Dendritic effect in polymer-supported catalysis of the intramolecular Pauson-Khand reaction

A remarkable increase in catalytic activity and selectivity in the intramolecular Pauson-Khand reaction is observed for Co complexes, immobilised on second- and third-generation dendron-functionalized polystyrene, as compared with their analogues on non-dendronized support.     

A perturbed pK(a) at the binding site of the nicotinic acetylcholine receptor: implications for nicotine binding

A series of tethered quaternary ammonium derivatives of Tyr have been incorporated into the binding site of the nicotinic acetylcholine receptor (nAChR) using the in vivo nonsense suppression method, producing constitutively active (self-gating) receptors. We have incorporated primary, secondary, and tertiary amine tethered agonists to give receptors whose constitutive activity can be modulated by pH. Lowering the pH protonates the tethered amine, giving it a positive charge and allowing it to reversibly activate the receptor. Tertiary and secondary tethered amines, TyrO3T and TyrO3S, have been successfully incorporated at alpha149 in the nAChR. Constitutive currents at pH 5.5 are 6 times those at pH 9.0. The pKa of TyrO3T in the binding site appears to be 6 or lower, differing substantially from its pKa in solution ( approximately 9.3). This local pKa perturbation has substantial implications for pharmacological research on the nAChR: of the tertiary agonists considered, noracetylcholine experiences this pKa perturbation, while nicotine does not.     

Solutions fondamentales pour un milieu élastique à isotropie transverse

The general solution of an axisymmetric problem for a homogeneous medium is given for a surface concentrated loading, and from it a closed form solution for a point force is deduced. The infinite and semi-infinite spaces are considered for various boundary conditions.     

Measurement of the radiative and nonradiative decay rates of single CdSe nanocrystals through a controlled modification of their spontaneous emission

We present a simple method to measure the radiative and nonradiative recombination rates of individual fluorescent emitters at room temperature. By placing a single molecule successively close and far from a dielectric interface and simultaneously measuring its photoluminescence decay and its orientation, both the radiative and nonradiative recombination rates can be determined. For CdSe nanocrystals, our results demonstrate that the fluorescence quantum efficiency, determined at the single-molecule level, is 98% in average, far above the value expected from conventional ensemble experiments. The bidimensional nature of the transition dipole is also directly evidenced from a single-particle measurement.     

Unprecedented (Cu2Ln)n complexes (Ln = Gd3+, Tb3+): a new "single chain magnet"

The magnetic study of a trinuclear Cu-Gd-Cu complex confirms that such basic units self-assemble to yield a high spin species. A nice fit of the magnetic data is obtained for an infinite chain of tetranuclear Gd(2)Cu(2) motifs linked through the Gd ions located at the opposite vertexes of the tetranuclear motifs according to two Cu-Gd coordination modes, a double bridging through phenoxo and alkoxo oxygen atoms and a single bridging through deprotonated amide functions. The two interaction pathways are ferromagnetic. Alternating current susceptibility measurements confirm that the equivalent copper-terbium entity is a single chain magnet with a barrier height for reversal of the magnetization equal to 28.5 K.     

Bunching and antibunching in the fluorescence of semiconductor nanocrystals

The fluorescence of single-colloidal CdSe quantum dots is investigated at room temperature by means of the autocorrelation function over a time scale of almost 12 orders of magnitude. Over a short time scale, the autocorrelation function shows complete antibunching, indicating single-photon emission and atomiclike behavior. Over longer time scales (up to tens of seconds), we measure a bunching effect that is due to fluorescence intermittency and that cannot be described by fluctuations between two states with constant rates. The autocorrelation function also exhibits nonstationary behavior related to power-law distributions of On and Off times.     

Compressibility of a zinc single crystal under confining pressure

Abstract

In the present study, the two compressibility moduli of a zinc single crystal were measured at room temperature up to 18 GPa. The results are compared with those obtained by Bridgman from a different experimental method. This study required a gaseous hydrostatic confining pressure, and then it has been necessary to develop an original diamond anvil cell with a large optical aperture allowing the use of sophisticated video imaging and image analysis techniques. A model is proposed for the zinc volume-pressure relation.