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One-electron Reduction (one-electron + reduction)
Selected AbstractsGeneration of Reactive Species by One-Electron Reduction of Fischer-Type Carbene Complexes of Group 6 Metals and Their Use for Carbon,Carbon Bond Formation.CHEMINFORM, Issue 21 2003Kohei Fuchibe Abstract For Abstract see ChemInform Abstract in Full Text. [source] The Role of One-Electron Reduction of Lipid Hydroperoxides in Causing DNA DamageCHEMISTRY - A EUROPEAN JOURNAL, Issue 40 2009Conor Crean Dr. Abstract The in vivo metabolism of plasma lipids generates lipid hydroperoxides that, upon one-electron reduction, give rise to a wide spectrum of genotoxic unsaturated aldehydes and epoxides. These metabolites react with cellular DNA to form a variety of pre-mutagenic DNA lesions. The mechanisms of action of the radical precursors of these genotoxic electrophiles are poorly understood. In this work we investigated the nature of DNA products formed by a one-electron reduction of (13S)-hydroperoxy-(9Z,11E)-octadecadienoic acid (13S -HPODE), a typical lipid molecule, and the reactions of the free radicals thus generated with neutral guanine radicals, G(,H).. A novel approach was devised to generate these intermediates in solution. The two-photon-induced ionization of 2-aminopurine (2AP) within the 2,-deoxyoligonucleotide 5,-d(CC[2AP]TCGCTACC) by intense nanosecond 308,nm excimer laser pulses was employed to simultaneously generate hydrated electrons and radical cations 2AP.+. The latter radicals either in cationic or neutral forms, rapidly oxidize the nearby G base to form G(,H).. In deoxygenated buffer solutions (pH,7.5), the hydrated electrons rapidly reduce 13S -HPODE and the highly unstable alkoxyl radicals formed undergo a prompt ,-scission to pentyl radicals that readily combine with G(,H).. Two novel guanine products in these oligonucleotides, 8-pentyl- and N2 -pentylguanine, were identified. It is shown that the DNA secondary structure significantly affects the ratio of 8-pentyl- and N2 -pentylguanine lesions that changes from 0.9:1 in single-stranded, to 1:0.2 in double-stranded oligonucleotides. The alkylation of guanine by alkyl radicals derived from lipid hydroperoxides might contribute to the genotoxic modification of cellular DNA under hypoxic conditions. Thus, further research is warranted on the detection of pentylguanine lesions and other alkylguanines in vivo. [source] Redox Activity and Structural Transition of Heptyl Viologen Adlayers on Cu(100),CHEMPHYSCHEM, Issue 7 2010Min Jiang Dr. Abstract The redox behaviour and potential-dependent adsorption structure of heptyl viologen (1,1,-diheptyl-4,4,-bipyridinium dichloride, DHV2+) on a Cu(100) electrode was investigated in a chloride-containing electrolyte solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC,STM). The dicationic DHV molecules generate a few pairs of current waves in CV measurements which are ascribed to two typical one-electron transfer steps. STM images obtained in a KCl-containing electrolyte solution disclose a well-ordered c(2×2) chloride adlayer on a Cu(100) electrode surface. After injecting DHV2+ molecules into the KCl electrolyte solution, a highly ordered 2D "dot-array" structure in STM images emerges on the c(2×2)-Cl modified Cu(100) electrode surface. DHV2+ molecules spontaneously arrange themselves with their molecular planes facing the electrode surface and their long molecular axis parallel to the step edge. Such adsorption structure can be described by mirror domains and rotational domains which stably exist between 200 mV and ,100 mV. One-electron reduction of the dications DHV2+ around ,150 mV causes a phase transition from a ,dot-array' assembly to a stripe pattern formed by DHV.+ radical monocations in STM images which has a bilayer structure. With a further decrease of the applied electrode potential, the structure of the DHV.+ adlayer undergoes a change from a loose stripe phase to a more compact stripe phase, a subsequent decay of the compact structure, and finally the formation of a new dimer phase. A further electron transfer reaction at ,400 mV causes the formation of an amorphous phase on the chloride free electrode surface. In a reverse anodic sweep, the reproduction of the ordered DHV.+ stacking phase occurs again on top of the chloride lattice. [source] Functional studies of frataxinACTA PAEDIATRICA, Issue 2004G Isaya Mitochondria generate adenosine triphosphate (ATP) but also dangerous reactive oxygen species (ROS). One-electron reduction of dioxygen in the early stages of the electron transport chain yields a superoxide radical that is detoxified by mitochondrial superoxide dismutase to give hydrogen peroxide. The hydroxyl radical is derived from decomposition of hydrogen peroxide via the Fenton reaction, catalyzed by Fe2+ ions. Mitochondria require a constant supply of Fe2+ for heme and iron-sulfur cluster biosyntheses and therefore are particularly susceptible to ROS attack. Two main antioxidant defenses are known in mitochondria: enzymes that catalytically remove ROS, e.g. superoxide dismutase and glutathione peroxidase, and low molecular weight agents that scavenge ROS, including coenzyme Q, glutathione, and vitamins E and C. An effective defensive system, however, should also involve means to control the availability of pro-oxidants such as Fe2+ ions. There is increasing evidence that this function may be carried out by the mitochondrial protein frataxin. Frataxin deficiency is the primary cause of Friedreich's ataxia (FRDA), an autosomal recessive degenerative disease. Frataxin is a highly conserved mitochondrial protein that plays a critical role in iron homeostasis. Respiratory deficits, abnormal cellular iron distribution and increased oxidative damage are associated with frataxin defects in yeast and mouse models of FRDA. The mechanism by which frataxin regulates iron metabolism is unknown. The yeast frataxin homologue (mYfhlp) is activated by Fe(II) in the presence of oxygen and assembles stepwise into a 48-subunit multimer (,48) that sequesters <2000 atoms of iron in a ferrihydrite mineral core. Assembly of mYfhlp is driven by two sequential iron oxidation reactions: a fast ferroxidase reaction catalyzed by mYfh1p induces the first assembly step (,,3), followed by a slower autoxidation reaction that promotes the assembly of higher order oligomers yielding ,48. Depending on the ionic environment, stepwise assembly is associated with the sequestration of 50,75 Fe(II)/subunit. This Fe(II) is initially loosely bound to mYfh1p and can be readily mobilized by chelators or made available to the mitochondrial enzyme ferrochelatase to synthesize heme. However, as iron oxidation and mineralization proceed, Fe(III) becomes progressively inaccessible and a stable iron-protein complex is produced. In conclusion, by coupling iron oxidation with stepwise assembly, frataxin can successively function as an iron chaperon or an iron store. Reduced iron availability and solubility and increased oxidative damage may therefore explain the pathogenesis of FRDA. [source] Electrochemical Approach to the Radical Anion Formation from 2,-Hydroxy Chalcone DerivativesELECTROANALYSIS, Issue 5 2006P. Quintana-Espinoza Abstract Three 2,-hydroxy chalcone derivatives were electrochemically reduced to the radical anion by a reversible one-electron transfer followed by a chemical dimerization reaction. Under suitable conditions of the medium, the one-electron reduction produces very well resolved cyclic voltammograms due to the formation of the radical anion. By using appropriately the wide versatility of the cyclic voltammetric technique, was possible to study the generation of the radical anion and its stability. [source] Mono- and Binuclear Arylnickel Complexes of the ,-Diimine Bridging Ligand 2,2,-Bipyrimidine (bpym)EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 6 2010Axel Klein Abstract The mono- and binuclear organometallic NiII complexes [(,-bpym){Ni(Mes)Br}n] (bpym = 2,2,-bipyrimidine; n = 1 or 2; Mes = mesityl = 2,4,6-trimethylphenyl) were prepared and characterised electrochemically and spectroscopically (NMR, UV/Vis/NIR) in detail. The long-wavelength absorptions for the binuclear complex reveal a marked electronic coupling of the two metal centres over the ligand bridge via their low-lying ,*-orbitals. While the mononuclear complex undergoes rapid dissociation of the bromido ligand after one-electron reduction the binuclear derivative exhibits reversible reductive electrochemistry and both of them yield stable radical anionic complexes with mainly bpym ligand centred spin density as shown by EPR spectroscopy of the free ligand bpym and the nickel complexes. The molecular structure of the binuclear bpym complex [(,-bpym){Ni(Mes)Br}2] was studied by EXAFS in comparison to the mononuclear analogue [(bpym)Ni(Mes)Br] revealing markedly increased Ni,C/N distance of the first coordination shell for the binuclear derivative suggesting an optimum overlap for the mononuclear complex, while two nickel complex fragments {Ni(Mes)Br} are seemingly too large to fit into the bis-chelate coordination site. [source] Theory of chemical bonds in metalloenzymes III: Full geometry optimization and vibration analysis of ferredoxin-type [2Fe,2S] clusterINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Mitsuo Shoji Abstract The nature of chemical bonds in a ferredoxin-type [2Fe,2S] cluster has been investigated on the basis of natural orbitals and several bond indices developed in Parts I and II of this study. The broken-symmetry hybrid density functional theory (BS-HDFT) with spin projection approach has been applied to elucidate the natural orbitals and occupation numbers for a model compound [Fe2S2(SCH3)4] (1), which is used to calculate the indices. The molecular structure, vibration frequencies, electronic structures, and magnetic properties in both oxidized and reduced forms of 1 have been calculated and compared with the experimental values. The optimized molecular structures after approximate spin projection have been in good agreement with experimental data. The structure changes upon one-electron reduction have been slight (<0.1 Å) and only limited around one side of the Fe atom. Raman and infrared (IR) spectra have been calculated, and their vibration modes have been assigned using the bridging 34S isotope substitution. Their magnetic properties have been examined in terms of spin Hamiltonians that contain exchange interactions and double exchange interactions. The BS-HDFT methods have provided the magnetic parameters; i.e., effective exchange integral (J) values and valence delocalization (B) values, which agree with the experimental results. It is found that large charge transfer (CT) from the bridging sulfur to the iron atoms has led to the strong antiferromagnetic interactions between iron atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] trans -Resveratrol Protects Embryonic Mesencephalic Cells from tert -Butyl HydroperoxideJOURNAL OF NEUROCHEMISTRY, Issue 1 2000Electron Param Abstract : In recent years, the antioxidant and other pharmacological properties of resveratrol, a natural product present in grapes and wine, have attracted considerable interest from the biomedical research community. In an examination of the potential neuroprotective properties of the compound, we have investigated the ability of resveratrol to protect rat embryonic mesencephalic tissue, rich in dopaminergic neurones, from the prooxidant tert -butyl hydroperoxide. Using the electron paramagnetic resonance (EPR) spin-trapping technique, the main radicals detected in cell suspensions were the tert -butoxyl radical and the methyl radical, indicating the one-electron reduction of the peroxide followed by a ,-scission reaction. The appearance of EPR signals from the trapped radicals preceded the onset of cytotoxicity, which was almost exclusively necrotic in nature. The inclusion of resveratrol in incubations resulted in the marked protection of cells from tert -butyl hydroperoxide. In parallel spin-trapping experiments, we were able to demonstrate the scavenging of radicals by resveratrol, which involved direct competition between resveratrol and the spin trap for reaction with the radicals. To our knowledge, this is the first example in which cytoprotection by resveratrol has been demonstrated by EPR spin-trapping competition kinetics to be due to its scavenging of the radicals responsible for the toxicity of a prooxidant. [source] Free-radical scavenging activity of wormwood (Artemisia absinthium L) extractsJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 2 2005Jasna M Canadanovic-Brunet Abstract In an effort to discover new antioxidant natural compounds, wormwood (Artemisia absinthium L) an aromatic-bitter herb, was screened. The sequential extraction was realized with five solvents of different polarities (70% methanol, petroleum ether, chloroform, ethyl acetate, n -butanol). The antioxidative activity was tested by measuring their ability to scavenge stable 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical and reactive hydroxyl radical during the Fenton reaction trapped by 5,5-dimethyl-1-pyrroline- N -oxide (DMPO), using electron spin resonance (ESR) spectroscopy. Results demonstrated that the antiradical and antioxidative activity depend on the type and concentration of applied extracts and increased in the order ethyl acetate > methanol > n -butanol > chloroform > petroleum ether > remaining water extracts. The investigation showed that the antiradical activity increased with increasing concentration of all extracts. The high contents of total phenolic compounds (25.6 mg g,1) and total flavonoids (13.06 mg g,1) indicated that these compounds contribute to the antiradical and antioxidative activity. In a model system, the formation of o -semiquinone radicals from quercetin and chlorogenic acid was obtained to prove the mechanism (hydrogen donating and/or one-electron reduction) of free-radical scavenging activity. Copyright © 2004 Society of Chemical Industry [source] Cluster ions of diquat and paraquat in electrospray ionization mass spectra and their collision-induced dissociation spectraRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 12 2003Boris L. Milman Cluster ions such as [Cat+X+nM]+ (n,=,0,4); [Cat-H+nM]+ (n,=,1,3); and [2(Cat-H)+X+nM]+ (n,=,0,2), where Cat, X, and M are the dication, anion, and neutral salt (CatX2), respectively, are observed in electrospray ionization (ESI) mass spectrometry of relatively concentrated solutions of diquat and paraquat. Collision-induced dissociation (CID) reactions of the clusters were observed by tandem mass spectrometry (MS/MS), including deprotonation to form [Cat-H]+, one-electron reduction of the dication to form Cat+., demethylation of the paraquat cation to form [Cat-CH3]+, and loss of neutral salt to produce smaller clusters. The difference in acidity and reduction power between diquat and paraquat, evaluated by thermodynamical estimates, can rationalize the different fractional yields of even-electron ([Cat-H]+ and its clusters) and odd-electron (mostly Cat+.) ions in ESI mass spectra of these pesticides. The [Cat+n,·,Solv]2+ doubly charged cluster ions, where n,,,2 and Solv is the solvent molecule (methanol and/or water), are only observed as very weak peaks in precursor ion CID spectra of the Cat2+ salt cation at low collision energy. The presence of an anion and a solvent molecule in a cluster is assumed to be related to existence of tight and loose ion pairs, respectively, in multiply charged droplets/ions formed by ESI. The results emphasize again the role of solution chemistry concepts such as acidity/basicity, redox power, and ion-pair formation, for ESI. Copyright © 2003 John Wiley & Sons, Ltd. [source] Purification, crystallization and X-ray crystallographic analysis of Archaeoglobus fulgidus neelaredoxinACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 3 2010Tiago M. Bandeiras Neelaredoxins are a type of superoxide reductase (SOR), which are blue 14,kDa metalloproteins with a catalytic nonhaem iron centre coordinated by four histidines and one cysteine in the ferrous form. Anaerobic organisms such as Archaeoglobus fulgidus, a hyperthermophilic sulfate-reducing archaeon, have developed defence mechanisms against toxic oxygen species in which superoxide reductases play a key role. SOR is responsible for scavenging toxic superoxide anion radicals (O2·,), catalysing the one-electron reduction of superoxide to hydrogen peroxide. Crystals of recombinant A. fulgidus neelaredoxin in the oxidized form (13.7,kDa, 125 residues) were obtained using polyethylene glycol and ammonium sulfate. These crystals diffracted to 1.9,Å resolution and belonged to the tetragonal space group P41212, with unit-cell parameters a = b = 75.72, c = 185.44,Å. Cell-content analysis indicated the presence of a tetramer in the asymmetric unit, with a Matthews coefficient (VM) of 2.36,Å3,Da,1 and an estimated solvent content of 48%. The three-dimensional structure was determined by the MAD method and is currently under refinement. [source] High-resolution structure of the antibiotic resistance protein NimA from Deinococcus radioduransACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 6 2008Hanna-Kirsti S. Leiros Many anaerobic human pathogenic bacteria are treated using 5-nitroimidazole-based (5-Ni) antibiotics, a class of inactive prodrugs that contain a nitro group. The nitro group must be activated in an anaerobic one-electron reduction and is therefore dependent on the redox system in the target cells. Antibiotic resistance towards 5-Ni drugs is found to be related to the nim genes (nimA, nimB, nimC, nimD, nimE and nimF), which are proposed to encode a reductase that is responsible for converting the nitro group of the antibiotic into a nonbactericidal amine. A mechanism for the Nim enzyme has been proposed in which two-electron reduction of the nitro group leads to the generation of nontoxic derivatives and confers resistance against these antibiotics. The cofactor was found to be important in the mechanism and was found to be covalently linked to the reactive His71. In this paper, the 1.2,Å atomic resolution crystal structure of the 5-nitroimidazole antibiotic resistance protein NimA from Deinococcus radiodurans (DrNimA) is presented. A planar cofactor is clearly visible and well defined in the electron-density map adjacent to His71, the identification of the cofactor and its properties are discussed. [source] A Novel Class of Antitumor Prodrug, 1-(2,-Oxopropyl)-5-fluorouracil (OFU001), That Releases 5-Fluorouracil upon Hypoxic IrradiationCANCER SCIENCE, Issue 4 2000Yuta Shibamoto We have been developing prodrugs of anticancer agents such as 5-fluorouracil (5-FU) that are activated by irradiation under hypoxic conditions via one-electron reduction. Among them, OFU001 [1-(2,-oxopropyl)-5-fluorouracil] is a prototype radiation-activated prodrug. In this study, we investigated the radiation chemical reactivity and the biological effects of OFU001. This prodrug is presumed to release 5-FU through incorporation of hydrated electrons into the antibonding ,* orbital of the C(1,)-N(1) bond. Hydrated electrons are active species derived from radiolysis of water, but are readily deactivated by O2 into superoxide anion radicals () under conditions of aerobic irradiation. Therefore, 5-FU release occurs highly specifically upon irradiation under hypoxic conditions. OFU001 dissolved in phosphate buffer released 5-FU with a G -value (mol number of molecules that are decomposed or produced by 1 J of absorbed radiation energy) of 1.9×10,7 mol/ J following hypoxic irradiation, while the G -value for 5-FU release was 1.0×10,8 mol/J following aerobic irradiation. However, the G -values for decomposition of OFU001 were almost the same, i.e., 3.4×10,7 mol/J following hypoxic irradiation and 2.5×10,7 mol/J following aerobic irradiation. When hypoxically irradiated (7.5,30 Gy) OFU001 was added to murine SCCVII cells for 1,24 h, a significant cell-killing effect was observed. The degree of this cytotoxicity was consistent with that of authentic 5-FU at the corresponding concentrations. On the other hand, cytotoxicity was minimal when the cells were treated with aerobically irradiated or unirradiated OFU001. This compound had no radiosensitizing effect against SCCVII cells under either aerobic or hypoxic conditions when the drug was removed immediately after irradiation. Since hypoxia is generally most marked in tumors and irradiation is applied at the tumor site, this concept of prodrug design appears to be potentially useful for selective tumor treatment with minimal adverse effects of anticancer agents. [source] The Role of One-Electron Reduction of Lipid Hydroperoxides in Causing DNA DamageCHEMISTRY - A EUROPEAN JOURNAL, Issue 40 2009Conor Crean Dr. Abstract The in vivo metabolism of plasma lipids generates lipid hydroperoxides that, upon one-electron reduction, give rise to a wide spectrum of genotoxic unsaturated aldehydes and epoxides. These metabolites react with cellular DNA to form a variety of pre-mutagenic DNA lesions. The mechanisms of action of the radical precursors of these genotoxic electrophiles are poorly understood. In this work we investigated the nature of DNA products formed by a one-electron reduction of (13S)-hydroperoxy-(9Z,11E)-octadecadienoic acid (13S -HPODE), a typical lipid molecule, and the reactions of the free radicals thus generated with neutral guanine radicals, G(,H).. A novel approach was devised to generate these intermediates in solution. The two-photon-induced ionization of 2-aminopurine (2AP) within the 2,-deoxyoligonucleotide 5,-d(CC[2AP]TCGCTACC) by intense nanosecond 308,nm excimer laser pulses was employed to simultaneously generate hydrated electrons and radical cations 2AP.+. The latter radicals either in cationic or neutral forms, rapidly oxidize the nearby G base to form G(,H).. In deoxygenated buffer solutions (pH,7.5), the hydrated electrons rapidly reduce 13S -HPODE and the highly unstable alkoxyl radicals formed undergo a prompt ,-scission to pentyl radicals that readily combine with G(,H).. Two novel guanine products in these oligonucleotides, 8-pentyl- and N2 -pentylguanine, were identified. It is shown that the DNA secondary structure significantly affects the ratio of 8-pentyl- and N2 -pentylguanine lesions that changes from 0.9:1 in single-stranded, to 1:0.2 in double-stranded oligonucleotides. The alkylation of guanine by alkyl radicals derived from lipid hydroperoxides might contribute to the genotoxic modification of cellular DNA under hypoxic conditions. Thus, further research is warranted on the detection of pentylguanine lesions and other alkylguanines in vivo. [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] Infrared Study of Intercomponent Interactions in a Switchable Hydrogen-Bonded RotaxaneCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2008Dhiredj Abstract The macrocycle in rotaxane 1 is preferentially hydrogen bonded to the succinamide station in the neutral form, but can be moved to the naphthalimide station by one-electron reduction of the latter. The hydrogen bonding between the amide NH groups of the macrocycle and the CO groups in the binding stations in the thread was studied with IR spectroscopy in different solvents in both states. In addition, the solvent effect on the vibrational frequencies was analyzed; a correlation with the solvent acceptor number (AN) was observed. The conformational switching upon reduction could be detected by monitoring the hydrogen-bond-induced shifts of the ,(CO) frequencies of the CO groups of the succinamide and the reduced naphthalimide stations. The macrocycle was found to shield the encapsulated station from the solvent: wavenumbers of ,(CO) bands of the CO groups residing inside the macrocycle cavity remain unaffected by the solvent polarity. [source] Ring Opening of the Cyclobutane in a Thymine Dimer Radical AnionCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007Chryssostomos Chatgilialoglu Dr. Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source] 2,5-Diphenyl-3,4-bis(2-pyridyl)cyclopenta-2,4-dien-1-one as a Redox-Active Chelating LigandCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2004Ulrich Siemeling Prof. Abstract 2,5-Diphenyl-3,4-bis(2-pyridyl)cyclopenta-2,4-dien-1-one (1), a close relative of tetraphenylcyclopentadienone, is a new ligand platform for use in redox switches and sensors. Compound 1 acts as a molecular electrochemical sensor towards a range of divalent metal ions and exhibits favourable two-wave behaviour. It forms chelates of the type [(1)MX2], whose stability is enhanced by five orders of magnitude upon one-electron reduction. The bite angle of 1 is close to 90° in these complexes. The attachment of the 14-valence-electron Cp*Co fragment to the cyclopentadienone , system reduces the bite angle and thus modulates the binding characteristics of 1. [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] Influence of pH on the Photochemical and Electrochemical Reduction of the Dinuclear Ruthenium Complex, [(phen)2Ru(tatpp)Ru(phen)2]Cl4, in Water: Proton-Coupled Sequential and Concerted Multi-Electron ReductionCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2005Norma R. de Tacconi Prof. Abstract The dinuclear ruthenium complex [(phen)2Ru(tatpp)Ru(phen)2]4+ (P; in which phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraaza tetrapyrido[3,2-a:2,3,-c:3,,,2,,-l:2,,,,3,,,]-pentacene) undergoes a photodriven two-electron reduction in aqueous solution, thus storing light energy as chemical potential within its structure. The mechanism of this reduction is strongly influenced by the pH, in that basic conditions favor a sequential process involving two one-electron reductions and neutral or slightly acidic conditions favor a proton-coupled, bielectronic process. In this complex, the central tatpp ligand is the site of electron storage and protonation of the central aza nitrogen atoms in the reduced products is observed as a function of the solution pH. The reduction mechanism and characterization of the rich array of products were determined by using a combination of cyclic and AC voltammetry along with UV-visible reflectance spectroelectrochemistry experiments. Both the reduction and protonation state of P could be followed as a function of pH and potential. From these data, estimates of the various reduced species' pKa values were obtained and the mechanism to form the doubly reduced, doubly protonated complex, [(phen)2Ru(H2tatpp)Ru(phen)2]4+ (H2P) at low pH (,7) could be shown to be a two-proton, two-electron process. Importantly, H2P is also formed in the photochemical reaction with sacrificial reducing agents, albeit at reduced yields relative to those at higher pH. [source] | |||||||||||||