Electrochemical approach to concerted proton and electron transfers. Reduction of the water-superoxide ion complex

Concerted proton and electron transfers (CPET) currently attract considerable theoretical and experimental attention, notably in view of their likely involvement in many enzymatic reactions. Electrochemistry, through techniques such as cyclic voltammetry, can provide a quite effective access to CPET in terms of diagnosis and quantitative kinetic characterization. The relationships expressing the rate constant of an electrochemical CPET are given. Besides the CPET standard potential, it depends on two main factors. One is the reorganization energy, which appears as the sum of an intramolecular contribution and two solvent reorganization energies corresponding to proton and electron transfers, respectively. The other is the pre-exponential factor that mainly depends on proton tunneling through the activation barrier. Procedures for estimating these various factors as well as the H/D kinetic isotope effect are described. Application of the theory is illustrated by the experimental results obtained for the cyclic voltammetric reduction of the water-superoxide ion complex in dimethylformamide and acetonitrile.     

Selection of internal standards for analysis of silicate rocks and limestones by laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES)

Included (Si, Ca) and added (Li, B) internal standards (IS) have been used comparatively in LA-ICP-AES of silicates and limestones to improve both precision and accuracy. A Q-switched Nd:YAG laser (355 nm, 10 Hz, 10 mJ per shot) has been applied for ablation. Samples have been prepared by fusion with Li(2)B(4)O(7) and measured using a Perkin-Elmer Optima 3000 ICP system. Both types of IS have given a relative standard deviation (RSD) less than 2-1%, which improved the repeatability by a factor of 2-10 and calibration graphs have been linear over the whole concentration range.     

A global indicator as a tool to follow airborne molecular contamination in a controlled environment

The impact of pollutants on production quality in nanotechnology necessitates reduction of contaminant levels in cleanrooms. So, devising a global airborne-pollutant indicator (GAPI) for rapid determination of the level of pollution and its danger to the process is justified. This tool used relative impact weights of the different molecules to quantify the pollution. A calculation of impact weight is proposed in this paper. Impact weights could take into account several characteristics of the molecules (molecular volume, sticking coefficient, ...). They could also be combined to be as close as possible to reality. An example of calculations of the impact of molecular volumes on air quality is given.     

Crystal Structure of 1,3-Alternate 25,26,27,28-Tetra(tert-butoxyethoxy)calix[4]arene: a Selective Ag+ Extractant

Alkylation of calix[4]arene by 2-tert-butoxyethyl bromide led to the tetraalkylatedcalix[4]arene in the 1,3-alternate, the conformation of which has been established byX-ray crystallography. This spatial structure included a cavity potentially useful forhost–guest complexes achieved with metal cations, especially with Ag⁺. The titlecompound crystallizes in the monoclinic space group Cc with cell constants a = 29.901(2),b = 8.139(1), c = 22.264(3) Å, = 90°, = 117.08(1)°and = 90°. This conformer represents an example for Ag⁺-tunnelingacross an aromatic cavity. This behaviour could lead to important implications with regardto the metal cation- interaction expected for metal transport through ion channels,metal inclusion in fullerenes, intercalation of metal cations into graphites, etc.     

Evidence for inverted region behavior in proton transfer to carbanions

The diphenylmethane-diphenylmethyl anion acid/base couple in N,N-dimethylformamide is taken as an example for investigating the dynamics of proton transfer at carbon in a system where the acid is not activated by an electron-withdrawing group or by removal of an electron. The laser flash electron photoinjection technique is applied to the determination of the rate constant for the protonation of diphenylmethyl anion by an extended series of acids that offers a range of driving forces encompassing over 1.2 eV. The plot of the rate constant versus the pK(a) difference between diphenylmethane and the acids or of the activation free energy versus the standard free energy of the reaction exhibits clear "inverted region" behavior (by a factor of 80 in terms of rate constants). While such behaviors have been predicted and observed for outersphere electron-transfer reactions, previous evidence for proton-transfer reactions was scarce. Entropic factors, derived from an investigation of the temperature dependence of the experimental rate constants, are also discussed.     

Solvent Polarity and pH Effects on the Spectroscopic Properties of Neutral Red: Application to Lysosomal Microenvironment Probing in Living Cells

Neutral red is a lysosomal probe and a biological pH indicator. In aqueous solutions, the protonated (NRW) and neutral (NR) forms of monomeric neutral red exhibit distinct absorption maxima (535 and 450 nm, respectively) but have the same fluorescence with a maximum at 637 nm and a quantum yield of 0.02. The similarity of the fluorescence spectra at acidic and basic pH suggests deprotonation of cationic species in the first singlet excited state. The NR fluorescence strongly depends on the solvent polarity as shown by addition of increasing amounts of water to pure dioxane, which gradually shifts the fluorescence maximum from 540 nm in pure dioxane to 637 nm in water. The fluorescence quantum yield increases from 0.17 in dioxane to 0.3 upon addition of 7% water and then decreases, reaching 0.02 in pure water. Immediately after incubation of human skin fibroblasts with neutral red, excitation with 435 nm light produces a fluorescence whose maximum is recorded at 575 nm. This fluorescence is located in the perinuclear region and originates from large fluorescent intracytoplasmic spots, suggesting staining of the endoplasmic reticulum-Golgi complex. At longer times, this fluorescence is shifted to 606 nm, suggesting slow diffusion of the lysosomotropic dye toward the more hydrated and acidic interior of ly-sosomes. Addition of a lysosomotropic detergent to cells previously incubated with neutral red shifts the fluorescence to the blue. Thus, in complex biological systems, this probe cannot be a good pH indicator but is a very sensitive probe of lysosomal rnicroenvironrnents.     

Jeux à champ moyen. I – Le cas stationnaire

We introduce here a general approach to model games with a large number of players. More precisely, we consider N players Nash equilibria for long term stochastic problems and establish rigorously the ‘mean field’ type equations as N goes to infinity. We also prove general uniqueness results and determine the deterministic limit. To cite this article: J.-M. Lasry, P.-L. Lions, C. R. Acad. Sci. Paris, Ser. I 343 (2006).     

The use of power transfer matrices in predicting system loss: Theory and experiment

The phase space diagram for parabolic and step index fibers leads to a graphic representation of the bound, leaky, and refracted rays of ray theory. This concept is used to predict the attenuation of typical components of local area networks. The technique uses power transfer matrices to track the evolution of power distribution in ray packets. In particular, we predict and then measure the power transfer of two ray packets for a step index fiber. The comparison is encouraging.