)-2′-deoxycytidine and 5-formyl-2′-deoxycytidine. This is the first report that halogenated quinoid carcinogens and H2O2 can induce potent methyl oxidation of 5mdC via a metal-independent mechanism, which may partly explain their potential carcinogenicity.

A Novel Mechanism for Metal-independent Hydroxyl Radical Production by Hydrogen Peroxide and Halogenated Quinones

The hydroxyl radical ((center dot)OH) has been considered to be one of the most reactive oxygen species that are produced in biological systems. Frequently, (center dot)OH formation is ascribed to the transition metal-catalyzed Fenton reaction. During the study of the molecular mechanism for the genotoxicity of the wood preservative pentachlorophenol (PCP), we found that (center dot)OH could be produced by H(2)O(2) and PCP metabolite tetrachloro-1,4-benzoquinone (TCBQ) independent of transition metal ions. Further studies showed that TCBQ, but not its corresponding semiquinone radical, is essential for (center dot)OH production. Metal-independent production of (center dot)OH could also be observed with other halogenated quinones and H(2)O(2). Based on these data, we propose that (center dot)OH production by TCBQ and H(2)O(2) is not through a semiquinone-dependent organic Fenton reaction, but rather through a novel nucleophilic substitution and homolytical decomposition mechanism. This represents a novel mechanism for (center dot)OH production not requiring the catalysis of redox-active transition metal ions, and may partly explain the potential carcinogenicity of the widely used biocides such as PCP and other polyhalogenated aromatic compounds.

A novel mechanism for metal-independent production of hydroxyl radicals

The hydroxyl radical (•OH) has been considered to be one of the most reactive oxygen species produced in biological systems. It has been shown that •OH can cause DNA, protein, and lipid oxidation. One of the most widely accepted mechanisms for •OH production is the transition metal- catalyzed Fenton reaction. Pentachlorophenol (PCP) has been widely used as a wood preservative. Using the salicylate hydroxylation assay and electron spin resonance (ESR) spin-trapping methods, we found that •OH can be produced by H2O2 and tetrachloro-1, 4-benzoquinone (TCBQ) (one of the major carcinogenic metabolites of PCP) independent of transition metal ions. Further studies showed that TCBQ, but not its corresponding semiquinone radical, the tetrachlorosemiquinone radical (TCSQ•), is essential for •OH production. The major reaction product between TCBQ and H2O2 was identified to be the ionic form of trichloro-hydroxy-1, 4-benzoquinone (TrCBQ-OH), and H2O2 was found to be the source and origin of the oxygen atom inserted into the reaction product TrCBQ-OH. Based on these data, we propose that •OH production by H2O2 and TCBQ is not through a semiquinone-dependent organic Fenton reaction, but rather through the following novel mechanism: a nucleophilic attack of H2O2 on TCBQ, forming a trichloro- hydroperoxyl-1, 4-benzoquinone (TrCBQ-OOH) intermediate, which decomposes homolytically to produce •OH. These findings represent a novel mechanism of •OH formation not requiring the involvement of redox-active transition metal ions, and may partly explain the potential carcinogenicity of widely used biocides such as PCP and other polyhalogenated aromatic environmental pollutants.

Potential Mechanism for Pentachlorophenol-Induced Carcinogenicity: A Novel Mechanism for Metal-Independent Production of Hydroxyl Radicals

The hydroxyl radical ((*)OH) has been considered to be one of the most reactive oxygen species produced in biological systems. It has been shown that (*)OH can cause DNA, protein, and lipid oxidation. One of the most widely accepted mechanisms for (*)OH production is through the transition metal-catalyzed Fenton reaction. Pentachlorophenol (PCP) was one of the most widely used biocides, primarily for wood preservation. PCP is now ubiquitously present in our environment and even found in people who are not occupationally exposed to it. PCP has been listed as a priority pollutant by the U.S. Environmental Protection Agency (EPA) and classified as a group 2B environmental carcinogen by the International Association for Research on Cancer (IARC). The genotoxicity of PCP has been attributed to its two major quinoid metabolites: tetrachlorohydroquinone and tetrachloro-1,4-benzoquinone (TCBQ). Although the redox cycling of PCP quinoid metabolites to generate reactive oxygen species is believed to play an important role, the exact molecular mechanism underlying PCP genotoxicity is not clear. Using the salicylate hydroxylation assay and electron spin resonance (ESR) secondary spin-trapping methods, we found that (*)OH can be produced by TCBQ and H(2)O(2) independent of transition metal ions. Further studies showed that TCBQ, but not its corresponding semiquinone radical, the tetrachlorosemiquinone radical (TCSQ(*)), is essential for (*)OH production. The major reaction product between TCBQ and H(2)O(2) was identified to be trichloro-hydroxy-1,4-benzoquinone (TrCBQ-OH), and H(2)O(2) was found to be the source and origin of the oxygen atom inserted into this reaction product. On the basis of these data, we propose that (*)OH production by TCBQ and H(2)O(2) is not through a semiquinone-dependent organic Fenton reaction but rather through the following novel mechanism: a nucleophilic attack of H(2)O(2) to TCBQ, leading to the formation of an unstable trichloro-hydroperoxyl-1,4-benzoquinone (TrCBQ-OOH) intermediate, which decomposes homolytically to produce (*)OH. These findings represent a novel mechanism of (*)OH formation not requiring the involvement of redox-active transition metal ions and may partly explain the potential carcinogenicity of the widely used biocides such as PCP and other polyhalogenated aromatic compounds.

Molecular mechanism for metal-independent production of hydroxyl radicals by hydrogen peroxide and halogenated quinones

We have shown previously that hydroxyl radicals (HO*) can be produced by H2O2 and halogenated quinones, independent of transition metal ions; however, the underlying molecular mechanism is still unclear. In the present study, using the electron spin resonance secondary radical spin-trapping method, we found that tetrachloro-1,4-benzoquinone (TCBQ), but not its corresponding semiquinone anion radical, the tetrachlorosemiquinone anion radical (TCSQ*-), is essential for HO* production. The major reaction product between TCBQ and H2O2 was identified by electrospray ionization quadrupole time-of-flight mass spectrometry to be the ionic form of trichlorohydroxy-1,4-benzoquinone (TrCBQ-OH), and H2O2 was found to be the source and origin of the oxygen atom inserted into the reaction product TrCBQ-OH. On the basis of these data, we propose that HO* production by H2O2 and TCBQ is not through a semiquinone-dependent organic Fenton reaction but rather through the following mechanism: a nucleophilic attack of H2O2 to TCBQ, forming a trichlorohydroperoxyl-1,4-benzoquinone (TrCBQ-OOH) intermediate, which decomposes homolytically to produce HO*. This represents a mechanism of HO* production that does not require redox-active transition metal ions.

A Straightforward Preparation of Fluorine‐Containing 1,2‐Dihydropyrimidines and Pyrimidines with 2,2‐Dihydropolyfluoroalkylaldehydes.

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A straightforward preparation of fluorine-containing 1,2-dihydropyrimidines and pyrimidines with 2,2-dihydropolyfluoroalkylaldehydes

The reactions of 2,2-dihydropolyfluoroalkylaldehydes with ammonia and enol ethers or carbonyl compounds are described. In the presence of zinc chloride, all reactions took place readily in THF at 50 °C to give fluorine-containing 1,2-dihydropyrimidines in moderate to good yields. Dehydrogenation of the resulting fluorine-containing 1,2-dihydropyrimidines with tetrachloro-1,4-benzoquinone (TCBQ) or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at room temperature afforded the corresponding fluorine-containing pyrimidines in good yields.

Photoinduced electron transfer at liquid|liquid interfaces: dynamics of the heterogeneous photoreduction of quinones by self-assembled porphyrin ion pairs.

The initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the water|1,2-dichloroethane (DCE) interface have been studied by ultrafast time-resolved spectroscopy and dynamic photoelectrochemical measurements. Photoexcitation of the water-soluble ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS(4)(-)) and zinc meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP(4+)) leads to a charge-separated state of the form ZnTPPS(3)(-)-ZnTMPyP(3+) within 40 ps. This charge-separated state is involved in the heterogeneous electron injection to acceptors in the organic phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estimations of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodynamic driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This analysis also suggests that the effective redox potential of BQ at the liquid|liquid boundary is shifted by 0.6 V toward positive potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is associated with the formation of hydrogen bonds at the liquid|liquid boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed.

Effects of Electrolytes in a Liquid Thin Layer System

The effects of electrolytes on electrochemical behavior from an oil thin layer interposed between a graphite electrode and an aqueous solution phase were examined. A hydrophobic electroactive species, tetrachloro-1,4-benzoquinone (TCQ), in a benzonitrile (EN) layer was employed to study ion transfer properties across the BN-water interface. Experimental results showed that hydrophobic cations as well as anions could be successfully used as ionic charge carriers. The addition of various salts into either the oil layers or the aqueous solutions offers deeper insight for the electrochemistry of the liquid thin layer system. When aqueous perchloric acid is interfaced with the BN films, the perchlorate ion of tetrahexylammonium perchlorate (THAP) substantially suppresses the dissociated proton concentration in the layer by the common ion effect while there is only a little change in the total acid concentration. Further approach by theoretical calculation makes it possible to quantitatively understand the effect of the electrolytes to the electrochemical responses of TCQ, which were previously reported (Anal. Chem. 73, 337 (2001)).

Metal-independent production of hydroxyl radicals by halogenated quinones and hydrogen peroxide: an ESR spin trapping study

The metal-independent production of hydroxyl radicals ( OH) from H 2O 2 and tetrachloro-1,4-benzoquinone (TCBQ), a carcinogenic metabolite of the widely used wood-preservative pentachlorophenol, was studied by electron spin resonance methods. When incubated with the spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO), TCBQ and H 2O 2 produced the DMPO/ OH adduct. The formation of DMPO/ OH was markedly inhibited by the OH scavenging agents dimethyl sulfoxide (DMSO), ethanol, formate, and azide, with the concomitant formation of the characteristic DMPO spin trapping adducts with CH 3, CH(CH 3)OH, COO −, and N 3, respectively. The formation of DMPO/ OH and DMPO/ CH 3 from TCBQ and H 2O 2 in the absence and presence, respectively, of DMSO was inhibited by the trihydroxamate compound desferrioxamine, accompanied by the formation of the desferrioxamine-nitroxide radical. In contrast, DMPO/ OH and DMPO/ CH 3 formation from TCBQ and H 2O 2 was not affected by the nonhydroxamate iron chelators bathophenanthroline disulfonate, ferrozine, and ferene, as well as the copper-specific chelator bathocuproine disulfonate. A comparative study with ferrous iron and H 2O 2, the classic Fenton system, strongly supports our conclusion that OH is produced by TCBQ and H 2O 2 through a metal-independent mechanism. Metal-independent production of OH from H 2O 2 was also observed with several other halogenated quinones.