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Second Electron Transfer (second + electron_transfer)
Selected AbstractsSonovoltammetric Elucidation of Electron Transfer Rates: The Oxidation of Dimethyl- p -phenylenediamine in Aqueous SolutionELECTROANALYSIS, Issue 4 2003Craig Abstract The electrochemical oxidation of dimethyl- p -phenylenediamine (DMPD) in aqueous solution (pH 7 phosphate buffer) has been studied under conventional hydrodynamic and microelectrode voltammetric conditions and found to undergo a two-electron electrochemically reversible oxidation. Upon the application of ultrasound to the system an observed shoulder emerges in the oxidation wave. This effect has been attributed to the resolution of the two-electron transfer processes occurring: the first a relatively fast electron transfer (0.1,cm s,1) followed by a second slower (10,3 cm s,1) electron transfer: under the very high mass transport rates induced by insonation an overpotential develops for the second electron transfer so leading to the observed voltammetric resolution. The range of mass transport conditions accessible via sonication allows the estimation of the two rate constants reported. [source] Transition metals as electron traps.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 10 2009Abstract Transition metal cations Co2+, Ni2+ and Zn2+ form 1 : 1 : 1 ternary complexes with 2,2,-bipyridine (bpy) and peptides in aqueous methanol solutions that have been studied for tripeptides GGG and GGL. Electrospray ionization of these solutions produced singly charged [Metal(bpy)(peptide , H)]+ and doubly charged [Metal(bpy)(peptide)]2+ ions (Metal = metal ion) that underwent charge reduction by glancing collisions with Cs atoms at 50 and 100 keV collision energies. Electron transfer to [Metal(bpy)(peptide)]2+ ions was less than 4.2 eV exoergic and formed abundant fractions of non-dissociated charge-reduced intermediates. Charge-reduced [Metal(bpy)(peptide)]+ ions dissociated by the loss of a hydrogen atom, ammonia, water and ligands that depended on the metal ion. The Ni and Co complexes mainly dissociated by the elimination of ammonia, water, and the peptide ligand. The Zn complex dissociated by the elimination of ammonia and bpy. A sequence-specific fragment was observed only for the Co complex. Electron transfer to [Metal(bpy)(peptide , H)]+ was 0.6,1.6 eV exoergic and formed intermediate radicals that were detected as stable anions after a second electron transfer from Cs. [Metal(bpy)(peptide , H)] neutrals and their anions dissociated by the loss of bpy and peptide ligands with branching ratios that depended on the metal ion. Optimized structures for several spin states, electron transfer and dissociation energies were addressed by combined density functional theory and Møller,Plesset perturbational calculations to aid interpretation of experimental data. The experimentally observed ligand loss and backbone cleavage in charge-reduced [Metal(bpy)(peptide)]+ complexes correlated with the dissociation energies at the present level of theory. The ligand loss in +CR, spectra showed overlap of dissociations in charge-reduced [Metal(bpy)(peptide , H)] complexes and their anionic counterparts which complicated spectra interpretation and correlation with calculated dissociation energies. Copyright © 2009 John Wiley & Sons, Ltd. [source] Alkyl halides reactions with cathodes or with magnesium.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2006Grignard reagent studied with radical clocks. Abstract In the mechanism of reaction of Grignard reagent formation for alkyl halides (RX), it is generally assumed that the alkyl radical, formed by the electron transfer from the metal to this halide, reacts rapidly with the paramagnetic MgX, species. The previous comparisons of aryl halides reactivity toward magnesium and their reactivity toward a cathode strongly suggested that MgX, species are not, for the aryl halides, compulsory to rationalise the observed facts. The aryl radicals formed by electron transfer from the metal to the aryl halide would undergo a rapid second electron transfer to yield carbanions transformed into RMgX by reaction with MgX2. In contrast, for the alkyl halides, the reduction of the rapidly formed alkyl radicals into carbanions has seldom been discussed as a possible fate for these radicals, the main discussed fates being dimerisation, disproportionation, hydrogen abstraction from the solvent, rearrangement or coupling with MgX, radicals. Two main differences distinguish the reactivity of alkyl halides from their aryl halides counterpart. First, the radical anions of aryl halides may have a given lifetime whereas electron transfer to alkyl halides is concerted with the cleavage of the molecule. Second, the aryl radicals display far stronger oxidising properties than the alkyl radicals. The counterpart of this property is that aryl carbanions display weaker reducing properties than the alkyl ones. In this report, putting in perspective Grignard reaction and the experimental results obtained with identical radical clocks in electrochemistry, we tentatively provide an answer to the question raised in the title. The comparison of electrochemical patterns of reactivity of selected alkyl halides and the evolutions of yields in the preparation of Grignard reagent suggest a new explanation for the lower yields generally observed when alkyl iodides are the starting substrates. It involves an autocatalytic reaction where carbanionic species formed from the alkyl radicals and diffusing away from the metal surface, transfer one electron to the alkyl halide; the result would be the creation of two radicals leading to an increased amount of by-products. If the carbanionic mechanism were to be retained for the formation of alkyl Grignard reagent one would have to admit that the magnesium surface behaves as a cathode displaying high current densities reminiscent of microelectrodes. Copyright © 2006 John Wiley & Sons, Ltd. [source] Hydroxycinnamic Acids as DNA-Cleaving Agents in the Presence of CuII Ions: Mechanism, Structure,Activity Relationship, and Biological ImplicationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 46 2009Gui-Juan Fan Abstract The effectiveness of hydroxycinnamic acids (HCAs), that is, caffeic acid (CaA), chlorogenic acid (ChA), sinapic acid (SA), ferulic acid (FA), 3-hydroxycinnamic acid (3-HCA), and 4-hydroxycinnamic acid (4-HCA), as pBR322 plasmid DNA-cleaving agents in the presence of CuII ions was investigated. Compounds bearing o -hydroxy or 3,5-dimethoxy groups on phenolic rings (CaA, SA, and ChA) were remarkably more effective at causing DNA damage than the compounds bearing no such groups; furthermore, CaA was the most active among the HCAs examined. The involvement of reactive oxygen species (ROS) and CuI ions in the DNA damage was affirmed by the inhibition of the DNA breakage by using specific scavengers of ROS and a CuI chelator. The interaction between CaA and CuII ions and the influence of ethylenediaminetetraacetic acid (EDTA), the solvent, and pH value on the interaction were also studied to help elucidate the detailed prooxidant mechanism by using UV/Vis spectroscopic analysis. On the basis of these observations, it is proposed that it is the CaA phenolate anion, instead of the parent molecule, that chelates with the CuII ion as a bidentate ligand, hence facilitating the intramolecular electron transfer to form the corresponding CaA semiquinone radical intermediate. The latter undergoes a second electron transfer with oxygen to form the corresponding o -quinone and a superoxide, which play a pivotal role in the DNA damage. The intermediacy of the semiquinone radical was supported by isolation of its dimer from the CuII -mediated oxidation products. Intriguingly, CaA was also the most cytotoxic compound among the HCAs toward human promyelocytic leukemia (HL-60) cell proliferation. Addition of exogenous CuII ions resulted in an effect dichotomy on cell viability depending on the concentration of CaA; that is, low concentrations of CaA enhanced the cell viability and, conversely, high concentrations of CaA almost completely inhibited the cell proliferation. On the other hand, when superoxide dismutase was added before, the two stimulation effects of exogenous CuII ions were significantly ameliorated, thus clearly indicating that the oxidative-stress level regulates cell proliferation and death. These findings provide direct evidence for the antioxidant/prooxidant mechanism of cancer chemoprevention. [source] | ||||||||||