Showing papers on "Quinone published in 1999"

Relation of structure of curcumin analogs to their potencies as inducers of Phase 2 detoxification enzymes.

Abstract: A series of naturally occurring as well as synthetic structural analogs of the dietary constituent curcumin were examined in order to elucidate which portions of the molecule are critical for the ability to induce Phase 2 detoxification enzymes in murine hepatoma cells, and hence to assess the chemoprotective potential of these compounds. Two groups of compounds were studied: classical Michael reaction acceptors such as curcumin and related beta-diketones such as dibenzoylmethane which lack direct Michael reactivity. The presence of two structural elements was found to be required for high inducer potency: (i) hydroxyl groups at ortho-position on the aromatic rings and (ii) the beta-diketone functionality. All curcuminoids elevate the specific activity of quinone reductase in both wild type and mutant cells defective in either the aryl hydrocarbon (Ah) receptor or cytochrome P4501A1 activity. This indicates that neither binding to this receptor, nor metabolic activation by P4501A1 are required for the signaling process originating from this family of electrophiles and ultimately resulting in Phase 2 enzyme induction.

Visualization of Dioxygen Bound to Copper During Enzyme Catalysis

Abstract: X-ray crystal structures of three species related to the oxidative half of the reaction of the copper-containing quinoprotein amine oxidase from Escherichia coli have been determined. Crystals were freeze-trapped either anaerobically or aerobically after exposure to substrate, and structures were determined to resolutions between 2.1 and 2.4 angstroms. The oxidation state of the quinone cofactor was investigated by single-crystal spectrophotometry. The structures reveal the site of bound dioxygen and the proton transfer pathways involved in oxygen reduction. The quinone cofactor is regenerated from the iminoquinone intermediate by hydrolysis involving Asp383, the catalytic base in the reductive half-reaction. Product aldehyde inhibits the hydrolysis, making release of product the rate-determining step of the reaction in the crystal.

One-electron oxidation of quercetin and quercetin derivatives in protic and non protic media

Abstract: Quercetin (3,3′,4′,5,7-pentahydroxyflavone) and quercetin derivatives (3-methylquercetin, rutin) are strong flavonoid antioxidants abundant in plants and in human diet. Their oxidation by DPPH, CAN or dioxygen (autoxidation) is studied in protic and non protic solvents. From kinetic investigations by UV–visible spectroscopy, oxidation rate constants are estimated. Fast disproportionation of flavonoid radicals is shown to give quinones which can be identified by their adducts with methanol (quercetin quinone) or sodium benzenesulfinate (rutin quinone). In strongly alkaline non aqueous conditions, the quercetin quinone can also be evidenced by strong charge transfer absorption bands in the range 700–800 nm.The consequences of these observations for the antioxidant properties of quercetin and quercetin derivatives are discussed.

Alkylation of Nucleic Acids by a Model Quinone Methide

Abstract: Quinone methides and related electrophiles represent a common class of intermediates that form during metabolism of drugs and xenobiotics and may lead to DNA alkylation. The intrinsic reactivity of these species has now been characterized using a stable model compound, O-(tert-butyldimethylsilyl)-2-bromomethylphenol, designed to generate an o-quinone methide in the presence of fluoride. The resulting deoxynucleoside adducts were assigned unambiguously through use of two-dimensional NMR and, in particular, heteronuclear multiple-bond connectivity (HMBC). Both purines, dG and dA, reacted at their exo-amino groups. In contrast, dC had previously been shown to react at its cyclic N3 position [Rokita, S. E.; Yang, J.; Pande, P.; Greenberg, W. A. J. Org. Chem. 1997, 62, 3010−3012], and the relatively nonnucleophilic T remained inert under all conditions examined. Surprisingly, the efficiency of cytosine modification exceeded that of adenine and guanine by more than 10-fold in competition studies with the deoxym...

Crystal structure of human quinone reductase type 2, a metalloflavoprotein.

Abstract: In mammals, two separate but homologous cytosolic quinone reductases have been identified: NAD(P)H:quinone oxidoreductase type 1 (QR1) (EC 1.6.99.2) and quinone reductase type 2 (QR2). Although QR1 and QR2 are nearly 50% identical in protein sequence, they display markedly different catalytic properties and substrate specificities. We report here two crystal structures of QR2: in its native form and bound to menadione (vitamin K3), a physiological substrate. Phases were obtained by molecular replacement, using our previously determined rat QR1 structure as the search model. QR2 shares the overall fold of the major catalytic domain of QR1, but lacks the smaller C-terminal domain. The FAD binding sites of QR1 and QR2 are very similar, but their hydride donor binding sites are considerably different. Unexpectedly, we found that QR2 contains a specific metal binding site, which is not present in QR1. Two histidine nitrogens, one cysteine thiol, and a main chain carbonyl group are involved in metal coordinat...

An Experimental and Theoretical Study of the Substituent Effects on the Redox Properties of 2-[(R-phenyl)amine]-1,4-naphthalenediones in Acetonitrile.

Abstract: We synthesized and analyzed 19 compounds of 3'- (meta-) and 4'- (para-) substituted 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) R = p-MeO, p-Me, p-Bu, p-Hex, p-Et, m-Me, m-Et, H, p-Cl, p-Br, m-F, m-Cl, p-COCH(3), m-CN, m-NO(2), m-COOH, and p-COOH. Despite the fact that the nitrogen atom, which binds the quinone with the meta- and para-substituted ring, interferes with the direct conjugation between both rings, the UV-vis spectra of these compounds show the existence of an intramolecular electronic transfer from the respective aniline to the p-naphthoquinone moiety. In accordance with this donor-acceptor character, the cyclic voltammograms of these compounds exhibit two, one-electron reduction waves corresponding to the formation of radical-anion and dianion, where the half-wave potential values vary linearly with the Hammett constants (sigma(x)). The analysis of the different voltammetric parameters (e.g., voltammetric function, anodic/cathodic peak currents ratio, and the separation between the anodic and cathodic potential peaks) show that with the exception of the carboxylic PAN derivatives, all compounds present the same reduction pathway. We investigated the molecular and electronic structures of these compounds using the semiempirical PM3 method and, within the framework of the Density Functional Theory, using the Becke 3LYP hybrid functional with a double zeta split valence basis set. Our theoretical calculations predict that, with the exception of the p-nitro compound, all the compounds are planar molecules where the conjugation degree of the nitrogen lone pair with the quinone system depends on the position and magnitude of the electronic effect of the substituent in the aniline ring. The Laplacians of the critical points (nabla(2)rho), for the C-O bonds, show that the first reduction wave corresponds to the carbonyl group in alpha-position to the aniline, and that the second one-electron transfer is due to the C(4)-O(2) carbonyl reduction. Thus, the higher reaction constant value (rho) obtained for the second one-electron transfer is due to the fact that the displacement of the nonshared electrons of the amino nitrogen merely modifies the electron density of C(4)-O(2) bond. The positive correlation between the LUMO energy values calculated for these compounds and the E(1/2) potentials corresponding to the C(1)-O(1) carbonyl reduction show that the electron addition takes place at the lowest unoccupied molecular orbital, supporting the fact that this wave is also prone to the substituent effect.

Oxidation of ochratoxin A by an Fe-porphyrin system: model for enzymatic activation and DNA cleavage.

Abstract: Ochratoxin A (OTA, 1) is a fungal toxin that facilitates single-strand DNA cleavage, DNA adduction, and lipid peroxidation when metabolically activated. To model the enzymatic activation of OTA, we have employed the water-soluble iron(III) meso-tetrakis(4-sulfonatophenyl)porphyrin (FeTPPS) oxidation system. In its presence, OTA has been found to facilitate single-strand cleavage of supercoiled plasmid DNA through production of reactive oxygen species (ROS) (i.e., the hydroxyl radical, HO(*)). The reaction of OTA with the FeTPPS oxidation system also generated three hydroxylated products (chlorine atom still attached), which was taken as evidence for production of the known hydroxylated metabolites (2-4) of OTA. This result suggested that the FeTPPS system served as a reasonable model for the enzymatic activation of OTA. When the reaction of OTA with FeTPPS was carried out in the presence of excess hydrogen peroxide (H(2)O(2)) and sodium ascorbate, a hydroquinone species (OTHQ, 5) was detected in which an OH group has replaced the chlorine atom of OTA. The production of OTHQ (5) was dependent on the presence of the reducing agent, sodium ascorbate, which suggested that the oxidation catalyst furnished the quinone derivative OTQ (6) that was subsequently reduced to OTHQ (5) by ascorbate. Utilizing a synthetic sample of OTHQ (5), the hydroquinone was found to undergo autoxidation with a t(1/2) of 11.1 h at pH 7.4, and to possess a pK(a) value of 8.03 for the phenolic oxygen ortho to the carbonyl groups. Our findings imply that the hydroquinone (OTHQ) and quinone (OTQ) metabolites of OTA have the ability to cause alkylation/redox damage and have allowed us to propose a viable pathway for oxidative damage by OTA.

Nonlinear Solvent Water Effects in the Excited-State (Formal) Intramolecular Proton Transfer (ESIPT) in m-Hydroxy-1,1-diaryl Alkenes: Efficient Formation of m-Quinone Methides1

Abstract: The photohydration of hydroxy-substituted 1,1-diaryl alkenes 1−3 has been studied in aqueous CH3CN solution. Evidence for formation of quinone methide intermediates was provided by product studies and by observation of its absorption spectrum by laser flash photolysis. For the meta isomers, the proposed mechanism of m-quinone methide formation probably involves a solvent-mediated (“proton-relay”) excited-state (formal) intramolecular proton transfer (ESIPT) from the phenol hydroxyl group to the β-carbon of the alkene moiety in neutral aqueous CH3CN solution, either in a concerted manner or via two very fast steps. The m-quinone methides are then trapped by water to form the corresponding diaryl ethanol product with high overall quantum yield. Evidence for the ESIPT pathway was provided by fluorescence and LFP measurements. The addition of small amounts of water (<0.8 M in CH3CN) decreased the fluorescence emissions of 1 and 2 with a concomitant increase in production of m-quinone methides. Stern−Volmer an...

Biochemical and electrochemical characterization of quinohemoprotein amine dehydrogenase from Paracoccus denitrificans.

Abstract: A new quinohemoprotein amine dehydrogenase from Paracoccus denitrificans IFO 12442 was isolated and characterized in views of biochemistry and electrochemistry. This enzyme exists in periplasm and catalyzes the oxidative deamination of primary aliphatic and aromatic amines. n-Butylamine or benzylamine as a carbon and energy source strongly induces the expression of the enzyme. Carbonyl reagents inhibit the enzyme activity irreversibly. This enzyme is a heterodimer constituted of alpha and beta subunits with the molecular mass of 59.5 and 36.5 kDa, respectively. UV-vis and EPR spectroscopy, and the quinone-dependent redox cycling and heme-dependent peroxidative stains of SDS-PAGE bands revealed that the alpha subunit contains one quinonoid cofactor and one heme c per molecule, while the beta subunit has no prosthetic group. The redox potential of the heme c moiety was determined to be 0.192 V vs NHE at pH 7.0 by a mediator-assisted continuous-flow column electrolytic spectroelectrochemical technique. The analysis of the substrate titration curve allowed the evaluation of the redox potential of the quinone/semiquinone and semiquinone/quinol redox couples as 0.19 and 0.11 V, respectively.

Oxidation of polycyclic aromatic hydrocarbons catalyzed by soybean peroxidase

Abstract: Soybean peroxidase (SBP) catalyzes the oxidation of a variety of polycyclic aromatic hydrocarbons (PAHs) in the presence of water-miscible organic cosolvents, including acetonitrile, tetrahydrofuran, and dimethylformamide (DMF). Oxidation was optimal at pH 2.0–2.5, with substantially lower reactivity at pH 1.5 as well as at pH > 3.0. Despite the low pH activity optimum, SBP had an observed half-life of 120 h at pH 2.5. Conversions of greater than 90% were observed with anthracene and 9-methylanthracene in the presence of 50% (v/v) DMF. Anthracene oxidation yielded exclusively anthraquinone, thereby demonstrating that SBP catalyzes a formal six-electron oxidation of the unactivated aromatic substrate to the quinone. A mechanism is proposed to account for this reaction that includes the initial one-electron oxidation of the PAH followed by addition of water to the oxidized PAH. 9-Methylanthracene was more reactive than anthracene, and its enzymatic oxidation yielded two products: anthraquinone and 9-methanol-9,10-dihydroanthracene. The former product indicates that loss of the methyl group occurs during enzymatic oxidation. These results suggest that SBP could be useful in the conversion of PAHs into more environmentally benign materials.

Excited-State Electron Transfer in a Chromophore-Quencher Complex. Spectroscopic Identification of a Redox-Separated State.

Abstract: In the chromophore-quencher complex fac-[Re(Aqphen)(CO)(3)(py-PTZ)](+) (Aqphen is 12,17-dihydronaphtho[2,3-h]dipyrido[3,2-a:2',3'-c]-phenazine-12,17-dione; py-PTZ is 10-(4-picolyl)phenothiazine), Aqphen is a dppz derivative, containing a pendant quinone acceptor at the terminus of a rigid ligand framework. This introduces a third, low-lying, ligand-based pi acceptor level localized largely on the quinone fragment. Laser flash excitation of fac-[Re(Aqphen)(CO)(3)(py-PTZ)](+) (354.7 nm; in 1,2-dichloroethane) results in the appearance of a relatively long-lived transient that decays with tau(298K) = 300 ns (k = 3.3 x 10(6) s(-)(1)). Application of transient absorption, time-resolved resonance Raman, and time-resolved infrared spectroscopies proves that this transient is the redox-separated state fac-[Re(I)(Aqphen(*)(-)())(CO)(3)(py-PTZ(*)(+)())](+) in which the excited electron is localized largely on the quinone portion of the Aqphen ligand.

Vibrational modes of ubiquinone in cytochrome bo3 from escherichia coli identified by fourier transform infrared difference spectroscopy and specific 13c labeling

Abstract: In this study we present the infrared spectroscopic characterization of the bound ubiquinone in cytochrome bo(3) from Escherichia coli. Electrochemically induced Fourier transform infrared (FTIR) difference spectra of DeltaUbiA (an oxidase devoid of bound ubiquinone) and DeltaUbiA reconstituted with ubiquinone 2 and with isotopically labeled ubiquinone 2, where (13)C was introduced either at the 1- or at the 4-position of the ring (C=O groups), have been obtained. The vibrational modes of the quinone bound to the discussed high-affinity binding site (Q(H)) are compared to those from the synthetic quinones in solution, leading to the assignment of the C=O modes to a split signal at 1658/1668 cm(-)(1), with both carbonyls similarly contributing. The FTIR spectra of DeltaUbiA reconstituted with the labeled quinones indicate an essentially symmetrical and weak hydrogen bonding of the two C=O groups from the neutral quinone with the protein and distinct conformations of the 2- and 3-methoxy groups. Perturbations of the vibrational modes of the 5-methyl side groups are discussed for a signal at 1452 cm(-)(1). Only negligible shifts of the aromatic ring modes can be reported for the reduced and the protonated form of the quinone. Alterations of the protein upon quinone binding are reflected in the electrochemically induced FTIR difference spectra. In particular, difference signals at 1640-1633 cm(-)(1) and 1700-1670 cm(-)(1) indicate variations of beta-sheet secondary structure elements and loops, bands at 1706 and 1678 cm(-)(1) are tentatively attributed to individual amino acids, and a difference signal a 1540 cm(-)(1) is discussed to reflect an influence on C=C modes of the porphyrin ring or on deprotonated propionate groups of the hemes. Further tentative assignments are presented and discussed. The (13)C labeling experiments allow the assignment of the vibrational modes of a bound ubiquinone 8 in the electrochemically induced FTIR difference spectra of wild-type bo(3).

Molecular Batteries Based on Carbon-Carbon Bond Formation and Cleavage in Titanium- and Vanadium-Schiff Base Complexes.

Abstract: A novel mode of storing and releasing electrons, based on the reversible formation and cleavage of C−C bonds, has been created. The C−C bonds formed by the reductive coupling of imino groups across two [M(salophen)] complexes (see structure) function as shuttle for two electrons, permitting long-range electron transfer to a variety of substrates (e.g. quinone, dioxygen, and azides). This electron transfer is mediated by the metal, which becomes the reactive site, while the C–C functionality is never directly involved.

Mechanism of the photochemical transformation of naphthalene in water

Abstract: Upon irradiation in air-saturated aqueous solution, naphthalene is mainly converted into 7-hydroxy-1,4-naphthoquinone, 2-formylcinnamaldehyde and 2-carboxycinnamaldehyde. The quantum yield of photolysis is equal to 0.0025±0.0005. Naphthalene is likely to be phototransformed via monophotonic ionization that occurs with a quantum yield equal to 0.02. The radical cations formed after the electron ejection can deprotonate or react with water yielding radicals that are oxidized by molecular oxygen. Finally, 2-formylcinnamaldehyde and 2-carboxycinnamaldehyde are yielded. The production of 7-hydroxy-1,4-naphthoquinone is consistent with the intermediary formation of photolabile 1,4-naphtoquinone.

Oxidation of 1-naphthol and related phenols with hydrogen peroxide and potassium superoxide catalysed by 5,10,15,20-tetraarylporphyrinatoiron(III)chlorides in different reaction conditions

Abstract: Reaction of 1-naphthol and related phenols with hydrogen peroxide catalysed by 5,10,15,20-tetra(pentafluorophenyl)porphyrinatoiron(III)chloride gives corresponding quinones and oxidative phenol coupled products, whereas the reaction of naphthols with hydrogen peroxide catalysed by 5,10,15,20-tetramesitylporphyrinatoiron(III)chloride give above products along with quinone epoxides in moderate yields. The reaction of quinone with potassium superoxide catalysed by Me12TPPFe(III)Cl and p-MeOTPPFe(III)Cl give higher yields of quinone epoxides than the reaction of quinone with hydrogen peroxide catalysed by 5,10,15,20-tetraarylporphyrinatoiron(III)chlorides.

Synthesis and in vitro antiprotozoal activity of thiophene ring-containing quinones.

Abstract: A series of quinones (3a-i, 4-9, 11) and aromatic compounds (2a, 2d, 2g) containing the thiophene ring were tested in vitro against the trypomastigote form of Trypanosoma cruzi and the promastigote forms of Leishmania. The quinones 3a-i, 4, 5a, b, 6 and 9 having the thiophene ring fused to a quinone nucleus were the most active members of the series. The electron affinities of the benzo[b]thiophene-4,7-quinones 3, evaluated by their LUMO energies and halfwave potentials, are reported.

Synthesis and Reactivity with beta-Lactamases of "Penicillin-like" Cyclic Depsipeptides.

Abstract: Several 7-carboxy-3-amido-3,4-dihydro-2H-1-benzopyran-2-ones have been synthesized as potential beta-lactamase substrates and/or mechanism-based inhibitors. Substituted o-tyrosine precursors were prepared by the Sorensen method and then heated in vacuo to give the lactones. These compounds are cyclic analogues of aryl phenaceturates which are known to be beta-lactamase substrates. The goal of incorporating the scissile ester group into a lactone was to retain the leaving group tethered to the acyl moiety at the acyl-enzyme stage of turnover by serine beta-lactamases, in a manner similar to that during penicillin turnover. Further, in two cases, a functionalized methylene group para to the leaving group phenoxide oxygen was incorporated. These molecules possess a latent p-quinone methide electrophile which could, in principle, be unmasked during enzymic turnover and react with an active site nucleophile. All of these compounds were found to be substrates of class A and C beta-lactamases, the first delta-lactones with such activity. Generally, k(cat) values were smaller than for the analogous acyclic depsipeptides, which suggests that the tethered leaving group may obstruct the attack of water on the acyl-enzymes. Further exploration of this structural theme might lead to quite inert acyl-enzymes and thus to significant inhibitors. Despite the apparent advantage offered by the longer-lived acyl-enzymes, the functionalized compounds were no better as irreversible inhibitors than comparable acyclic compounds [Cabaret, D.; Liu, J.; Wakselman, M.; Pratt, R. F.; Xu, Y. Bioorg. Med. Chem. 1994, 2, 757-771]. Thus, even tethered quinone methides, at least when placed as dictated by the structures of the present compounds, were unable to efficiently trap a nucleophile at serine beta-lactamase active sites.

Oxetanes from [2+2] Cycloaddition of Stilbenes to Quinone via Photoinduced Electron Transfer†

Abstract: The photochemical coupling of various stilbenes (S) and chloranil (Q) is effected by the specific charge-transfer (CT) activation of the precursor electron donor−acceptor (EDA) complex [S, Q], and the [2+2] cycloaddition is established by X-ray structure elucidation of the crystalline trans-oxetanes formed selectively in high yields. Time-resolved (fs/ps) spectroscopy reveals the (singlet) ion-radical pair 1[S•+, Q•-] to be the primary reaction intermediate and thus unambiguously establishes for the first time the electron-transfer pathway for this typical Paterno−Buchi transformation. The alternative cycloaddition via the specific activation of the carbonyl component (as a commonly applied procedure in Paterno−Buchi couplings) leads to the same oxetane regioisomers in identical molar ratios. As such, we conclude that a common electron-transfer mechanism applies via the quenching of the photoactivated quinone acceptor by the stilbene donor to afford triplet ion-radical pairs 3[S•+, Q•-] which appear on th...

Structure and Conformation of Photosynthetic Pigments and Related Compounds. 12. A Crystallographic Analysis of Porphyrin‐quinones and Their Precursors

Abstract: . A comprehensive crystallographic analysis of 10 porphyrin quinone precursors (dimethoxybenzene derivatives), and six porphyrin quinones has been performed. The free bases and zinc(II) complexes of the porphyrin quinones are of the 5,10,15-triaryl/alkyl-20-quinone-porphyrin type and carry various bridging and quinone units. The structural and conformational parameters were determined for all compounds; the donor-acceptor separation distances range from 6.3 to 10.9 A. Knowledge of these data is a prerequisite for a detailed interpretation of theoretical and spectroscopic studies on such systems. Despite the obvious influence of the type and geometry of the bridging unit and quinone on the spatial arrangement of the donor and acceptor components, a large variety of different packing arrangements in the crystal were observed. These include π stacking, aggregate formation and axial ligation in the zinc(II) porphyrins. The latter often utilized the quinone (or dimethoxy) oxygen atoms for coordination to zinc(II) centers leading to porphyrin quinone dimers and even polymers.

Modulation of the toxicity and macromolecular binding of benzene metabolites by NAD(P)H:Quinone oxidoreductase in transfected HL-60 cells.

Abstract: Benzene is oxidized in the liver to produce a series of hydroxylated metabolites, including hydroquinone and 1,2,4-benzenetriol These metabolites are activated to toxic and genotoxic species in the bone marrow via oxidation by myeloperoxidase (MPO) NAD(P)H:quinone oxidoreductase (NQO1) is an enzyme capable of reducing the oxidized quinone metabolites and thereby potentially reducing their toxicities We introduced the NQO1 gene into the HL-60 cell line to create a high MPO-, high NQO1-expressing cell line, and tested its response in assays of benzene metabolite toxicity NQO1 expression reduced a class of hydroquinone- and benzenetriol-induced DNA adducts by 79-86% The cytotoxicity and apoptosis caused by hydroquinone were modestly reduced, while protein binding was unchanged and the rate of glutathione depletion increased NQO1's activity in reducing a class of benzene metabolite-induced DNA adducts may be related to its known activities in maintaining membrane-bound endogenous antioxidants in reduced form Alternatively, NQO1 activity may prevent the formation of adducts which result from polymerized products of the quinones In either case, this protection by NQO1 may be an important mechanism in the observation that a lack of NQO1 activity affords an increased risk of benzene poisoning in exposed individuals [Rothman, N, et al (1997) Cancer Res 57, 2839-2842]

The Course of the Copper-Catalyzed Oxidative Polymerization of 2,6-Dimethylphenol. Analysis of Oligomeric Phenols during the Coupling Reaction

Abstract: The course of the copper-catalyzed oxidative coupling of 2,6-dimethylphenol (DMP) has been studied by HPLC-analysis of reaction mixtures which had started with either DMP itself, or with its C-O coupled dimer, 4-(2',6'-dimethylphenoxy)-2,6-dimethylphenol, under both aerobic and anaerobic conditions. These measurements have provided information on how the actual phenol coupling step takes place and how the polymerization reaction proceeds. In reactions started with the monomer, no or at most very small amounts of oligomers of DMP are detected, apart from a precipitate of polymeric material in the aerobic experiments. Reactions started with the dimer only result in swift formation of significant amounts of monomer in addition to oligomers. This difference in behavior can be ascribed to the higher reactivity of oligomeric phenols compared to the monomer. The fact that monomer phenol is formed from dimer phenol is strong evidence for a reaction pathway in which a quinone ketal is formed by C -O coupling of two phenolic moieties. It is believed that as long as this quinone remains coordinated to the copper, it can decompose by (probably heterolytic) fission of one of the ether bonds to generate two (new) species, de facto resulting in redistribution of the oligomers. Once the quinone dissociates from the copper catalyst, a rearrangement may occur to afford the C-O coupled phenol. Methylated phenols (anisoles) do not take part in either redistribution or rearrangement reactions, since a quinone ketal can only be formed from species that can be deprotonated, i.e., from phenols.

Investigation on the detergent role in the function of secondary quinone in bacterial reaction centers

Abstract: In this paper are reported studies on the detergent role in isolated reaction centers (RC) from Rhodobacter sphaeroides, over a large range of lauryldimethylamino-N-oxide (LDAO) concentrations, in influencing the thermodynamics of the quinone exchange reaction as well as the protein aggregation. The occurrence of the quinone exchange reaction between the QB-binding site (where QB is the second quinone molecule of two in the RC) and the ubiquinone 0 dissolved in the different environments (water, LDAO micelles and detergent phase of the protein-detergent complex) has also been analyzed. Measurements carried out in QB-depleted RC to which exogenous quinone has been added show that the relative amplitudes of the slow and fast phase of the recombination reaction depend on this parameter. The overall amount of the restored QB-functionality is affected by the concentration of the LDAO in solution. Interpolation of the titration curves with a quadratic function obtained by simple considerations allowed the binding constant of UQ0 to the QB-binding site to be calculated. From the fitting procedure, the distribution of the quinone in the different environments present in solution was evaluated, indicating that the exchange reaction can take place only between the QB-site and the detergent phase. The dependence of the quinone pool size upon the volume of the phase in which the interacting quinone is solubilized is also discussed. The increasing difficulty in saturating the QB-pocket above the LDAO critical micellar concentration is finally related to the association of protein-detergent complexes to form large protein clusters.

Photochemical generation and lifetimes in water of p-aryloxy- and p-alkoxyphenylnitrenium ions

Abstract: This paper describes product and flash photolysis studies following irradiation in aqueous solution of 4X–C6H4N3 [X = MeO (12a), EtO (12b), PriO (12c), ButO (12d), C6H5O (12e), 4-MeOC6H4O (12f), F, Cl] and 4-methoxy-1-naphthyl azide (15). p-Benzoquinone (or 1,4-naphthoquinone) is observed as a product, in yields of 70–90% with 12a–d, 15, 40% with 12e, 26% with 4-F and 15% with 4-Cl. The quinone arises by a pathway whereby the initially-formed singlet arylnitrene is quenched by protonation by a solvent water molecule to form a nitrenium ion. Hydration of this cation at the para position leads through a hemiacetal (or halohydrin) to the quinone imine, whose hydrolysis results in the final quinone product. Three kinetic processes are observed, the nitrenium hydration on the µs time scale, the hemiacetal breakdown on the ms time scale, and the imine hydrolysis on the minutes time scale. The nitrenium ions have lifetimes in aqueous solution of 50 ns (4-PhO), 70 ns (4-MeOC6H4O), 370 ns (4-MeO), 550 ns (4-EtO), 1.25 µs (4-PriO), 1.56 µs (4-ButO) and 1.35 µs (4-methoxynaphthyl). A nitrenium transient is not observed with the 4-halophenyl azides, probably because the lifetime is too short for detection with ns laser flash photolysis (LFP). The alkoxyphenylnitrenium ions are argued to be better represented as oxocarbocations derived from O-alkylation of the quinone imine. The 4-ethoxyphenylnitrenium ion is not quenched by 0.01 mol dm–3 2′-deoxyguanosine, so that k2(dG) is less than 2 × 107 mol –1 dm3 s–1. This contrasts with the 4-biphenylylnitrenium ion, which has a similar solvent reactivity, but reacts with k2(dG) = 2 × 109 mol–1 dm3 s–1. The localization of the positive charge in the alkoxy system is a possible explanation behind this difference.

The metal function in the reactions of bovine serum amine oxidase with substrates and hydrazine inhibitors.

Abstract: Bovine serum amine oxidase (BSAO) reacts with 2-hydrazinopyridine, which binds the organic cofactor 2,4,5-trihydroxyphenylalanine quinone, forming a band at 435 nm. The band shifts to 526 nm around 60 °C, to 415 nm upon denaturation, but only shifts to 429 nm upon Cu2+ depletion. Its wavelength and intensity suggest that the adduct has the azo conformation, whilst the same adduct of crystallineEscherichia coli amine oxidase (ECAO) shows the hydrazone conformation in the X-ray structure. The steady state kinetics of aminomethyl- and aminoethylpyridines confirm that the formation of the product Schiff base, analogous to the azo form of the 2-hydrazinopyridine adduct, is not hindered in solution. The structural stability of the adduct in the absence of Cu2+ is taken to imply hydrogen bonding of the pyridyl nitrogen to a conserved aspartate, as in the ECAO adduct. Thus the ECAO adduct provides a good model for a transient intermediate leading to formation of the BSAO azo adduct. On the basis of this model and of the catalytic competence of Co2+-substituted BSAO, confirmed by the present data, a catalytic reaction scheme is proposed.