Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25°C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH ₂ -as well as to the number of hydrogen atoms, n _H , belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V ₂ ^o and C _p2 ^o , depending on the substitution of alkyl and cyclooligomethylene groups.     

Chemical characterization of alpha-oxohydrazone ligation on colloids: toward grafting molecular addresses onto biological vectors

New mild and specific chemical strategies have been developed recently for the selective coupling of biological macromolecules. Among them, the hydrazone ligation strategy offers high chemoselectivity and versatility. We intended to use hydrazone ligation to target the controlled release of therapeutic agents by biological vectors (multilamellar vesicles called onion vectors). An accurate measure of ligation bond stability was needed to ensure that the ligation bond would stand long exposures to physiological conditions. In this study, we have completed a kinetic and thermodynamic characterization of hydrazone formation on a model reaction. The mechanism of the reaction in solution as well as in different self-organized systems (micelles, liposomes and multilamellar vesicles) was investigated. In solution, submicromolar stability was achieved as well as half-lives of several weeks. The kinetics and stability were both enhanced in colloidal media thanks to autoassociation effects. The results were expanded to the realistic case of RGD-peptide coupling to onion vectors. The RGD grafted onion vectors were then tested for their ability to bind endothelial cells in vitro.     

Molecular Interactions of Macrocycles with Dipeptides in Aqueous Solutions. Partial Molar Volumes and Heat Capacities of Transfer of a Chiral 18-Crown-6 and of a Calix[4]resorcinarene Derivative from Water to Aqueous Dipeptide Solutions at 25°C

Densities and specific heat capacities of ternary aqueous systems containing a dipeptide (alanyl-alanine, alanyl-glutamic acid, alanyl-serine or L-seryl-L-leucine) and a macrocycle (D--manno-naphtho-18-crown-6-ether or 2,8,14,20-tetrakis[-methyl (aminoformyl)]-4,6,10,12,16,18,22,24-octahydroxycalix[4]arene) were determined at 25°C by flow densimetry and flow calorimetry. The partial molar volume and heat capacity of transfer of a macrocycle from water to the dipeptide solution was determined as a function of the dipeptide concentration. Positive values for transfer volumes and transfer heat capacities are observed with all the solutions studied. With the crown ether, except for alanyl-glutamic acid where a 1:1 complex is clearly evidenced due to specific interactions of the side-chain functional group of the peptide with the crown ether, no stoichiometric complexes are confirmed and the partial molar quantities of transfer increase with the hydrophobic character of the dipeptide. Partial quantities of transfer are smaller with the calixarene than with the crown ether and stoichiometric complexes [calixarene]/[dipeptide] from 2:1 to 1:4 are evidenced, depending on the nature and the concentration of the dipeptide.     

Chemical profiling and classification of illicit heroin by principal component analysis, calculation of inter sample correlation and artificial neural networks

Artificial neural networks (ANNs) were utilised to validate illicit drug classification in the profiling method used at “Institut de Police Scientifique” of the University of Lausanne (IPS). This method established links between samples using a combination of principal component analysis (PCA) and calculation of a correlation value between samples.Heroin seizures sent to the IPS laboratory were analysed using gas chromatography (GC) to separate the major alkaloids present in illicit heroin. Statistical analysis was then performed on 3371 samples. Initially, PCA was performed as a preliminary screen to identify samples of a similar chemical profile. A correlation value was then calculated for each sample previously identified with PCA. This correlation value was used to determine links between drug samples. These links were then recorded in an Ibase^® database. From this database the notion of “chemical class” arises, where samples with similar chemical profiles are grouped together. Currently, about 20 “chemical classes” have been identified.The normalised peak areas of six target compounds were then used to train an ANN to classify each sample into its appropriate class. Four hundred and sixty-eight samples were used as a training data set. Sixty samples were treated as blinds and 370 as non-linked samples. The results show that in 96% of cases the neural network attributed the seizure to the right “chemical class”.The application of a neural network was found to be a useful tool to validate the classification of new drug seizures in existing chemical classes. This tool should be increasingly used in such situations involving profile comparisons and classifications.     

Potential energy function for cation–peptide interactions: An ab initio study

A potential energy function is developed to represent the interaction of small monovalent cations, Li⁺, Na⁺, and K⁺, with the backbone of polypeptides. The results are based on ab initio calculations up to the 6-31G* level of the interactions of the ions with acetamide and N-methylacetamide. Basis set superposition errors are corrected with the counterpoise method. A systematic overestimate of the bond polarities is taken into account by an empirical scaling procedure that uses the ratio of the experimental to ab initio dipole moment. The calculated binding energies obtained with this procedure show consistent convergence with different basis sets and are in good agreement with experimental data on cation–water and cation–dimethylformamide systems. Investigations of the calculated ab initio potential energy surface indicate that the cation–peptide interaction is dominated by electrostatics and includes a nonnegligible contribution from polarization of the peptide group by the ion. The induced polarization results in a steeper-than-Coulombic interaction and cannot be described by fixed ion–peptide partial charges electrostatics. Atomic polarizabilities located on the atoms of the ligand molecule are introduced to account for the induced polarization in the empirical energy function. A ～1/r⁴ attractive interaction appears in the potential function. The resulting radial and angular dependence of the potential energy surface is well reproduced. © 1995 by John Wiley & Sons, Inc.     

Stability of alkali-atom clusters

A detailed study of the stability of alkali-atom clusters is presented and discussed. Free clusters are produced in an unseeded adiabatic expansion and ionized by a pulsed laser beam. A tandem time-of-flight system provides a size selection of ionized species and mass spectrometry of fragments. We have investigated for sodium and potassium clusters containing up to fifty atoms: the metastable decay following ionization, the photo-induced dissociation, the collisional induced dissociation and the collisional charge exchange. Experimental evidence is shown for dissociative process involving evaporation of monomers and dimers. This is explained by the statistical unimolecular theory and by the energetics of the systems.     

Viability of Bacteria in Hybrid Aqueous Silica Gels

Whole E. coli bacteria have been trapped within silica gels obtained via the acidification of sodium silicate and silica nanoparticles solutions. Their -galactosidase enzymatic activity increases with time, suggesting that their membrane is partially lysed during the encapsulation process. Such a lysis can be greatly reduced when encapsulation is performed in the presence of gelatin. The biocatalytic activity of trapped bacteria remains almost constant for more than a week. Moreover bacteria trapped in such gels remain able to incorporate glucose, showing that their viability has been preserved.     

Méthodes multipas pour des équations paraboliques non linéaires

Summary We study the error due to the discretization in time of a nonlinear parabolic problem by a multistep method. Error estimates are obtained if the method is of the orderp (p>1) and stronglyA()-stable . The method is also applied to the Navier-Stokes equations in two dimensions.