Synthesis and electrochemical properties of fullerene-rich nanoclusters synthesized by cobalt-catalyzed cyclotrimerization of bis(aryl)alkyne fullerodendrimers.
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Alkynes↗
Biomedical subjects
Christian Amatore
Publications and source records attributed to Christian Amatore.
Rate and mechanism of the reaction of alkenes with aryl palladium complexes ligated by a bidentate P,P ligand in Heck reactions.
The regioselectivity of the Heck reaction is supposed to be highly affected by the electronic properties of the alkene and the ionic or neutral character of the aryl palladium(II) complexes involved in the reaction with alkenes. In Heck reactions performed in dmf, [Pd(dppp){dppp(O)}Ph](+) (dppp=1,2-bis(diphenylphosphino)propane) is generated in the oxidative addition of PhI with [Pd(0)(dppp)(OAc)](-) formed in situ from Pd(OAc)(2) associated to two equivalents of dppp. [Pd(dppp){dppp(O)}Ph](+) is not very reactive with alkenes (styrene or methyl acrylate); however, it reacts with iodide ions (released in the catalytic reactions) to give [Pd(dppp)IPh] and with acetate ions (used as base) to give [Pd(dppp)(OAc)Ph]. [Pd(dppp)(OAc)Ph] reacts with styrene and methyl acrylate exclusively by an ionic mechanism, that is, via the cationic complex [Pd(dppp)(dmf)Ph](+) formed by dissociation of the acetate ion. The reaction of [Pd(dppp)IPh] is more complex and substrate dependent. It reacts with styrene exclusively by the ionic mechanism via [Pd(dppp)(dmf)Ph](+). [Pd(dppp)IPh] (neutral mechanism) and [Pd(dppp)(dmf)Ph](+) (ionic mechanism) react in parallel with methyl acrylate. [Pd(dppp)(dmf)Ph](+) is more reactive than [Pd(dppp)IPh] but is always generated at lower concentration.
Journal Article↗
Electrocarboxylation of benzyl halides through redox catalysis on the preparative scale.
The electrocarboxylation of benzyl halides to the corresponding carboxylic acids through homogeneous charge-transfer catalysis was investigated both theoretically and experimentally to determine the influence of the operative parameters on the yield of the process and on the catalyst consumption. Theoretical considerations, based on fast kinetics of redox catalysis, were confirmed by the electrocarboxylation of 1-phenyl-1-chloroethane catalyzed by 1,3-benzenedicarboxylic acid dimethyl ester performed at a carbon cathode under different operative conditions. We obtained high yields of the target carboxylic acid and experienced a low catalyst consumption by operating with optimized [RX]bulk/[CO2]bulk and [RX]bulk/[catalyst] ratios.
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Glutamatergic Control of Microvascular Tone by Distinct GABA Neurons in the Cerebellum.
The tight coupling between increased neuronal activity and local cerebral blood flow, known as functional hyperemia, is essential for normal brain function. However, its cellular and molecular mechanisms remain poorly understood. In the cerebellum, functional hyperemia depends almost exclusively on nitric oxide (NO). Here, we investigated the role of different neuronal populations in the control of microvascular tone by in situ amperometric detection of NO and infrared videomicroscopy of microvessel movements in rat cerebellar slices. Bath application of an NO donor induced both NO flux and vasodilation. Surprisingly, endogenous release of NO elicited by glutamate was accompanied by vasoconstriction that was abolished by inhibition of Ca2+-phopholipase A2 and impaired by cyclooxygenase and thromboxane synthase inhibition and endothelin A receptor blockade, indicating a role for prostanoids and endothelin 1 in this response. Interestingly, direct stimulation of single endothelin 1-immunopositive Purkinje cells elicited constriction of neighboring microvessels. In contrast to glutamate, NMDA induced both NO flux and vasodilation that were abolished by treatment with a NO synthase inhibitor or with tetrodotoxin. These findings indicate that NO derived from neuronal origin is necessary for vasodilation induced by NMDA and, furthermore, that NO-producing interneurons mediate this vasomotor response. Correspondingly, electrophysiological stimulation of single nitrergic stellate cells by patch clamp was sufficient to release NO and dilate both intraparenchymal and upstream pial microvessels. These findings demonstrate that cerebellar stellate and Purkinje cells dilate and constrict, respectively, neighboring microvessels and highlight distinct roles for different neurons in neurovascular coupling.
Animals↗
Coupling of electrochemistry and fluorescence microscopy at indium tin oxide microelectrodes for the analysis of single exocytotic events.
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Barium↗
Mapping electrochemiluminescence as generated at double-band microelectrodes by confocal microscopy under steady state.
A confocal microspectrometer was used to collect spatially resolved electrochemiluminescence (ECL) spectra from the annihilation reaction between Ru(bpy)3+ and Ru(bpy)3(3+), each species being generated at a platinum band microelectrode and separated by a micrometric insulating gap. The use of a confocal microspectrometer allows in situ photon detection of the ECL reaction with micrometric spatial resolution and observes the concentration distribution of the photon source in the vicinity of the interelectrode gap.
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Mechanism of the palladium-catalyzed homocoupling of arylboronic acids: key involvement of a palladium peroxo complex.
The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2) (Ar = 4-Z-C(6)H(4) with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fully elucidated. The peroxo complex (eta(2)-O(2))PdL(2) (L = PPh(3)), generated in the reaction of dioxygen with the Pd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generate an adduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid) characterized by (31)P NMR spectroscopy and ab initio calculations. This adduct reacts with a second molecule of arylboronic acid to generate trans-ArPd(OH)L(2) complexes. A transmetalation by the arylboronic acid gives trans-ArPdArL(2) complexes. The biaryl is then released in a reductive elimination. This reaction is at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki-Miyaura reactions when they are not conducted under oxygen-free atmosphere.
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Modelling release of nitric oxide in a slice of rat's brain: describing stimulated functional hyperemia with diffusion-reaction equations.
The physicochemical process of nitric oxide (NO degrees ) release from an active neuron is modelled based on the results obtained experimentally in independent series of experiments reported elsewhere in which the NO degrees release elicited by patch-clamping a single neuron (stellate neuron from cerebellum area) is monitored by an ultramicroelectrode introduced into a slice of living rat's brain. This process is believed to be central to brain behaviour by coupling neuronal activity with the blood supply to active areas of the living brain through precise control of NO degrees -mediated dilatation of blood capillary vessels. This work, based on the conformal mapping approach initially proposed in a previous work, aims to model the overall physicochemical and diffusional processes giving rise to the release of NO degrees by a neuron and during its collection at an electrode sensor. Fitting simulated currents to experimental ones published previously yields indeed the gross kinetic information which represents the overall neuron activation and defines the instant value of the concentration of NO degrees at the neuron surface. This allows reconstructing the NO degrees fluxes around the neuron body as they would have been in the absence of the electrode sensor. This permits one to appreciate how far NO degrees is released by the neuron at concentrations which greatly exceed their basal values. The success of this procedure is exemplified using a set of three experimental data reported elsewhere.
Animals↗
In situ and online monitoring of hydrodynamic flow profiles in microfluidic channels based upon microelectrochemistry: optimization of electrode locations.
Herein, we extend our previous approach concerning the reconstruction of profiles of pressure-driven hydrodynamic flow in microfluidic channels based on current measurements at band electrode(s) [see the preceding paper ChemPhysChem 2005, 6, 1581]. We address the central issue of optimization of geometrical parameters describing the electrode(s) assembly (a single band and two bands working in generator-collector mode) within the channel flow cell to enhance the speed and precision of the flow profile reconstruction method.
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Nitric oxide release during evoked neuronal activity in cerebellum slices: detection with platinized carbon-fiber microelectrodes.
Nitric oxide is an important biological messenger that particularly induces the relaxation of smooth muscle cells surrounding vessels, and, hence, controls the flow of blood. This mechanism is essential for brain function, and its fine control, termed functional hyperemia, is supposed to be realized by certain neurons that may release bursts of NO*. The aim of the present study is to examine the advantages of platinized carbon-fiber microelectrodes (5-7 microm tip diameter) for the direct and in situ electrochemical detection of NO* released by neurons into ex vivo cerebellum slices. After establishing the different analytical properties of the platinized carbon-fiber microelectrodes in vitro on NO* solutions at 50 nM to 1 mM concentration, they were characterized using DEA-NONOate solutions that chemically decompose into NO*, and therefore mimic the measurement of transient variations of NO* concentration in biological samples. This validated the present approach, so that direct, in situ ex vivo measurements of nitric oxide released by neurons in a rat cerebellar slice are presented and discussed.
Animals↗
Monitoring in real time with a microelectrode the release of reactive oxygen and nitrogen species by a single macrophage stimulated by its membrane mechanical depolarization.
Macrophages are key cells of the immune system. During phagocytosis, the macrophage engulfs a foreign bacterium, virus, or particle into a vacuole, the phagosome, wherein oxidants are produced to neutralize and decompose the threatening element. These oxidants derive from in situ production of superoxide and nitric oxide by specific enzymes. However, the chemical nature and sequence of release of these compounds is far from being completely determined. The aim of the present work was to study the fundamental mechanism of oxidant release by macrophages at the level of a single cell, in real time and quantitatively. The tip of a microelectrode was positioned at a micrometric distance from a macrophage in a culture to measure oxidative-burst release by the cell when it was submitted to physical stimulation. The ensuing release of electroactive reactive oxygen and nitrogen species was detected by amperometry and the exact nature of the compounds was characterized through comparison with in vitro electrochemical oxidation of H2O2, ONOO-, NO*, and NO2(-) solutions. These results enabled the calculation of time variations of emission flux for each species and the reconstruction of the original flux of production of primary species, O2*- and NO*, by the macrophage.
Animals↗
Regulation of exocytosis in chromaffin cells by trans-insertion of lysophosphatidylcholine and arachidonic acid into the outer leaflet of the cell membrane.
Vesicular exocytosis is an important complex process in the communication between cells in organisms. It controls the release of chemical and biochemical messengers stored in an emitting cell. In this report, exocytosis is studied amperometrically (at carbon fiber ultramicroelectrodes) at adrenal chromaffin cells, which release catecholamines after appropriate stimulation, while testing the effects due to trans-insertion of two exogenous compounds (lysophosphatidylcholine (LPC) and arachidonic acid (AA)) on the kinetics of exocytotic events. Amperometric analyses showed that, under the present conditions (short incubation times and micromolar LPC or AA solutions), LPC favors catecholamine release (rate, event frequency, charge released) while AA disfavors the exocytotic processes. The observed kinetic features are rationalized quantitatively by considering a stalk model, for the fusion pore formation, and the physical constraints applied to the cell membrane by the presence of small fractions of LPC and AA diluted in its external leaflet (trans-insertion). We also observed that the detected amount of neurotransmitters in the presence of LPC was larger than under control conditions, while the opposite trend is observed with AA.
Action Potentials↗
Ferrocene-mediated proton-coupled electron transfer in a series of ferrocifen-type breast-cancer drug candidates.
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Antineoplastic Agents↗
In situ and online monitoring of hydrodynamic flow profiles in microfluidic channels based upon microelectrochemistry: concept, theory, and validation.
Herein, we propose a method for reconstructing any plausible macroscopic hydrodynamic flow profile occurring locally within a rectangular microfluidic channel. The method is based on experimental currents measured at single or double microband electrodes embedded in one channel wall. A perfectly adequate quasiconformal mapping of spatial coordinates introduced in our previous work [Electrochem. Commun. 2004, 6, 1123] and an exponentially expanding time grid, initially proposed [J. Electroanal. Chem. 2003, 557, 75] in conjunction with the solution of the corresponding variational problem approached by the Ritz method are used for the numerical reconstruction of flow profiles. Herein, the concept of the method is presented and developed theoretically and its validity is tested on the basis of the use of pseudoexperimental currents emulated by simulation of the diffusion-convection problem in a channel flow cell, to which a random Gaussian current noise is added. The flow profiles reconstructed by our method compare successfully with those introduced a priori into the simulations, even when these include significant distortions compared with either classical Poiseuille or electro-osmotic flows.
Journal Article↗
When voltammetry reaches nanoseconds.
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Journal Article↗
Correlation between vesicle quantal size and fusion pore release in chromaffin cell exocytosis.
A significant number of exocytosis events recorded with amperometry demonstrate a prespike feature termed a "foot" and this foot has been correlated with messengers released via a transitory fusion pore before full exocytosis. We have compared amperometric spikes with a foot with spikes without a foot at chromaffin cells and found that the probability of detecting a distinct foot event is correlated to the amount of catecholamine released. The mean charge of the spikes with a foot was found to be twice that of the spikes without a foot, and the frequency of spikes displaying a foot was zero for small spikes increasing to approximately 50% for large spikes. It is hypothesized that in chromaffin cells, where the dense core is believed to nearly fill the vesicle, the expanding core is a controlling factor in opening the fusion pore, that prefusion of two smaller vesicles leads to excess membrane, and that this slows pore expansion leading to an increased observation of events with a foot. Clearly, the physicochemical properties of vesicles are key factors in the control of the dynamics of release through the fusion pore and the high and variable frequency of this release makes it highly significant.
Animals↗
Remote fluorescence imaging of dynamic concentration profiles with micrometer resolution using a coherent optical fiber bundle.
Dynamic concentration profiles within the diffusion layer of an electrode were imaged in situ using fluorescence detection through a multichannel imaging fiber. In this work, a coherent optical fiber bundle is positioned orthogonal to the surface of an electrode and is used to report spatial and temporal micrometric changes in the fluorescence intensity of an initial fluorescent species. The fluorescence signal is directly related to the local concentration of a redox fluorescent reagent, which is electrochemically modulated by the electrode. Fluorescence images are collected through the optical fiber bundle during the oxidation of tris(2,2'-bipyridine)ruthenium(II) to ruthenium(III) at a diffusion-limited rate and allow the concentration profiles of Ru(II) reagent to be monitored in situ as a function of time. Tris(2,2'-bipyridine)ruthenium(II) is excited at 485 nm and emits fluorescence at 605 nm, whereas the Ru(III) oxidation state is not fluorescent. Our experiments emphasize the influence of two parameters on the micrometer spatial resolution: the numerical aperture of optical fibers within the bundle and the Ru(II) bulk concentration. The extent of the volume probed by each individual fiber of the bundle is discussed qualitatively in terms of a primary inner-filter effect and refractive index gradient. Experimentally measured fluorescence intensity profiles were found to be in very good agreement with concentration profiles predicted upon considering planar diffusion and thus validate the concept of this new application of imaging fibers. The originality of this remote approach is to provide a global view of the entire diffusion layer at a given time through one single image and to allow the time expansion of the diffusion layer to be followed quantitatively in real time.
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Active anionic zero-valent palladium catalysts: characterization by density functional calculations.
This works uses DFT (B3LYP/LACVP*(+)//B3LYP/LACVP* level) to ascertain the existence of the tricoordinate, anionic zero-valent palladium complexes that were postulated as the active species in the catalytic cycles of Pd-catalyzed Heck and cross-coupling reactions. The variety of complexes studied (1 and 2), include [Pd(PR(3))(2)X](-) species, in which R=H, Me, vinyl, and phenyl, and X=Cl, Br, I, AcO, and TFA, as well as bidentate complexes, [Pd[Ph(2)P(CH(2))(n)Ph(2)P]X](-), in which X=Cl, AcO and n=3-6. The study shows that these complexes exist as distinct minima in the gas phase as well as in THF. In addition, it provides geometric features and Pd--X(-) dissociation energies for all these complexes as well as some NMR and IR data, which show a clear distinction in these features between the tri- and dicoordinate Pd(0) species. An orbital interaction model and perturbation theory arguments account for the bonding mechanism and rationalize all the trends in the stability of the Pd--X bond. These trends include the effects of variation of X, R, and the length of the linker in the bidentate ligands.
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