Designs in Grassmannian Spaces and Lattices

We introduce the notion of a t-design on the Grassmann manifold of the m-subspaces of the Euclidean space ⁿ . It generalizes the notion of antipodal spherical design which was introduced by P. Delsarte, J.-M. Goethals and J.-J. Seidel. We characterize the finite subgroups of the orthogonal group which have the property that all its orbits are t-designs. Generalizing a result due to B. Venkov, we prove that, if the minimal m-sections of a lattice L form a 4-design, then L is a local maximum for the Rankin function _n,m .     

On-site determination of polynuclear aromatic hydrocarbons in seawater by stir bar sorptive extraction (SBSE) and thermal desorption GC-MS

The identification and quantification of semi-volatile contaminants dissolved in water is currently done in laboratory after a sampling step. This procedure is not satisfactory first because risks of samples contamination and analytes losses remain, in particular when these are present in ultra-trace concentrations, and secondly because procedures are time-consuming. The coupling of the stir bar sorptive extraction (SBSE), a new device of extraction technique, and a new generation of gas chromatography mass spectrometry (GC-MS), the field apparatus EM 640 S from Bruker, could be an answer to the challenge of on-site analysis. This analytical system was used to analyze 24 PAHs, among them 15 EPA priority pollutants. It was shown that this coupling led to encouraging results with LODs around the sub-ppt level for most of the compounds and R.S.D. included between 1 and 48%. The existence of competition phenomena between the various analytes inside the absorbent phase was demonstrated with the release (up to 80%) of light compounds. This result shows the necessity to work on the kinetic domain rather than on the thermodynamic equilibrium that is influenced by nature and concentration of other compounds. The matrix effects were also studied through the comparative analysis of ultrapure water, artificial and natural seawaters spiked with PAHs and the influence of ionic strength and particulate organic matter was investigated.     

Recognition of RNA by amide modified backbone nucleic acids: molecular dynamics simulations of DNA-RNA hybrids in aqueous solution

Thermodynamic and structural properties of a chemically modified DNA-RNA hybrid in which a phosphodiester linkage is replaced by a neutral amide-3 linkage (3'-CH(2)-CONH-5') were investigated using UV melting experiments, molecular dynamics simulations in explicit water, and continuum solvent models. van't Hoff analysis of the experimental UV melting curves suggests that the significant increase of the thermodynamic stability of a 15-mer DNA-RNA with seven alternated amide-3 modifications (+11 degrees C) is mainly due to an increased binding enthalpy. To further evaluate the origin in the observed affinities differences, the electrostatic contribution to the binding free energy was calculated by solving the Poisson-Boltzmann equation numerically. The nonelectrostatic contribution was estimated as the product of a hydrophobic surface tension coefficient and the surface area that is buried upon double strand formation. Structures were taken from 10 ns molecular dynamics simulations computed in a consistent fashion using explicit solvent, counterions, and the particle-mesh Ewald procedure. The present preliminary thermodynamic study suggests that the favorable binding free energy of the amide-3 DNA single strand to the complementary RNA is equally driven by electrostatic and nonpolar contributions to the binding compared to their natural analogues. In addition, molecular dynamics simulations in explicit water were performed on an amide-3 DNA single strand and the corresponding natural DNA. Results from the conformations cluster analysis of the simulated amide-3 DNA single strand ensembles suggest that the 25% of the population sampled within 10 ns has a pre-organized conformation where the sugar C3' endo pucker is favored at the 3'-flanking nucleotides. These structural and thermodynamic features contribute to the understanding of the observed increased affinities of the amide-3 DNA-RNA hybrids at the microscopic level.     

Dramatic Effect of Magnetite Particles on the Dynamics of Photogenerated Free Radicals

Abstract— Polymeric particles of about 1 u.m in diameter and containing around 40% magnetite have a dramatic influence on the dynamics of triplet radical pair reactions occurring in micelles in their vicinity. The effect has been monitored with laser flash photolysis techniques and results in a decrease of the number of radicals that separate from their geminate partner and an acceleration of radical pair geminate reactions. The effect involves remote interactions because the solution contains ca 10¹¹micelles for each polymeric particle. Magnetite also perturbs the way in which the radicals interact with an external magnetic field.     

Efficient Control of the Stereoselectivity in Reactions of 2-Oxy-Substituted Benzylic Radicals

The stereoselectivity in reactions of 2-oxy-substituted radicals of type B was investigated. As expected, minimization of allylic 1,3-strain was the major controlling factor. Under standard conditions, only a modest level of stereoselectivity was observed. E.g., deuteration of the benzylated radical (R¹ = benzyl) gave diastereoisomer ratios ≤ 2:1. Use of a bulky protective group on the O-atom (R¹ = (t-Bu)Ph₂Si) enhanced slightly the selectivity (ratio 4.1:1). However, a dramatic increase of the stereoselectivity (ratio 13:1) was obtained, when the reaction was performed with the free alcohol after treatment with bulky methylaluminium bis(phenoxide) derivatives (methyl-aluminium bis[2,6-di(tert-butyl)-4-methylphenoxide] (MAD) and methylaluminium bis(2,6-diphenylphenoxide) (MAPH)).     

Capillary electrophoresis combined with microdialysis in the human spinal cord: a new tool for monitoring rapid peroperative changes in amino acid neurotransmitters within the dorsal horn

A method originally developed for the separation of the three neurotransmitters gamma-aminobutyric acid (GABA), glutamate (Glu) and L-aspartate (L-Asp) in microdialysis samples from rat brain (Sauvinet et al., Electrophoresis 2003, 24, 3187-3196) was applied to human spinal dialysates obtained during peroperative microdialysis from patients undergoing surgery against chronic pain. Molecules were tagged on their primary amine function with the fluorogene agent, naphthalene-2,3-dicarboxaldehyde (NDA), and, after separation by capillary electrophoresis (CE, 75 mmol/L borate buffer, pH 9.2, containing 70 mmol/L sodium dodecyl sulfate and 10 mmol/L hydroxypropyl-beta-cyclodextrin, + 25 kV voltage), were detected by laser-induced fluorescence detection (LIFD) using a 442 nm helium-cadmium laser. The complete method, including microdialysis sampling and analysis by CE-LIFD, has been validated for the analysis of human spinal microdialysates. The analytical detection limits were 1, 3.7 and 17 nmol/L for GABA, Glu and L-Asp respectively. This method allows an accurate measurement of the three amino acid neurotransmitters during an in vivo monitoring performed as rapidly as every minute in the human spinal dorsal horn. In addition, the effect of a brief peroperative electrical stimulation of the dorsal rootlets was investigated. The results obtained illustrate the advantages of combining microdialysis with CE-LIFD for studying neurotransmitters with such a high sampling rate.     

Preparation of ortho-aryl-benzaldehyde derivatives via free-radical ipso-substitution of an amidomethyl group

Preparation of 2-biarylcarbaldehydes using an intramolecular free-radical ispo-substitution is described. The two aryl moieties to be coupled are pre-associated using a glycolamide derivative. An unusual amidomethyl leaving group was successfully employed in this process.     

Radical chain monoalkylation of pyridines

The monoalkylation of N-methoxypyridinium salts with alkyl radicals generated from alkenes (via hydroboration with catecholborane), alkyl iodides (via iodine atom transfer) and xanthates is reported. The reaction proceeds under neutral conditions since no acid is needed to activate the heterocycle and no external oxidant is required. A rate constant for the addition of a primary radical to N-methoxylepidinium >10⁷ M⁻¹ s⁻¹ was experimentally determined. This rate constant is more than one order of magnitude larger than the one measured for the addition of primary alkyl radicals to protonated lepidine demonstrating the remarkable reactivity of methoxypyridinium salts towards radicals. The reaction has been used for the preparation of unique pyridinylated terpenoids and was extended to a three-component carbopyridinylation of electron-rich alkenes including enol esters, enol ethers and enamides.

N-Methoxypyridinium salts are exceptionally reactive radical traps that can be used in efficient radical chain reactions with organoboranes.