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One-electron Oxidation (one-electron + oxidation)
Selected AbstractsRe-evaluation of intramolecular long-range electron transfer between tyrosine and tryptophan in lysozymesFEBS JOURNAL, Issue 17 2003Evidence for the participation of other residues One-electron oxidation of six different c-type lysozymes from hen egg white, turkey egg white, human milk, horse milk, camel stomach and tortoise was studied by gamma- and pulse-radiolysis. In the first step, one tryptophan side chain is oxidized to indolyl free radical, which is produced quantitatively. As shown already, the indolyl radical subsequently oxidizes a tyrosine side chain to the phenoxy radical in an intramolecular reaction. However this reaction is not total and its stoichiometry depends on the protein. Rate constants also vary between proteins, from 120·s,1 to 1000·s,1 at pH 7.0 and room temperature [extremes are hen and turkey egg white (120·s,1) and human milk (1000·s,1)]. In hen and turkey egg white lysozymes we show that another reactive site is the Asn103,Gly104 peptidic bond, which gets broken radiolytically. Tryptic digestion followed by HPLC separation and identification of the peptides was performed for nonirradiated and irradiated hen lysozyme. Fluorescence spectra of the peptides indicate that Trp108 and/or 111 remain oxidized and that Tyr20 and 53 give bityrosine. Tyr23 appears not to be involved in the process. Thus new features of long-range intramolecular electron transfer in proteins appear: it is only partial and other groups are involved which are silent in pulse radiolysis. [source] Synthesis and Electrochemical Study of an Original Copper(II)-Capped Salen,Cyclodextrin ComplexEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29 2010Elise Deunf Abstract A new metallocapped cyclodextrin (CD) was synthesized by the regioselective debenzylation, induced by diisobutylaluminium hydride (DIBAL-H), of perbenzylated cyclodextrins. This reaction allowed for the efficient preparation of an unprecedented CD,salen type copper(II) complex. The electrochemical behavior of both the bound and unbound CD,salen compounds was investigated by cyclic voltammetry. Notably, it was shown that the presence of tert -butyl groups at the ortho - and para -positions of the salen aromatic rings stabilized the copper(II) phenoxyl radical species that was generated upon the one-electron oxidation of the starting compound. Importantly, this stabilization remained effective when the salen-type ligand was covalently attached to the CD. This allowed for investigations of the reactivity of the copper(II) phenoxyl radical complex towards a primary alcohol to be performed by cyclic voltammetry. This reaction can be considered as mimicking the behavior of galactose oxidase. However, under these conditions, no reactivity was observed in the presence of benzyl alcohol. This may be due to distortion, either of the initially square planar salen ligand after its grafting to the CD primary face, and/or of the CD itself. On the other hand, the electrochemical reduction of the un-grafted copper(II) salen-type ligand led to a transient anionic species that exhibited significant stability on the time-scale of the slow cyclic voltammetry measurement in the absence of the CD, but was unstable in the presence of the CD. In the latter case, it was demonstrated that the anionic species was protonated by the CD. Importantly, this protonation was not fast enough to prevent catalytic activation of iodomethane by the electro-generated copper(I)-capped salen CD complex. [source] NO synthase isoforms specifically modify peroxynitrite reactivityFEBS JOURNAL, Issue 19 2010Amandine Maréchal Nitric oxide synthases (NOSs) are multi-domain hemothiolate proteins that are the sole source of nitric oxide (NO) in mammals. NOSs can also be a source or a sink for peroxynitrite (PN), an oxidant that is suspected to be involved in numerous physiopathological processes. In a previous study, we showed that the oxygenase domain of the inducible NOS (iNOSoxy) reacts with PN and changes its oxidative reactivity [Maréchal A, Mattioli TA, Stuehr DJ & Santolini J (2007) J Biol Chem282, 14101,14112]. Here we report a similar analysis on two other NOS isoforms, neuronal NOS (nNOS) and a bacterial NOS-like protein (bsNOS). All NOSs accelerated PN decomposition, with accumulation of a similar heme intermediate. The kinetics of PN decomposition and heme transitions were comparable among NOSs. However, their effects on PN reactivity differ greatly. All isoforms suppressed PN two-electron oxidative activity, but iNOSoxy enhanced PN one-electron oxidation and nitration potencies, the oxygenase domain of nNOS (nNOSoxy) affected them minimally, and bsNOS abolished all PN reactivities. This led to the loss of both NOS and PN decomposition activities for nNOSoxy and iNOSoxy, which may be linked to the reported alterations in their electronic absorption spectra. Bacterial bsNOS was affected to a lesser extent by reaction with PN. We propose that these differences in PN reactivity among NOSs might arise from subtle differences in their heme pockets, and could reflect the physiological specificity of each NOS isoform, ranging from oxidative stress amplification (iNOS) to detoxification (bsNOS). [source] Theoretical study of the oxidative polymerization of aniline with peroxydisulfate: Tetramer formationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2008-Marjanovi, Gordana Abstract Semi-empirical quantum chemical study of the oxidative polymerization of aniline with ammonium peroxydisulfate, in aqueous solutions without added acid, has been based on the MNDO-PM3 computations of thermodynamic, redox, and acid,base properties of reactive species and the intermediates, combined with the MM2 molecular mechanics force-field method and conductor-like screening model of solvation. The main reaction routes of aniline tetramerization are proposed. The regioselectivity of the formation of aniline tetramers by redox and electrophilic aromatic substitution reactions is analyzed. It was proved that the linear NC4 coupled tetra-aniline is formed as a dominant product by three different pathways: comproportionation redox reaction between N -phenyl-1,4-benzoquinonediimine and 4-aminodiphenylamine, the one-electron oxidation of aniline with its half-oxidized NC4 coupled trimer, and the electrophilic aromatic substitution reaction of aniline with fully oxidized NC4 coupled trianiline nitrenium cation. The electrophilic aromatic substitution reaction of the NC4 coupled aniline trimer with aniline nitrenium cation, as well as the oxidation of aniline with half-oxidized branched trimer, lead to the branched aniline tetramers. The competing character of different tetramerization routes is highlighted. The oxidative intramolecular cyclization of branched oligoanilines and polyaniline, leading to the generation of substituted phenazine units, has been predicted to accompany the classical routes of the polymerization of aniline. Various molecular (branched vs. linear) oligomeric structures produced at different level of acidity during the course of polymerization and their impact on the formation of supramolecular structures of conducting polyaniline (nanorods and nanotubes vs. granular morphology), are discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Stereoelectronic effects in radical processes,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8-9 2006Paolo Brandi Abstract A kinetic study of the H-abstraction reaction from cyclic and acyclic alkylarene substrates by the nitroxyl radical (dubbed BTNO) of 1-hydroxy-benzotriazole (HBT) has been carried out in MeCN solution at 25°C. BTNO was generated from one-electron oxidation of HBT by cerium(IV) ammonium nitrate. The H-abstraction reactivity measured with the cyclic alkylarenes is invariably higher than that with the acyclic counterparts. This is explained as the contribution of hyperconjugation between the aromatic ,-system and the scissile benzylic CH bond of the substrate, which weakens the CH bond in the transition state and promotes its cleavage. Stereoelectronic considerations enable to appreciate why the weakening effect is more pronounced in the cyclic system than in the acyclic counterpart, thereby justifying the higher reactivity of the former. Evidence for the intervention of stereoelectronic effects is embodied by the dissociation energies of the CH bonds, having always lower values for the cyclic substrates investigated. Copyright © 2006 John Wiley & Sons, Ltd. [source] The formation of neutral CCCO2H and HCCCO2 molecules from anionic precursors in the gas phase: a joint experimental and theoretical studyRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 24 2005Mark Fitzgerald Calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that the anions ,CCCO2H and HCCCO are stable species in their singlet states. Upon collision-induced, vertical one-electron oxidation under neutralisation-reionisation (,NR+) conditions, they produce the neutral molecules CCCO2H and HCCCO2, respectively. Some of the CCCO2H neutrals should be stable for the duration of the neutralisation-reionisation experiment (10,6,s), while others will dissociate to CCCO and OH (requires 125,kJ,mol,1). In contrast, neutral HCCCO2 is expected to be much less stable, and dissociate to HCC and CO2 (37,kJ,mol,1). Neither CCCO2H nor HCCCO2 is expected to interconvert, or to rearrange to other isomers. The anions ,CCCO2H and HCCCO have been formed in the ion source of the mass spectrometer by the reactions between (CH3)3SiCCCO2H and F, and HCCCO2Si(CH3)3 and F,, respectively. The ,NR+ spectrum of ,CCCO2H shows a recovery signal and also indicates that the lowest energy dissociation pathway of neutral CCCO2H corresponds to the loss of OH. The ,NR+ spectrum of HCCCO2 displays little or no recovery signal, and the spectrum is dominated by the [CO2]+ ion. The experimental observations are in agreement with the predictions of the extensive theoretical studies. Copyright © 2005 John Wiley & Sons, Ltd. [source] Modified Microperoxidases Exhibit Different Reactivity Towards Phenolic SubstratesCHEMBIOCHEM, Issue 12 2004Corrado Dallacosta Dr. Abstract The reactivity of several microperoxidase derivatives with different distal-site environments has been studied. The distal-site environments of these heme peptides include a positively charged one, an uncharged environment, two bulky and doubly or triply positively charged ones, and one containing aromatic apolar residues. The reactivity in the catalytic oxidation of two representative phenols, carrying opposite charges, by hydrogen peroxide has been investigated. This allows the determination of the binding constants and of the electron-transfer rate from the phenol to the catalyst in the substrate/microperoxidase complex. The electron-transfer rates scarcely depend on the redox and charge properties of the phenol, but depend strongly on the microperoxidase. Information on the disposition of the substrate in the adducts with the microperoxidases has been obtained through determination of the paramagnetic contribution to the1H NMR relaxation rates of the protons of the bound substrates. The data show that the electron-transfer rate drops when the substrate binds too far away from the iron and that the phenols bind to microperoxidases at similar distances to those observed with peroxidases. While the reaction rate of microperoxidases with peroxide is significantly smaller than that of the enzymes, the efficiency in the one-electron oxidation of phenolic substrates is almost comparable. Interestingly, the oxyferryl form of the triply positively charged microperoxidases shows a reactivity larger than that exhibited by horseradish peroxidase. [source] Stereoselective Synthesis of Bicyclic Nitrocyclopropanes by a Radical,Anion Domino ReactionCHEMISTRY - A EUROPEAN JOURNAL, Issue 40 2009Akio Kamimura Prof. A novel one-step radical-ionic cyclopropanation was carried out to prepare aza- or oxabicyclo[3.1.0]hexane from ,-nitroamides or ,-nitroethers in a stereoselective manner (see scheme). The present procedure involves a higher-order domino process that includes one-electron oxidation, radical cyclization,intramolecular SN2 reaction. [source] A Novel Tripodal Ligand Containing Three Different N -Heterocyclic Donor Functions and Its Application in Catechol Dioxygenase MimickingCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2009Marit Wagner Dipl.-Chem. Abstract Prominent donors: A pyridyl, an imidazolyl, and a pyrazolyl donor function are part of the novel tripodal ligand depicted, which thus combines three of the most prominent donors applied in ligands for bioinorganic chemistry within one coordination unit. To exploit its behaviour and potential, first investigations have been carried out in relation to catechol dioxygenase mimicry. We describe a novel chiral ligand, L, in which three different N -donor functions are linked to a methoxymethine unit: a methylpyrazole derivative, a methylimidazole unit, and a pyridyl residue. Complexes with FeCl2, FeBr2, and FeCl3 have been synthesized and fully characterized, including with respect to their molecular structures. While in combination with FeCl3L coordinates in a tripodal fashion, with FeX2 (X=Cl, Br) it binds only through two functions and the pyridyl unit remains dangling. For potential modelling of intradiol and extradiol catechol dioxygenase reactivity, the complexes [LFeCl2], 1, and [LFeCl3], 3, have been treated with 3,5-di- tert -butylcatechol, triethylamine, and O2. Both complexes yielded similar results in such investigations, since the LFeII,catecholate complex reacts with O2 through one-electron oxidation in the first step. Employing 3 in acetonitrile solution, intradiol cleavage occurred, although the undesired quinone was formed as the main product. If reagents were added (NaBPh4, H+) or reaction conditions were chosen (CH2Cl2 instead of CH3CN as the solvent) that made the coordination sphere at the iron centre more accessible for a third substrate donor function, an alternative reaction route, presumably involving O2 binding at the metal, became more important, which led to extradiol cleavage. In the extreme case (CH2Cl2 as the solvent and with the addition of NaBPh4), mainly the extradiol cleavage products were formed; the intradiol products were only observed as side products then and quinone formation became negligible. Protonated base functions in the second coordination sphere increased the efficiency of extradiol cleavage only slightly. The obtained results are in line with current understanding of the function of intradiol/extradiol dioxygenases. [source] Polysulfurated Pyrene-Cored Dendrimers: Luminescent and Electrochromic PropertiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2008Marc Gingras Prof. Abstract We have synthesized a novel class of dendrimers, consisting of a polysulfurated pyrene core with appended poly(thiophenylene) dendrons (PyG0, PyG1, and PyG2, see Scheme,1), which exhibit remarkable photophysical and redox properties. In dichloromethane or cyclohexane solution they show a strong, dendron-localized absorption band with a maximum at around 260,nm and a band in the visible region with a maximum at 435,nm, which can be assigned to the pyrene core strongly perturbed by the four sulfur substituents. The dendrimers exhibit a strong (,=0.6), short-lived (,=2.5,ns) core-localized fluorescence band with maximum at approximately 460,nm in cyclohexane solution at 293,K. A strong fluorescence is also observed in dichloromethane solution at 293,K, in dichloromethane/chloroform rigid matrix at 77,K, and in the solid state (powder) at room temperature. The dendrimers undergo reversible chemical and electrochemical one-electron oxidation with formation of a strongly colored deep blue radical cation. A second, reversible one-electron oxidation is observed at more positive potential values. The photophysical and redox properties of the three dendrimers are finely tuned by the length of their branches. The strong blue fluorescence and the yellow to deep blue color change upon reversible one-electron oxidation can be exploited for optoelectronic devices. [source] Spectroelectrochemical and Computational Studies on the Mechanism of Hypoxia Selectivity of Copper RadiopharmaceuticalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2008Jason Abstract Detailed chemical, spectroelectrochemical and computational studies have been used to investigate the mechanism of hypoxia selectivity of a range of copper radiopharmaceuticals. A revised mechanism involving a delicate balance between cellular uptake, intracellular reduction, reoxidation, protonation and ligand dissociation is proposed. This mechanism accounts for observed differences in the reported cellular uptake and washout of related copper bis(thiosemicarbazonato) complexes. Three copper and zinc complexes have been characterised by X-ray crystallography and the redox chemistry of a series of copper complexes has been investigated by using electronic absorption and EPR spectroelectrochemistry. Time-dependent density functional theory (TD-DFT) calculations have also been used to probe the electronic structures of intermediate species and assign the electronic absorption spectra. DFT calculations also show that one-electron oxidation is ligand-based, leading to the formation of cationic triplet species. In the absence of protons, metal-centred one-electron reduction gives the reduced anionic copper(I) species, [CuIATSM],, and for the first time it is shown that molecular oxygen can reoxidise this anion to give the neutral, lipophilic parent complexes, which can wash out of cells. The electrochemistry is pH dependent and in the presence of stronger acids both chemical and electrochemical reduction leads to quantitative and rapid dissociation of copper(I) ions from the mono- or diprotonated complexes, [CuIATSMH] and [CuIATSMH2]+. In addition, a range of protonated intermediate species have been identified at lower acid concentrations. The one-electron reduction potential, rate of reoxidation of the copper(I) anionic species and ease of protonation are dependent on the structure of the ligand, which also governs their observed behaviour in vivo. [source] Evaluation of the Efficiency of the Photocatalytic One-Electron Oxidation Reaction of Aromatic Compounds Adsorbed on a TiO2 SurfaceCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2004Takashi Tachikawa Dr. Abstract The TiO2 photocatalytic one-electron oxidation mechanism of aromatic sulfides with a methylene bridging group (-(CH2)n -, n=0,4) between the 4-(methylthio)phenyl chromophore and the carboxylate binding group on the surface of a TiO2 powder slurried in acetonitrile (MeCN) has been investigated by time-resolved diffuse reflectance (TDR) spectroscopy. The electronic coupling element (HDA) between the hole donor and acceptor, which was estimated from the spectroscopic characteristics of the charge transfer (CT) complexes of the substrates (S) and the TiO2 surface, exhibited an exponential decline with the increasing of the methylene number of S. The determined decay factor (,) of 9 nm,1 also supports the fact that the 4-(methylthio)phenyl chromophore is separated from the TiO2 surface. The efficiency of the one-electron oxidation of S adsorbed on the TiO2 surface, which was determined from the relationship between the amount of adsorbates and the concentration of the generated radical cations, significantly depended on the HDA value, but not on the oxidation potential of S determined in homogeneous solution. [source] Highly Conjugated p -Quinonoid ,-Extended Tetrathiafulvalene Derivatives: A Class of Highly Distorted Electron DonorsCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2004Marta C. Díaz Abstract A new class of ,-extended TTF-type electron donors (11,a,c) has been synthesized by Wittig,Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10,a,c). In cyclic voltammetry experiments, donors 11,a,c reveal a single, electrochemically irreversible oxidation,yielding the corresponding dicationic products,at relatively low oxidation potentials (,0.7,0.8 V). Theoretical calculations, performed at the DFT level (B3,P86/6-31,G*), predict a highly-folded C2h structure for 11,a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO,LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11,a.+ retains the folded C2h structure predicted for the neutral molecule as the most stable conformation, the dication 11,a2+ has a fully aromatic D2 structure, formed by an orthogonal 9,9,-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11,a,c was shown to lead to the radical cation species (11,a,c.+), which were found to disproportionate with generation of the respective dication species (11,a,c2+) and the neutral molecules (11,a,c). Una nueva familia de moléculas dadoras de electrones de tipo TTF , -extendido, altamente conjugadas, (11,a,c) se han sintetizado mediante la reacción de olefinación de Wittig,Horner de la biantrona (9) con fosfonatos de 1,3-ditiol (10,a,c). En los experimentos de voltamperometría cíclica, los dadores 11,a,c muestran una única onda de oxidación electroquímicamente irreversible,dando lugar a los productos dicatiónicos,a potenciales relativamente bajos (,0.7,0.8 V). Cálculos teóricos, llevados a cabo a nivel DFT (B3,P86/6-31,G*), predicen una estructuraC2haltamente distorsionada para 11,a. La molécula adopta una conformación en forma de doble mariposa para aliviar el impedimento estérico. Los cálculos sugieren que la transferencia de carga intramolecular asociada a la transición HOMO,LUMO es responsable de la banda de absorción observada por encima de 400 nm en el espectro electrónico. El catión radical 11,a.+retiene la estructura C2hplegada predicha para la molécula neutra como la conformación más estable. Por el contrario, el dicatión 11,a2+muestra una estructuraD2totalmente aromática,formada por una unidad central de 9,9,-biantrilo ortogonal, unida a los anillos cargados de ditiol. Los profundos cambios conformacionales que experimentan los compuestos 11 tras la oxidación explican sus propiedades electroquímicas. Medidas de radiólisis de pulso, esto es, la oxidación monoelectrónica de 11,a,c inducida por radicales, conduce a las especies catión radical (11,a,c.+), las cuales dismutan para generar las respectivas especies dicatiónicas (11,a,c2+) y la molécula neutra (11,a,c). [source] Synthesis and Characterization of p -[Perfluoro-1-(2-fluorosulfonylethoxy)] ethylated PolystyreneCHINESE JOURNAL OF CHEMISTRY, Issue 4 2003Guan Chuan-Jin Abstract A new fluorinated polystyrene bearing a p -substituted perfluoro[1-(2-fluorosulfonylethoxy)] ethyl group was synthesized via one-electron oxidation of polystyrene by perfluoro [2-(2-fluorosulfonylethoxy)] propionyl peroxide at different peroxide to polystyrene molar ratios. The yield of perfluoroalkylation decreases with the increase of the reactant molar ratio. The modified polymer has been characterized by various techniques: the ring perfluoro[l-(2-fluorosulfonylethoxy)] ethylation has been proved by FT-IR and 19F NMR; the X-ray photoelectron spectra (XPS) show the maximum binding energy of Fls, Ols Cls(two kinds of carbon atoms, namely C-H and C-F) and S2p, respectively; desulfonylation of the fluorinated polystyrene appearing at 217 °C has been found by its thermogravimetric analysis (TGA). The determinations of contact angle, refractive index and glass transition temperature of the modified polymer have disclosed that when the contact angle increases with the increase of the molar ratio, the refractive index and glass transition temperature decrease. The polydispersity values indicate that the degradation of the polymer chains did not occur during the reaction. [source] Comparative Electrochemical Study of Unsubstituted and Substituted Bis(phthalocyaninato) Rare Earth(III) ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2004Peihua Zhu Abstract The electrochemistry of homoleptic substituted phthalocyaninato rare earth double-decker complexes M(TBPc)2 and M(OOPc)2 [M = Y, La...Lu except Pm; H2TBPc = 3(4),12(13),21(22),30(31)-tetra- tert -butylphthalocyanine, H2OOPc = 3,4,12,13,21,22,30,31-octakis(octyloxy)phthalocyanine] has been comparatively studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in CH2Cl2 containing 0.1 M tetra- n -butylammonium perchlorate (TBAP). Two quasi-reversible one-electron oxidations and three or four quasi-reversible one-electron reductions have been revealed for these neutral double-deckers of two series of substituted complexes, respectively. For comparison, unsubstituted bis(phthalocyaninato) rare earth analogues M(Pc)2 (M = Y, La...Lu except Pm; H2Pc = phthalocyanine) have also been electrochemically investigated. Two quasi-reversible one-electron oxidations and up to five quasi-reversible one-electron reductions have been revealed for these neutral double-decker compounds. The three bis(phthalocyaninato)cerium compounds display one cerium-centered redox wave between the first ligand-based oxidation and reduction. The half-wave potentials of the first and second oxidations and first reduction for double-deckers of the tervalent rare earths depend on the size of the metal center. The difference between the redox potentials of the second and third reductions for MIII(Pc)2, which represents the potential difference between the first oxidation and first reduction of [MIII(Pc)2],, lies in the range 1.08,1.37 V and also gradually diminishes along with the lanthanide contraction, indicating enhanced ,,, interactions in the double-deckers connected by the smaller, lanthanides. This corresponds well with the red-shift of the lowest energy band observed in the electronic absorption spectra of reduced double-decker [MIII(Pc,)2], (Pc, = Pc, TBPc, OOPc). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] | ||||||||||