Photoreactions of p-benzo-, p-naphtho- and p-anthraquinones with ascorbic acid.

The photoreduction of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives, e.g. dimethylBQ, trimethylBQ, duroquinone, bromoNQ, methoxyNQ, methylAQ and dimethylAQ in acetonitrile-water by ascorbate was studied by time-resolved UV-vis spectroscopy using 20 ns laser pulses at 308 nm and continuous 254 nm irradiation. The semiquinone radical (*QH/Q*(-)) is formed after H-atom transfer from ascorbate to the quinone triplet state. The rate constant for quenching is k(q)=(2-9) x 10(9) M(-1) s(-1). Termination of the radicals takes place in the micros-ms range. The results are compared with those initiated by electron transfer from DABCO under similar conditions, where the k(q) values are similar, but the termination of Q*(-) takes place by electron back transfer not yielding hydroquinones. Specific properties of the quinone triplet state, e.g. self-quenching, nucleophilic water addition and the effects of structure are discussed.     

Formation of electronically excited states during the interaction of p-benzoquinone with hydrogen peroxide

The reaction between p-benzoquinone and H₂O₂ in slightly alkaline solutions yields three major quinoid products that accumulate in the reaction mixture: (a) 2,3-epoxy-p-benzoquinone, (b) 2-hydroxy-p-benzoquinone and (c) p-benzohydroquinone. The reaction is accompanied by photoemission, probably originating from excited triplet 2-hydroxy-p-benzoquinone. These products originate from hydrogen peroxide and hydroxide nucleophilic addition to the C₂?C₃ double bond, as well as secondary redox interactions. The hydroxy substituent and the epoxide ring exert a substantial influence on the electronic distribution in the p-benzoquinone molecule leading to a decrease in the half-wave potential, as compared to the parent p-benzoquinone. The generation of electronically excited states is the result of reactions secondary to the nucleophilic additions involving 2-hydroxy-p-benzosemiquinone, H₂O₂ and hydroxyl radical. The process involves the primary oxidation of 2-hydroxy-p-benzosemiquinone by hydrogen peroxide, followed by oxidation of the semiquinone by hydroxyl radical leading to the formation of the electronically excited quinone. The decay of the excited triplet to the ground state is accompanied by photoemission with maximal intensity at 485–530 nm. Thermodynamic calculations along with an observed increase of photoemission intensity in anaerobiosis point to the triplet (n, π*) multiplicity of the excited state. The efficiency of chemiluminescence could be calculated as 10^?8 photons/2-hydroxy-p-benzoquinone molecule formed. Photoemission arising from the p-benzoquinone/H₂O₂ reaction was inhibited efficiently by addition of GSH to the reaction mixture. This may be due to deactivation of the triplet quinone by a 2-glutathionyl-p-benzohydroquinone adduct, involving thioether α-hydrogen atom-transfer to the triplet ketone.     

Studies on Effects of Certain Quinones: II. Photosynthetic Incorporation of CO(2) by Chlorella

The effects of various quinone herbicides and fungicides on the photosynthetic ¹⁴CO₂ fixation and the incorporation of ¹⁴C among the products of photosynthesis in Chlorella pyrenoidosa was investigated. Addition of 30 μm 2,3-dichloro-1,4-naphthoquinone (dichlone), 2-amino-3-chloro-1,4-naphthoquinone (06K-quinone), or 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) inhibited CO₂ fixation, whereas 1,4-benzoquinone had no effect. Treatment with 3 μm or higher concentrations of dichlone, 06K-quinone or 1,4-benzoquinone also produced marked changes in the pattern of ¹⁴C distribution. A noticeable effect was an increase in the proportion of ¹⁴C in sucrose and glycine accompanied by a reduction in ¹⁴C lipids and glutamic acid. These changes appear to occur as a result of shifts in the flow of carbon along various biosynthetic pathways of photosynthetic CO₂ fixation. It is suggested that inactivation of coenzyme A and shortage of reduced triphosphopyridine nucleotide in the quinone-treated cells inhibited the synthesis of lipids and glutamic acid, thereby diverting more carbon into sucrose and glycine.     

β-D-Ribofuranosyl-1,4-benzoquinone - antibacterial agent with showdomycin-like mode of action

β-D-Ribofuranosyl-1,4-benzoquinone is toxic in wild-type E. coli while mutants deficient in constitutive nucleoside, permease are resistant; the toxic action may be abolished by 2-chloro-2-deoxyuridine known to inhibit nucleoside permease. α-D-Ribofuranosyl-1,4-benzoquinone and 4(β-D-ribofuranosyl)-1,2-benzoquinone are inactive. 1,4-Dihydroxy-2-β-D-ribofuranosylbenzene does not interact with nucleoside permease. It appears that nucleoside analogs with 1,4-benzoquinone ring are transported by nucleoside permease and their mode of action resembles that of showdomycin.     

Interaction of electron acceptors with thylakoids from halophytic and non-halophytic species

Thylakoid membranes isolated from halophytic species showed differences in their interactions with ionic and lipophilic electron acceptors when compared to thylakoids from non-halophytes. FeCN was considerably less efficient as electron acceptor with halophyte thylakoids, supporting much lower rates of O₂ evolution and having a lower affinity. FeCN accepted electrons at a different, DMMIB insensitive, site with these thylakoids. 1,4-Benzo-quinones with less positive midpoint potentials were less effective in accepting electrons from halophyte thylakoids compared to nonhalophyte thylakoids, also reflected in lower rates of O₂ evolution and lower affinity. Considering the lipolphilic nature and the fact that there was no apparent change in the site donating electrons to the quinones, an alteration in the midpoint potential of this site by about +100mV is postulated for the halophyte thylakoids.Abbreviations AMPD 2-amino-2-methyl-1,3-propanediol - Cyt b₆/f cytochrome b₆/f complex - DBMIB 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone - DCBQ 2,6-dichloro-1,4-benzoquinone - DCIP 2,6-dichlorophenol-indolphenol - DMBQ 2,5-dimethyl-1,4-benzoquinone - E_m7 midpoint redox potential at pH 7.0, FeCN-K₃Fe(CN)₆ - HNQ 5-hydroxy-1,4-naphthoquinone - MV methylviologen - NQ 1,4-naphthoquinone - PBQ phenyl-1,4-benzoquinone - PC plastocyanin - PQ plastoquinone     

Unfolding pathway and intermolecular interactions of the cytochrome subunit in the bacterial photosynthetic reaction center

Precise folding of photosynthetic proteins and organization of multicomponent assemblies to form functional entities are fundamental to efficient photosynthetic electron transfer. The bacteriochlorophyll b-producing purple bacterium Blastochloris viridis possesses a simplified photosynthetic apparatus. The light-harvesting (LH) antenna complex surrounds the photosynthetic reaction center (RC) to form the RC-LH1 complex. A non-membranous tetraheme cytochrome (4Hcyt) subunit is anchored at the periplasmic surface of the RC, functioning as the electron donor to transfer electrons from mobile electron carriers to the RC. Here, we use atomic force microscopy (AFM) and single-molecule force spectroscopy (SMFS) to probe the long-range organization of the photosynthetic apparatus from Blc. viridis and the unfolding pathway of the 4Hcyt subunit in its native supramolecular assembly with its functional partners. AFM images reveal that the RC-LH1 complexes are densely organized in the photosynthetic membranes, with restricted lateral protein diffusion. Unfolding of the 4Hcyt subunit represents a multi-step process and the unfolding forces of the 4Hcyt α-helices are approximately 121 picoNewtons. Pulling of 4Hcyt could also result in the unfolding of the RC L subunit that binds with the N-terminus of 4Hcyt, suggesting strong interactions between RC subunits. This study provides new insights into the protein folding and interactions of photosynthetic multicomponent complexes, which are essential for their structural and functional integrity to conduct photosynthetic electron flow.     

Cyclin E and cyclin A are likely targets of Src for PDGF-induced DNA synthesis in fibroblasts

The effects of benzoquinone analogues, phenyl-1,4-benzoquinone (PBQ) and 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (DBMIB), on state transitions in Synechocystis sp. PCC 6803 were investigated. PBQ induced a transition from state 2 to state 1 in the absence of actinic light whereas DBMIB caused a state 2 transition. 3-(3,4-Dichlorophenyl)-1,1-dimethyl urea could not eliminate the effects of PBQ and DBMIB. These results imply that the redox state of the plastoquinone pool controls the state transitions in vivo and cytochrome b6f complex is involved in this process. As a working hypothesis, we propose that the occupancy of the quinol oxidation site and the movement of the Rieske protein may be pivotal in this regulation.     

Quinone-induced inhibition of urease: elucidation of its mechanisms by probing thiol groups of the enzyme

In this work we studied the reaction of four quinones, 1,4-benzoquinone (1,4-BQ), 2,5-dimethyl-1,4-benzoquinone (2,5-DM-1,4-BQ), tetrachloro-1,4-benzoquinone (TC-1,4-BQ) and 1,4-naphthoquinone (1,4-NQ) with jack bean urease in phosphate buffer, pH 7.8. The enzyme was allowed to react with different concentrations of the quinones during different incubation times in aerobic conditions. Upon incubation the samples had their residual activities assayed and their thiol content titrated. The titration carried out with use of 5,5'-di-thiobis(2-nitrobenzoic) acid was done to examine the involvement of urease thiol groups in the quinone-induced inhibition. The quinones under investigation showed two distinct patterns of behaviour, one by 1,4-BQ, 2,5-DM-1,4-BQ and TC-1,4-BQ, and the other by 1,4-NQ. The former consisted of a concentration-dependent inactivation of urease where the enzyme-inhibitor equilibrium was achieved in no longer than 10min, and of the residual activity of the enzyme being linearly correlated with the number of modified thiols in urease. We concluded that arylation of the thiols in urease by these quinones resulting in conformational changes in the enzyme molecule is responsible for the inhibition. The other pattern of behaviour observed for 1,4-NQ consisted of time- and concentration-dependent inactivation of urease with a nonlinear residual activity-modified thiols dependence. This suggests that in 1,4-NQ inhibition, in addition to the arylation of thiols, operative are other reactions, most likely oxidations of thiols provoked by 1,4-NQ-catalyzed redox cycling. In terms of the inhibitory strength, the quinones studied formed a series: 1,4-NQ approximately 2,5-DM-1,4-BQ<1,4-BQ相似文献   

Photosystem I with benzoquinone analogues incorporated into the A<Subscript>1</Subscript> binding site

Time-resolved FTIR difference spectroscopy has been used to study photosystem I (PSI) particles with three different benzoquinones [plastoquinone-9 (PQ), 2,6-dimethyl-1,4-benzoquinone (DMBQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (Cl₄BQ)] incorporated into the A₁ binding site. If PSI samples are cooled in the dark to 77 K, the incorporated benzoquinones are shown to be functional, allowing the production of time-resolved (P700⁺A₁^??P700A₁) FTIR difference spectra. If samples are subjected to repetitive flash illumination at room temperature prior to cooling, however, the time-resolved FTIR difference spectra at 77 K display contributions typical of the P700 triplet state (³P700), indicating a loss of functionality of the incorporated benzoquinones, that occurs because of double protonation of the incorporated benzoquinones. The benzoquinone protonation mechanism likely involves nearby water molecules but does not involve the terminal iron–sulfur clusters F_A and F_B. These results and conclusions resolve discrepancies between results from previous low-temperature FTIR and EPR studies on similar PSI samples with PQ incorporated.     

Metabolite profiling of tubers of an early- and a late-maturing potato line and their grafts

Metabolomics - The 2,6-dichloro-1,4-benzoquinone (DCBQ) and its derivative 2,6-dichloro-3-hydroxy-1,4-benzoquinone (DCBQ-OH) are disinfection by-products (DBPs) and emerging pollutants in the...     

Characterization and Genetic Analysis of Mutant Strains of Escherichia coli K-12 Accumulating the Ubiquinone Precursors 2-Octaprenyl-6-Methoxy-1,4-Benzoquinone and 2-Octaprenyl-3-Methyl-6-Methoxy-1,4-Benzoquinone

The ubiquinone precursors, 2-octaprenyl-6-methoxy-1,4-benzoquinone and 2-octaprenyl-3-methyl-6-methoxy-1,4-benzoquinone, were isolated from ubiquinone-deficient mutants of Escherichia coli and identified by nuclear magnetic resonance and mass spectrometry. Mutants accumulating 2-octaprenyl-6-methoxy-1,4-benzoquinone and 2-octaprenyl-3-methyl-6-methoxy-1,4-benzoquinone were shown to carry mutations in genes designated ubiE and ubiF, respectively. The ubiE gene was shown to be cotransducible with metE (minute 75) and close to two other genes concerned with ubiquinone biosynthesis. The ubiF gene was located close to minute 16 by cotransduction with the lip, gltA, and entA genes.     

Studies on Effect of Certain Quinones: I. Electron Transport, Photophosphorylation, and CO(2) Fixation in Isolated Chloroplasts

The effect of quinone herbicides and fungicides on photosynthetic reactions in isolated spinach (Spinacia oleracea) chloroplasts was investigated. 2,3-Dichloro-1,4-naphthoquinone (dichlone), 2-amino-3-chloro-1,4-naphthoquinone (06K-quinone), and 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) inhibited ferricyanide reduction as well as ATP formation. Benzoquinone had little or no effect on these reactions. The two reactions showed a differential sensitivity to these inhibitors. Dichlone was a strong inhibitor of both photosystems I and II; photosystem I was more sensitive to 06K-quinone than was photosystem II, whereas the reverse was true of chloranil. Chloranil and 06K-quinone inhibited ferricyanide reduction and the coupled photophosphorylation to the same extent, whereas dichlone affected photophosphorylation to a greater extent than the ferricyanide reduction.     

Males of Hylamorpha elegans burmeister (Coleoptera: Scarabaeidae) are attracted to odors released from conspecific females

The behavioral responses of Hylamorpha elegans L. (Coleoptera: Scarabaeidae, Rutelinae) to the semiochemicals released from conspecific individual adults were studied, with particular attention paid to female attraction of males. Odors released from virgin females significantly attracted male conspecifics in both the field and laboratory olfactometer and wind tunnel bioassays. However, females did not attract other females, and males attracted no one. The response of male H. elegans to (1) compounds (1,4-hydroquinone and 1,4-benzoquinone) released only by unmated females; (2) the essential oil of the secondary host (Nothofagus obliqua); and (3) the blend of 1,4-hydroquinone and 1,4-benzoquinone with N. obliqua essential oil was studied. The blend of 1,4-benzoquinone mixed with essential oil at the trial concentration was attractive with males. The same response was found with 1,4-hydroquinone alone. The essential oil did not have the expected attractant effect on conspecific males. These results suggest that, when combined with essential oil, 1,4-benzoquinone may function in the sexual behavior of males and females. These findings are discussed in terms of the ecological role of this putative sexual pheromone and its potential use in a strategy of control of this pest.     

What is beta-carotene doing in the photosystem II reaction centre?

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.     

Photo-oxidation of di-n-butylsulfide by various electron transfer sensitizers in oxygenated acetonitrile.

The selective activation of different photosensitizers has been carried out under comparable conditions and their efficiency towards di-n-butylsulfide oxidation in oxygenated acetonitrile compared from the product distribution after 150 minutes of irradiation. As expected, the best selectivity towards sulfoxide is obtained with a conventional energy transfer sensitizer such as Rose Bengal (RB), but also with a quinone with a low-lying triplet state, 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil or CHLO) and with 9,10-dicyanoanthracene (DCA). More significant yields in sulfonic and sulfuric acids are obtained under sensitization with 9,10-anthraquinone (ANT) or a derivative of benzophenone, 4-benzoyl benzoic acid (4-BB), with which additional experiments were carried out in order to discuss the involvement of either singlet oxygen or superoxide radical anion. Triphenyl pyrylium tetrafluoroborate (TPT+) is inefficient under the selected conditions and sulfide photo-oxidation can only be achieved with higher TPT+ concentrations with simultaneous total TPT+ bleaching. With TPT+, 1,2,4,5-tetracyanobenzene (TCNB) and TiO2, the product distribution and the low selectivity as well as the formation of numerous common by-products are indicative of radical mechanisms. All these results are discussed according to the possible formation of activated oxygen species, such as singlet oxygen, superoxide radical anion or alkylperoxy radicals.     

Thymoquinone,a biologically active component of Nigella sativa,induces mitochondrial production of reactive oxygen species and programmed death of tumor cells

Biophysics - Mechanisms of tumor-cell responses to 2-isopropyl-5-methyl-1,4-benzoquinone (thymoquinone) and 1,4-benzoquinone were studied using fluorescence and the inhibition assay. It was shown...     

The formation and characterization of copper(II) 1,4-benzenediolate and p-semiquinonate complexes

Strongly oxidizing p-quinones such as tetrachloro-1,4-benzoquinone and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone undergo stepwise oxidative addition reactions with copper(I) chloride and bromide in pyridine resulting in copper(II) p-semiquinone and dinuclear copper(II) 1,4-benzenediolate pyridine complexes.     

Double mutant studies identify electrostatic interactions that are important for docking cytochrome c2 onto the bacterial reaction center

Cytochrome c2 (cyt) is the mobile electron donor to the reaction center (RC) in photosynthetic bacteria. The electrostatic interactions involved in the dynamics of docking of cyt onto the RC were examined by double mutant studies of the rates of electron transfer between six modified Rhodobacter sphaeroides RCs in which negatively charged acid residues were replaced with Lys and five modified Rhodobacter capsulatus Cyt c2 molecules in which positively charged Lys residues were replaced with Glu. We measured the second-order rate constant, k2, for electron transfer from the reduced cyt to the oxidized primary donor on the RC, which reflects the energy of the transition state for the formation of the active electron transfer complex. Strong interactions were found between Lys C99 and Asp M184/Glu M95, and between Lys C54 and Asp L261/Asp L257. The interacting residues were found to be located close to each other in the recently determined crystal structure of the cyt-RC complex [Axelrod, H., et al. (2002) J. Mol. Biol. (in press)]. The interaction energies were approximately inversely proportional to the distances between charges. These results support earlier suggestions [Tetreault, M., et al. (2001) Biochemistry 40, 8452-8462] that the structure of the transition state in solution resembles the structure of the cyt-RC complex in the cocrystal and indicate that specific electrostatic interactions facilitate docking of the cyt onto the RC in a configuration optimized for both binding and electron transfer. The specific interaction between Asp M184 and Lys C99 may help to nucleate short-range hydrophobic contacts.     

Population of the triplet states of bacteriorhodopsin and of related model compounds by intramolecular energy transfer

In variance with chlorophyll-based photosynthetic pigments, the triplet states of rhodopsins, either visual or photosynthetic, have not been observed experimentally. This is due to the ultrafast crossing from S1 to S0, which effectively competes with intersystem crossing to the triplet (T1) state. In order to populate T1 indirectly, laser photolysis experiments are performed with model protonated Schiff bases of retinal in solution, in which both inter- and intramolecular energy transfer to the polyene chromophore are carried out from an appropriate triplet energy donor. The experiments are then extended to bacteriorhodopsin (bR) by detaching the native retinal chromophore from the protein-binding site and replacing it by an analogous (synthetic) protonated Schiff base polyene, attached in a nonconjugated way to a naphthone triplet donor. Pulsed laser excitation of the latter moiety led, for the first time, to the observation of the triplet state of a rhodopsin. Possible locations and roles of the T1 state in bR and in visual pigments are discussed briefly.     

Different mechanisms of the binding of soluble electron donors to the photosynthetic reaction center of Rubrivivax gelatinosus and Blastochloris viridis

The tetraheme cytochrome subunits of the photosynthetic reaction centers (RCs) in two species of purple bacteria, Rubrivivax gelatinosus and Blastochloris (Rhodopseudomonas) viridis, were compared in terms of their capabilities to bind different electron-donor proteins. The wild-type RCs from both species and mutated forms of R. gelatinosus RCs (with amino acid substitutions introduced to the binding domain for electron-donor proteins) were tested for their reactivity with soluble cytochromes and high potential iron-sulfur protein. Cytochromes from both species were good electron donors to the B. viridis RC and the R. gelatinosus RC. The reactivity in the R. gelatinosus RC showed a clear dependence on the polarity of the charges introduced to the binding domain, indicating the importance of the electrostatic interactions. In contrast, high potential iron-sulfur protein, presumed to operate according to the hydrophobic mechanism of binding, reacted significantly only with the R. gelatinosus RC. Evolutionary substitution of amino acids in a region of the binding domain on the cytochrome subunit surface probably caused the change in the principal mode of protein-protein interactions in the electron-transfer chains.