Optimisation d’une sélection in vitro d’enzymes

Optimisation of an in vitro enzyme selection. Isolation of a catalyst for a given chemical reaction may be achieved by in vitro selection of enzymes from a protein library. Here, we investigate the polymerisation reaction on filamentous phage and the cross-linking of substrate on phage to optimise an in vitro selection for DNA polymerase activity. The efficiency of the optimised selection is measured by enrichment factors up to 3.8 × 10³, the highest described so far for an in vitro selection of proteins for catalysis. It should be useful for directed polymerase evolution towards novel catalytic activities. To cite this article: E. Orsi, J.-L. Jestin, C. R. Chimie 6 (2003).     

Nouvelle méthode de synthèse d’acides et d’esters arylboroniques par électroréduction de dérivés aromatiques halogénés en présence d’agents boratants

Novel synthesis of arylboronic acids and esters by electroreduction of aromatic halides in the presence of borating agents. A novel strategy for the one-step synthesis of arylboronic acids and esters by an electrochemical coupling reaction is described. It is based on the reductive coupling between the aromatic halides and a borating agent (trialkyl borate or pinacolborane). The reactions are carried out in DMF or THF with the use of sacrificial magnesium or aluminium anodes in a single-compartment cell. Arylboronic acids and esters are obtained with moderate-to-good isolated yields. To cite this article: C. Laza, E. Duñach, C. R. Chimie 6 (2003).     

Influence d’alcools aliphatiques sur l’adsorption d’acides aminés sur charbon actif

Adsorption of aqueous solution of two amino acids – phenylalanine (Phe) and tyrosine (Tyr) – on powdered activated carbon (PAC) has been studied in the presence of linear or branched alcohols. Methanol has no influence on the adsorption. For the other alcohols, the adsorption decreases when the alcohol chain gets longer, from ethanol to 1-propanol, up to 85 %. Alcohol branching reduces this phenomenon. Adsorption kinetics have been tested according two models, allowing good representation of results and determination of kinetic constants.     

Préparation d’orthoesters originaux par addition d’acides carboxyliques sur des acétals cycliques de cétènes

Original orthoesters have been prepared by addition of carboxylic acids on the exocyclic double bond of 2- alkylidene -benzo[1,3]dioxin-4-ones. To cite this article: P. Babin et al., C. R. Chimie 6 (2003).     

L’importance des phénomènes d’oxydo-réduction dans le verre

Oxydo-reduction phenomenon concerns glass manufacturing as well as its properties. Various oxidation scales have been used by glassmakers, the most widely used being the ratio between Fe²⁺ and the total iron content. Comparisons are made with others, such as the partial pressure of oxygen or the redox potential. The influence of the chemical composition of the glass is also studied as well as that of the temperature, through the results obtained in Saint-Gobain Recherche and in other laboratories. In industrial glass manufacturing, the oxidation equilibrium is usually achieved during the fining process, an intense ‘bubbling’ being necessary to obtain a homogeneous glass. Thus, the oxidoreduction state of the glass does not depend on the atmosphere under which it was made, but depends on the mixture of gases generated during the fining process, which is described through the reactions generated by the two main fining systems, arsenic oxide or antimony oxide + sodium nitrate and sodium sulphate + a reducing agent. The former provides oxygen and the latter, a mixture of oxygen and sulphur dioxide. Consequently, a glass melted with arsenic oxide is more oxidised than a glass melted with sodium sulphate. The equilibrium constant used to interpret the latter system, mostly used for soda lime silica glasses fining, is given as a function of the temperature and some of the consequences are examined. Obviously, the coloration by iron oxide impurities is influenced by the manufacturing process. Some other colouring oxides are submitted to a change of oxidation state and thus to a modification of their colouring action. A quick review is done of the transition metal coloration, of the electron-charge-transfer absorption due to sulphides or selenides, and of aggregate formation inducing the well-known red ruby glass, for instance.     

Réaction d’isomérisation de l’acide urocanique et de ses dérivés: stabilité comparée des isomères Z et E

Thermal and photochemical isomérisation of urocanic acid and its derivatives (O- and/or N-alkylated) have been studied. The data show the wavelength and the molar extinction coefficient-dependent photochemistry of E urocanic acid and its derivatives. Thermal isomérisation of these compounds is difficult and undergoes a “one-way” Z → E reaction. Thus, E isomers are thermodynamically more stable than the Z ones. Under basic conditions, derivatives having an unsubstitued imidazole ring (NH function free) exist in their anionic forms, which lowers the energy barrier without changing the isomérisation pathway.     

Radicaux imidazoline-nitroxyde,briques moléculaires paramagnétiques pour la construction d’édifices magnétiques

The synthesis, characterization and magnetic properties of new series of metal complexes based on spin-labelled pyridine, bipyridine, terpyridine, polyaza-macrocycles and calix[4]arene scaffolds have been described. Magneto-structural correlations revealed that the nature and the amplitude of the magnetic interactions strongly depend on the torsion angles between the radical and the metallic centre. In the 6,6’-bis-NIT-2,2’-bipyridine/Cu²⁺ case, the interaction is antiferromagnetic, with J ≈ –167 cm^–1, while with Ni²⁺ a large range of ferromagnetic interactions is found, depending on the geometry of the first coordination sphere [from 40 to 7 cm^–1], NIT for a nitronyle-nitroxyde radical. The presence of a pyridine spacer between the NIT radical and the metallic centre (4’-NIT-2,2’:6’,6’’-terpyridine case) notably mitigates the interaction, which remains antiferromagnetic in all cases (J ≈ –7 cm^–1 with Ni²⁺). In the podand bipyridine-ligand, one nitroxide arm is bound equatorially and the second arm is axially coordinated to Cu²⁺. With the Ni²⁺ and Zn²⁺ cations, both radicals are axially bounded. In all complexes, the bipyridine/aminomethyl fragment is meridionally complexed to the metal centre.     

Le calcul du déplacement chimique,un outil de validation des structures d’acides nucléiques

The large majority of analytical NMR methods in chemistry or biochemistry are based on the quality of the chemical shift dispersion. The purpose of this work is to show that theoretical chemical shift back calculation starting from structures can be used to select molecular modeling structures in order to differentiate several conformational possibilities. Here we report the result of the chemical shift calculation carried out on two original structures corresponding to two DNA ‘kissing complexes’. The 46 nucleotides sequence corresponds to the RNA deoxyribose analogous implied in the HIV-1_Lai dimerization process. It is interesting to note that, even in cases we are very far from the classical helical structure (loop–loop interaction, AA base pairing, base stacking, misappariement...), the theoretical chemical shift is in very good agreement with the experimental chemical shift (±0.25 ppm). The satisfactory results obtained enable us to conclude that the comparison of the proton chemical shifts is an invaluable tool making it possible to select or to validate oligonucleotides structures.     

Effets chimiques et magnétovolumiques compétitifs dans les nitrures d’insertion

An analysis of the chemical and magnetovolume effects due to nitrogen insertion is carried out in order to establish trends for the Fe–N system. Computations are based on the density functional theory and use is made of the augmented spherical wave method. Taking the Fe₄N nitride as a representative member, the chemical bonding study shows that nitrogen selectively contracts covalent bond with one of the two iron sublattices, with the consequence of lowering the magnetic moment. The magnetovolume effects are examined for a comparative study of the volume-dependent magnetic moments within Fe₄N as with respect to γ-Fe, in its two magnetic states ‘high-moment–large volume’ and ‘low moment–low volume’. The insertion of nitrogen ‘prepares’ the volume in such a manner in the nitride that the two states are simultaneously present at the two Fe crystal sites. The trends for an increase of the Fe atomic volume within the series Fe₈N, Fe₄N, Fe₃N, Fe₂N should favour the enhancement of the magnetisation; this is, however, hindered by the Fe–N chemical bond, involving an increasing number of Fe sites leading to a lowering of the moments.     

L’acide déhydracétique,précurseur de nouvelles pyridopyrimidines,thiazolopyrimidines et pyridones

A new synthetic approach to the novel pyrimidine and pyridone is described. The structures of the obtained adducts by opening of dehydroacetic acid ring have been assigned by means of spectroscopic measurements.     

Caractérisation des propriétés électriques des parois de pores d’une membrane

The characterisation of the surface electrical properties of membrane materials is critical for understanding and predicting the filtration performances of membranes. In this paper, four simple experimental methods – streaming potential, electro-osmosis, pore conductivity and membrane potential –, allowing the characterisation of the charge state of membrane pore walls are presented. The four experimental parameters provide information about the sign of the electrical net charge. Examples illustrating the influence of the pH, electrolyte concentration and nature on the four experimental parameters are given. The zeta potential can be then deduced from these measurements by means of a model.     

Géométrie de l'intermittence en turbulence développée

A geometrical interpretation of intermittency in fully developed turbulence is realized through an hierarchy of fractal structures Ω_p of dimensions Δ_p linked each other by the relations Ω_{p + 1} − Ω_p (i.e. Δ_{p + 1} < Δ_p) and γ = (Δ_{p + 1} − Δ_∞)/(Δ_p − Δ_∞) with γ = ((1 + 3/√8)^1/3 + (1 − 3/√8)^1/3)³ and Δ_∞ = 1 and Δ_∞ = 1. This is obtained by the introduction of an entropy jump, defined at the scale r, ΔS_p(r) = (Δ_{p + 1} − Δ_p) ln (r/r₀) characterizing the order level of each sub-structure Ω_p and verifying a linear relation ΔS_p(r) = γ ΔS_{p − 1}(r).     

Récents développements liés à l'enthalpie d'immersion

The two calorimetric methods of determination of the enthalpies of immersion and of wetting are described in this text. The equations linking the adsorption enthalpies to the immersion enthalpies and to the wetting enthalpies are derived. Three experimental examples are shortly described in order to give the experimental limits and ability of these techniques. In the second part, a general model for the interpretation of adsorption and wetting is developed. This model uses three parameters related to the dispersive, acid and basic components of the energy. Some experimental examples are given, in order to show that it is possible to compute the surface energy of divided solids. From these results, it is also possible to predict the macroscopic behaviour of these solids when facing pure fluids.     

Étude physicochimique de nouveaux composés formés par incorporation directe d’hétéropolyacide dans une silice mésoporeuse organisée

Heteropolyacids (HPW) are known as active catalysts for various redox reactions. The direct incorporation of HPW species in a mesoporous organised silica molecular sieve during the synthesis was studied. The association of two type of surfactants is essential for the preparation of such solids. The silica-organised materials comprising heteropoly compounds thus obtained were thermally stable porous materials with mesopores and large surface areas.     

Caractérisation par émission acoustique de l’adhérence et de l’endommagement d’un revêtement : cas d’un revêtement WC–Co sur acier

This note presents a characterization method of the ‘adhesion’ of a coating by acoustic emission technique. In situ acoustic emission measurements which were investigated in terms of amplitude, absolute energy, position of the event, etc. were performed on WC–Co coated specimens prepared by HVOF (high velocity oxy fuel) during four-point bending tests. The microscopic observations of the specimens show two types of cracks: transversal cracks (regularly spaced on the coating surface) and interfacial cracks. Acoustic emission results show two different types of acoustic events in terms of absolute energy and amplitude, which are representative of two cracking mechanisms.     

Composés d’intercalation du graphite : des binaires aux ternaires

Graphite intercalation compounds: from binaries to ternaries. Graphite, which is an amphoteric host structure, easily reacts with electropositive elements and especially with alkali metals, leading to binary graphite–metal intercalation compounds. Alkali metals can also help elements unable to react alone with graphite to intercalate. Therefore, ternary intercalation compounds are obtained using various synthesis routes. To cite this article: C. Herold, P. Lagrange, C. R. Chimie 6 (2003).     

Algorithme de dénombrement des graphes chiraux et achiraux d’un cycloalcane monocyclique hétérosubstitué de formule brute CnHm1Xm2Ym3

A general pattern inventory is given for the enumeration of chiral and achiral graphs of any monocyclic cycloalkane with a multiple heterosubstitution of binary type X and Y and an empirical formula C_nH_m1X_m2Y_m3, where m₁ + m₂ + m₃ = 2 n.     

Simulation numérique déterministe de la turbulence

We first present techniques of large-eddy simulation (LES) in physical und Fourier spaces. We show how LES allows vortex control in mixing layers, and good prediction of velocity statistics in a turbulent channel. We study also in a developed boundary layer the dynamics of high- and low-speed streaks at the wall, and the associated pressure fluctuations. Following this, we show in rotating shear flows a universal behaviour ol the mean velocity in regions whosw local Rossby number is sirictly inferior to — 1. Finally, we present for LES of compressible flows a formalism based on Favre averages.