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Radical Species (radical + species)
Kinds of Radical Species Selected AbstractsPhotodynamic therapy against cyanobacteriaENVIRONMENTAL TOXICOLOGY, Issue 1 2007M. Drábková Abstract This study explores the use of photosensitizers and reactive oxygen species (ROS) to limit growth of cyanobacteria. We chose 12 phthalocyanines, tetraphenol porphyrine, and methylene blue as compounds producing singlet oxygen. Hydrogen peroxide was chosen as another source of ROS. These compounds were tested using algal toxicity tests in microplates on three cultures of green algae (Pseudokirchneriella subcapitata, Scenedesmus quadricauda, and Chlorella kessleri) and on three cultures of cyanobacteria (Synechococcus nidulans, Microcystis incerta, and Anabaena sp.). Results indicate that photosensitizers and singlet oxygen could be highly toxic for some selected phytoplankton species. Green alga Scenedesmus quadricauda was highly sensitive (EC50 = 0.07 mg/L) to compounds producing singlet oxygen, although it was not sensitive to hydrogen peroxide, which was about 10 times more toxic for cyanobacteria. We conclude that the compounds producing hydroxyl radical species seems to be more promising to treat cyanobacterial blooms than the compounds producing the singlet oxygen. © 2007 Wiley Periodicals, Inc. Environ Toxicol 22: 112,115, 2007. [source] Zonisamide Reduces the Increase in 8-Hydroxy-2,-Deoxyguanosine Levels Formed During Iron-Induced Epileptogenesis in the Brains of RatsEPILEPSIA, Issue 9 2000M. Komatsu Summary: Purpose: To examine the change of 8,-hydroxy-2,-deoxyguanosine (8-OHdG) levels, which are used as a marker for oxidative DNA damage, in iron-induced epileptogenic foci of the rat cerebrum. Method: Male Wistar rats were given a cortical injection of ferric chloride, and their 8-OHdG levels were determined over time. Additional animals were pretreated with the antiepileptic drug zonisamide (ZNS) before the ferric chloride injection, and their 8-OHdG levels were compared with the nonpretreated rats. Results: Fifteen minutes after ferric chloride solution injection, the level of 8-OHdG increased, reaching a maximum 30 minutes after injection. Sixty minutes after injection, the levels coincided with those of controls. ZNS, in concentrations of 50 and 100 mg/kg body weight, prevented the increase of 8-OHdG levels within the cerebrum 30 minutes after iron solution injection. Conclusions: These results indicate that the formation of iron-induced epileptogenic foci in rats is related to DNA-damage-induced reactive oxygen species and that the inhibition of 8-OHdG formation by ZNS after iron injection may be due to the drug's antioxidant activity. The data suggest that free radical species known to be formed during iron salts,induced focal epileptogenesis cause damage to isocortical DNA. Furthermore, ZNS appears to inhibit the focal injuring response to DNA that occurs following iron salts,induced acute epileptogenesis. [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] A Rigid Molecular Scaffold Affixing a (Polypyridine)ruthenium(II)- and a Nickel(II)-Containing Complex: Spectroscopic Evidence for a Weakly Coupled Bichromophoric SystemEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2003Yann Pellegrin Abstract The synthesis of DppztBuSalH2 (7), a rigid conjugated ditopic ligand containing a Dppz (dipyrido[3,2- a:2,,3,- c]phenazine) skeleton and a salophen-type chelate, is reported. The complexes DppztBuSalNi (10), [Ru(bpy)2(DppztBuSalH2)]2+ (11), and [Ru(bpy)2(DppztBuSalNi)]2+ (12) have been prepared and characterised using common spectroscopic methods. Electrochemical, UV/Vis spectroelectrochemical and EPR studies were conducted on compounds 7, 10, 11, and 12. The singly reduced radical forms of 7 and 10 can be generated electrochemically, with the lone electron located on the low-lying phenazine ,*-molecular orbital. Complexes 11 and 12 show several reduction waves and electronic and EPR data obtained for the electrogenerated singly reduced species show them to be closely related to the radical species 7·, and 10·,, respectively. The presence of nickel(II) in compound 12 renders the addition of the second electron on the phenazine group reversible. Both 11 and 12 show common features on the cathodic side of their cyclic voltammograms, with reversible one-electron ruthenium-centred oxidation. An additional low-potential reversible-oxidation wave is observed for 12, and this is ascribed to oxidation of the nickel(II) ion. The combined spectroscopic data best describe the ruthenium-containing complexes as weakly coupled bichromophoric systems. Photophysical studies attest to the formation of a charge-separated state for 11, whereas a strong quenching is detected for 12. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Respiratory oscillations in yeast: mitochondrial reactive oxygen species, apoptosis and time; a hypothesisFEMS YEAST RESEARCH, Issue 4 2003David Lloyd Abstract Oscillatory metabolic activities occur more widely than is generally realised; detectability requires observation over extended times of single yeast cells or synchrony of individuals to provide a coherent population. Where oscillations in intracellular metabolite concentrations are observed, the phenomenon has been ascribed to sloppy control, energetic optimisation, signalling, temporal compartmentation of incompatible reactions, or timekeeping functions. Here we emphasise the consequences of respiratory oscillations as a source of mitochondrially generated reactive O2 metabolites. Temporal co-ordination of intracellular activities necessitates a time base. This is provided by an ultradian clock, and one result of its long-term operation is cyclic energisation of mitochondria, and thereby the generation of deleterious free radical species. Our hypothesis is that unrepaired cellular constituents and components (especially mitochondria) eventually lead to cellular senescence and apoptosis when a finite number of respiratory cycles has occurred. [source] Insect glutathione transferases and insecticide resistanceINSECT MOLECULAR BIOLOGY, Issue 1 2005A. A. Enayati Abstract Glutathione transferases (GSTs) are a diverse family of enzymes found ubiquitously in aerobic organisms. They play a central role in the detoxification of both endogenous and xenobiotic compounds and are also involved in intracellular transport, biosynthesis of hormones and protection against oxidative stress. Interest in insect GSTs has primarily focused on their role in insecticide resistance. GSTs can metabolize insecticides by facilitating their reductive dehydrochlorination or by conjugation reactions with reduced glutathione, to produce water-soluble metabolites that are more readily excreted. In addition, they contribute to the removal of toxic oxygen free radical species produced through the action of pesticides. Annotation of the Anopheles gambiae and Drosophila melanogaster genomes has revealed the full extent of this enzyme family in insects. This mini review describes the insect GST enzyme family, focusing specifically on their role in conferring insecticide resistance. [source] Kinetics of aerobic and anaerobic oxidations of ethanol by Fenton's reagentINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2008Mordechai L. Kremer For the first time, the time dependence of [H2O2] and [Fe2+] was followed during the aerobic oxidation of ethanol by Fenton's reagent. It was found that part of the ethanol was oxidized by dissolved O2 via the transient formation of H2O2. A model was set up based on FeO2+ as the key intermediate. Both one- and two-equivalent oxidations of ethanol occur, the former producing radical species derived from ethanol. No free radicals derived from H2O2 play part in the system. The relevant rate constants or their ratios were determined. The mechanism accounted successfully also for the anaerobic oxidation of ethanol. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 541,553, 2008 [source] Use of electron spin resonance measurements on irradiated sperma lentil seeds to indicate accidental irradiationINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 1 2003Mustafa Korkmaz Summary The results of electron spin resonance studies on ,-irradiated micro- and macrosperma lentil seeds are reported. Spectra of non-irradiated intact sperma were composed of an equally spaced sextet originating from the presence of Mn2+ ions and a single weak resonance line. Irradiation produced a linear increase in the radical signal intensity in the radiation dose range (0.5,5 kGy) studied, without affecting the Mn2+ signal. Signal intensities of both sperma followed compound exponential decay curves originating from the presence of three different radical species. Heating the sperma cause irreversible decreases in both radical and Mn2+ signal intensities. Two radical species, described in the present work, and a radical of unknown origin were used to explain the experimental results. [source] Extended Hartree,Fock theory of chemical reactions.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2008VIII. Abstract We have investigated the reaction pathways for the primary hydroxylation reaction of trimethylmethane by a high-valent Fe(IV)O porphyrin ,-cation radical species known as compound I at the B3LYP/CEP-31G level. The isoelectronic analogy of the Fe(IV)O core of compound I to a molecular oxygen (O2) has been successfully used to clarify the important roles of the singlet excited state of the Fe(IV)O core in the alkane hydroxylation, which has hitherto been neglected. The reaction is initiated by the rate-determining hydrogen-atom abstraction from the substrate to give a discrete radical intermediate complex, in accordance with the conventional radical rebound mechanism. Similar to the chemistry of O2, however, one of the singlet excited states, i.e., the diradical component of the 1, state of the Fe(IV)O core intercepts the triplet ground state (the 3, state) in the region of the transition state for the hydrogen abstraction. Our findings strongly indicate that the exchange polarization or intersystem crossing for the nonradiative transition to the locally singlet state is highly important to enhance the reactivity of compound I. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Antioxidant polyphenols from the mycelial culture of the medicinal fungi Inonotus xeranticus and Phellinus linteusJOURNAL OF APPLIED MICROBIOLOGY, Issue 6 2008J.-Y. Jung Abstract Aims:, The medicinal fungi Inonotus xeranticus and Phellinus linteus in the family Hymenochaetaceae have been used as traditional medicines for the treatment of various diseases. However, the compound responsible for the antioxidant activity is still unknown. Therefore, this study was conducted to characterize the antioxidant substances present in cultured broths made from these fungi. Methods and Results:, Antioxidant fractions of the cultured broths obtained from I. xeranticus and P. linteus were analysed using reversed-phase HPLC, which revealed several peaks that exhibited a potent free radical scavenging activity. To identify these antioxidant peaks, an I. xeranticus strain was mass-cultured, and the cultured broth was separated using antioxidant activity-guided fractionation. Four major active substances were purified and identified as hispidin and its dimers, 3,14,-bihispidinyl, hypholomine B, and 1,1-distyrylpyrylethan based on spectroscopic analyses. All compounds exhibited a significant scavenging activity against these radical species in a concentration-dependent manner. Conclusions:, Antioxidant substances found in the cultured broths of the medicinal fungi I. xeranticus and P. linteus were identified as hispidin and its dimers, 3,14,-bihispidinyl, hypholomine B, and 1,1-distyrylpyrylethan. Significance and Impact of the Study:, Polyphenol antioxidants were isolated from the cultured broth of the medicinal fungi I. xeranticus and P. linteus and identified based on extensive spectroscopic analyses. These compounds exhibited a strong antioxidant activity. [source] Gas-phase-assisted surface polymerization of methyl methacrylate with Fe(0)/TsCl initiator systemJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2007Yoshito Andou Abstract To obtain a high polymer coated Fe(0) surface, gas-phase-assisted surface polymerization (GASP) of methyl methacrylate (MMA) was investigated using a zero-valent iron (Fe(0))/p -toluene sulfonylchloride (TsCl) initiator system, resulting in successful high polymer production on the solid surface. GASP was found to be initiated by radical species that might have been generated via redox reactions with Fe(0), Fe(II), Fe(III), and TsCl. From 1H-NMR analysis, the p -toluene sulfonyl group was found at one end of the polymer chain. The molecular weight of obtained PMMA drastically decreased with increase in the composition ratio of Fe(0) in the initiator system, and increased with increase in polymer yield. From the results, it was assumed that the physically controlled polymerization of MMA proceeded by immobilized active species at gas,solid interfaces. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1879,1886, 2007 [source] Quinolinic acid modulates the activity of src family kinases in rat striatum: in vivo and in vitro studiesJOURNAL OF NEUROCHEMISTRY, Issue 5 2006Alessio Metere Abstract Quinolinic acid (QA) has been shown to evoke neurotoxic events via NMDA receptor (NMDAR) overactivation and oxidative stress. NMDARs are particularly vulnerable to free radicals, which can modulate protein tyrosine kinase (PTK) and phosphotyrosine phosphatase (PTP) activities. The src family of tyrosine kinases are associated with the NMDAR complex and regulate NMDA channel function. Because QA is an NMDAR agonist as well as a pro-oxidant agent, we investigated whether it may affect the activity of PTKs and PTPs in vivo and in vitro. In synaptosomes prepared from striata dissected 15 min, 30 min or 15 days after bilateral injection of QA we observed modulation of the phosphotyrosine pattern; a significant decrease in PTP activity; and a sustained increase in c-src and lyn activity at 15 and 30 min after treatment with QA, followed by a decrease 2 weeks later. Striatal synaptosomes treated in vitro with QA showed time- and dose-dependent modulation of c-src and lyn kinase activities. Moreover, the nitric oxide synthase inhibitor NG -nitro- l -arginine-methyl ester, the NMDAR antagonist d -2-amino-5-phosphonovaleric acid and pyruvate suppressed the QA-induced modulation of c-src activity. These findings suggest a novel feature of QA in regulating src kinase activity through the formation of reactive radical species and/or NMDAR overactivation. [source] Peroxynitrite and nitrosoperoxycarbonate, a tightly connected oxidizing-nitrating couple in the reactive nitrogen-oxygen species family: new perspectives for protection from radical-promoted injury by flavonoidsJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 12 2007Radmila Pavlovic Peroxynitrite is the product of the reaction of nitric oxide with superoxide radical and is implicated in the pathogenesis of a wide variety of human diseases, being responsible for in-vivo oxidation/nitration events. Nitrosoperoxycarbonate anion, formed by the interaction of peroxynitrite with CO2/bicarbonate at physiological concentrations, provides a new interpretation of oxidative/nitrative processes formerly attributed to peroxynitrite. The aim of this review is to summarize the chemistry and biology of peroxynitrite and radical species related to nitrosoperoxycarbonate anion, as well as the information available regarding the molecular mechanisms that determine and regulate radical-promoted injury by the two tightly connected species at physiological concentrations. Interception of carbonate and nitro radicals produced by interaction of peroxynitrite with CO2/bicarbonate, as in-vivo prevention of pathological events, creates new perspectives for the evaluation of safe scavengers of oxidative/nitrative stress at the physiological level. In this respect, natural products such as flavonoids hold a preeminent position among the vast array of compounds endowed with such properties. [source] Photoamination of 1-hydroxyanthraquinone in a water-acetonitrile mixed solventJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2009Masahiro Tajima Abstract We investigated the photoamination kinetics of 1-hydroxyanthraquinones in a water-acetonitrile mixed solvent by stationary light irradiation. The progression of the reaction under an inert atmosphere demonstrated the existence of additional reaction pathways. The excited triplet state of the base form of the 1-hydroxyanthraquinones was found to be responsible for the additional pathways. On the other hand, the same reaction under air involved an attack of the amine-derived radical species (aminium and amino radicals) on the ground state of the 1-hydroxyanthraquinones. One of the pathways preferentially led to 2-amination, and the other led exclusively to 4-amination. Results also indicate the attainment of an equilibrium state between the aminium and amino radicals. Copyright © 2008 John Wiley & Sons, Ltd. [source] An ESR and electrochemical approach to the unusual reactivity of ferrocenoylsilanes with organometallic compoundsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2004A. Alberti Abstract ESR studies carried out during the reaction of ferrocenoylsilanes with Grignard reagents at low temperature led to the detection of the radical anions of the former compounds. Cyclic voltammetric experiments indicate that the observed radical anions are likely to originate through an inner-sphere electron transfer process. Different radical species were observed upon annealing, originating from destruction of the ferrocenyl moiety. Paramagnetic species were also detected on reacting ferrocenoylsilanes with LiAlH4. Copyright © 2004 John Wiley & Sons, Ltd. [source] Photopolymerization of 1,6-hexanedioldiacrylate initiated by three-component systems based on N -arylphthalimidesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 16 2004T. Brian Cavitt Abstract Three-component photoinitiators comprised of an N -arylphthalimide, a diarylketone, and a tertiary amine were investigated for their initiation efficiency of acrylate polymerization. The use of an electron-deficient N -arylphthalimide resulted in a greater acrylate polymerization rate than an electron-rich N -arylphthalimide. Triplet energies of each N -arylphthalimide, determined from their phosphorescence spectra, and the respective rate constants for triplet quenching by the N -arylphthalimide derivatives (acquired via laser flash photolysis) indicated that an electron,proton transfer from an intermediate radical species to the N -arylphthalimide (not energy transfer from triplet sensitization) is responsible for generating the initiating radicals under the conditions and species concentrations used for polymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4009,4015, 2004 [source] Photoprotection by Porcine Eumelanin Against Singlet Oxygen Production,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2008Alice Wang Melanin, a major pigment found in retinal pigment epithelium (RPE) cells, is considered to function in dual roles, one protective and one destructive. By quenching free radical species and reactive oxygen species (ROS) melanin counteracts harmful redox stress. However, melanin is also thought to be capable of creating ROS. In this destructive role, melanin increases redox strain in the cell. This study uses readily available eumelanin extracted from porcine RPE cells as a more authentic model than synthetic melanin to determine specific mechanisms of melanin activity with regard to singlet oxygen in the presence and absence of rose bengal, a singlet-oxygen photosensitizer. Optical detection of singlet-oxygen was determined by monitoring the bleaching of p -nitrosodimethylaniline in the presence of histidine. Production of singlet oxygen in aqueous oxygen-saturated solutions of rose bengal without eumelanin was readily accomplished. In contrast, detection of singlet oxygen in oxygen-saturated solutions of eumelanin without rose bengal failed, consistent with results of others. However, a significant decrease in singlet oxygen production by rose bengal was observed in the presence of eumelanin. After correction for light absorption and chemical bleaching of eumelanin, the results show that eumelanin also provides a photoprotective mode arising from chemistry, that is, not just the physical process of light absorption followed by energy dissipation as heat. [source] Electron capture dissociation mass spectrometry of peptide cations containing a lysine homologue: a mobile proton model for explaining the observation of b-type product ionsRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 21 2006Sunyoung Lee Eleven doubly protonated peptides with a residue homologous to lysine were investigated by electron capture dissociation mass spectrometry (ECD-MS). Lysine homologues provide the unique opportunity to examine the ECD fragmentation behavior by allowing us to vary the length of the lysine side chain, with minimal structural change. The lysine homologue has a primary amine side chain with a length that successively decreases by one methylene (CH2) unit from the CH2CH2CH2CH2NH2 of lysine and the accompanying decrease of its proton affinities: lysine (K), 1006.5(±7.2) kJ/mol; ornithine (K*), 1001.1(±6.6) kJ/mol; 2,4-diaminobutanoic acid (K**), 975.8(±7.4) kJ/mol; 2,3-diaminopropanoic acid (K***), 950.2(±7.2) kJ/mol. In general, the lysine-homologous peptides exhibited overall ECD fragmentation patterns similar to that of the lysine-containing peptides in terms of the locations, abundances, and ion types of products, such as yielding c+ and z+. ions as the dominant product ions. However, a close inspection of product ion mass spectra showed that ECD-MS for the alanine-rich peptides with an ornithinyl or 2,4-diaminobutanoyl residue gave rise to b ions, while the lysinyl-residue-containing peptides did not, in most cases, produce any b ions. The peptide selectivity in the generation of b+ ions could be understood from within the framework of the mobile proton model in ECD-MS, previously proposed by Cooper (Ref. 29). The exact mass analysis of the resultant b ions reveals that these b ions are not radical species but rather the cationic species with R-CO+ structure (or protonated oxozalone ion), that is, b+ ions. The absence of [M+2H]+. species in the ECD mass spectra and the selective b+ -ion formation are evidence that the peptides underwent H-atom loss upon electron capture, and then the resulting reduced species dissociated following typical MS/MS fragmentation pathways. This explanation was further supported by extensive b+ ions generated in the ECD of alanine-based peptides with extended conformations. Copyright © 2006 John Wiley & Sons, Ltd. [source] Insight into the free-radical-scavenging mechanism of hydroxyl-substituent Schiff bases in the free-radical-induced hemolysis of erythrocytesCELL BIOCHEMISTRY AND FUNCTION, Issue 6 2007You-Zhi Tang Abstract This work aimed to explore the mechanism by which hydroxyl-substituent Schiff bases scavenge free-radicals. Thus, four Schiff bases, that is benzylidene aniline (BAN), 2-(phenyliminomethyl)phenol (BAH), 4-benzimidoylphenol (PBH) and 2-benzimidoylphenol (OBH), were applied to protect human erythrocytes against 2,2,-azobis(2-amidinopropane hydrochloride) (AAPH)-induced hemolysis. The results revealed that the OH attached to the ortho -position of methylene in Schiff base scavenges 1.46 radicals per molecule, the OH attached to the para -position of the N atom scavenges 2.94 radicals and the OH attached to the ortho -position of the N atom scavenges 3.63 radicals. In addition, four Schiff bases were used together with some familiar antioxidants, such as 6-hydroxyl-2,5,7,8-tetramethyl chroman-2-carboxylic acid (Trolox), L -ascorbic acid (VC), ,-tocopherol (TOH) and L -ascorbyl-6-laurate (VC-12) in AAPH-induced hemolysis of erythrocytes. It was found that, except for BAN+VC-12, BAH,+,VC-12, OBH,+,VC-12 and PBH+TOH, all the other combinations protected erythrocytes more perfectly than when used individually. This result demonstrated that a promotive protection existed between Schiff base and other antioxidants and this improved their ability to scavenge free-radicals. Finally, IC50 values of the aforementioned Schiff bases together with 2-((o -hydroxylphenylimino) methyl)phenol (OSAP) and 2-((p -hydroxylphenylimino)methyl)phenol (PSAP) were determined by reaction with two radical species, that is, 2,2,-azinobis(3-ethylbenzothiazoline-6-sulfonate) radical (ABTS+·) and 2,2,-diphenyl-1-picrylhydrazyl (DPPH). The results implied that the molecular framework of a Schiff base and an OH attached to the ortho -position of methylene were apt to reduce radicals, but the OH attached to the aniline ring in a Schiff base was prone to scavenge radicals directly. Copyright © 2006 John Wiley & Sons, Ltd. [source] Influence of the Protonic State of an Imidazole-Containing Ligand on the Electrochemical and Photophysical Properties of a Ruthenium(II),Polypyridine-Type ComplexCHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2007Annamaria Quaranta Dr. Abstract The synthesis and characterisation of [Ru(bpy)2(PhenImHPh)]2+ where PhenImHPh represents the 2-(3,5-di- tert -butylphenyl)imidazo[4,5- f][1,10]phenanthroline ligand are described. The compounds issued from the three different protonic states of the imidazole ring [Ru(bpy)2(PhenImPh)]+ (I), [Ru(bpy)2(PhenImHPh)]2+ (II) and [Ru(bpy)2(PhenImH2Ph)]3+ (III) were isolated and spectroscopically characterised. The X-ray structures of [Ru(bpy)2(PhenImPh)](PF6),H2O,6,MeOH, [Ru(bpy)2(PhenImHPh)](NO3)2,H2O,3,MeOH and [Ru(bpy)2(PhenImH2Ph)](PF6)3, 5,H2O are reported. Electrochemical data obtained on these complexes indicate almost no potential shift for the RuIII/II redox couple. Therefore a Coulombic effect between the imidazole ring and the metal centre can be ruled out. The monooxidised forms of I and II have been characterised by EPR spectroscopy and are reminiscent of the presence of a radical species. The emission properties of the parent compound [Ru(bpy)2(PhenImHPh)]2+ were studied as a function of pH and both the lifetimes and intensities decreased upon deprotonation. Photophysical properties, investigated in the absence and presence of an electron acceptor (methylviologen), were distinctly different for the three compounds. Transient absorption features indicate that unique excited states are involved. Theoretical data obtained from DFT calculations in water on the three protonic forms are presented and discussed in the light of the experimental results. [source] Partial Oxidation of 4- tert -Butyltoluene Catalyzed by Homogeneous Cobalt and Cerium Acetate Catalysts in the Br,/H2O2/Acetic Acid System: Insights into Selectivity and MechanismCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2007Abstract The partial oxidation of 4- tert -butyltoluene to 4- tert -butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4- tert -butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4- tert -butylbenzyl bromide and its hydrolysis and solvolysis products 4- tert -butylbenzyl alcohol and 4- tert -butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)2 and Co(acac)2 are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75,80,% selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50,% due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72,% (cerium(III) acetate) or 58,% (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields. [source] |