Determination of aluminum by electrothermal atomic absorption spectroscopy in lubricating oils emulsified in a sequential injection analysis system

The sequential injection (SIA) technique was applied for the on-line preparation of an “oil in water” microemulsion and for the determination of aluminum in new and used lubricating oils by electrothermal atomic absorption spectrometry (ET AAS) with Zeeman-effect background correction. Respectively, 1.0, 0.5 and 1.0 ml of surfactants mixture, sample and co-surfactant (sec-butanol) solutions were sequentially aspirated to a holding coil. The sonication and repetitive change of the flowing direction improved the stability of the different emulsion types (oil in water, water in oil and microemulsion). The emulsified zone was pumped to fill the sampling arm of the spectrometer with a sub-sample of 200 μl. Then, 10 μl of this sample solution were introduced by means of air displacement in the graphite tube atomizer. This sequence was timed to synchronize with the previous introduction of 15 μg of Mg(NO₃)₂ (in a 10 μl) by the spectrometer autosampler. The entire SIA system was controlled by a computer, independent of the spectrometer. The furnace program was carried out by employing a heating cycle in four steps: drying (two steps at 110 and 130 °C), pyrolisis (at 1500 °C), atomization (at 2400 °C) and cleaning (at 2400 °C). The calibration graph was linear from 7.7 to 120 μg Al l⁻¹. The characteristic mass (mo) was 33.2 pg/0.0044 s and the detection limit was 2.3 μg Al l⁻¹. The relative standard (RSD) of the method, evaluated by replicate analyses of different lubricating oil samples varied in all cases between 1.5 and 1.7%, and the recovery values found in the analysis of spiked samples ranged from 97.2 to 100.4%. The agreement between the observed and reference values obtained from two NIST Standard Certified Materials was good. The method was simple and satisfactory for determining aluminum in new and used lubricating oils.     

A Self-consistent Reaction Field Model of Solvation Using Distributed Multipoles. II: Second Energy Derivatives and Application to Vibrational Spectra

The use of distributed multipoles in the formalism of the reaction field factors allowed us to develop a computational scheme adapted to quantum chemical computations on a molecule solvated by a dielectric continuum. The algorithm used to compute the second energy derivatives is developed and, after its implementation in a quantum chemical computational code, permits the computation of the vibrational frequencies of the solute. This approach has been tested successfully on two test molecules and applied to the study of the solvent effect on the structure and the vibrational spectrum of a series of parasubstituted benzonitriles.     

Inhibition of mitosis by glycopeptide dendrimer conjugates of colchicine

Glycopeptide dendrimers have been prepared bearing four or eight identical glycoside moieties at their surface (beta-glucose, alpha-galactose, alpha-N-acetyl-galactose, or lactose), natural amino acids within the branches (Ser, Thr, His, Asp, Glu, Leu, Val, Phe), 2,3-diaminopropionic acid as the branching unit, and a cysteine residue at the core. These dendrimers have been used as drug-delivery devices for colchicine. Colchicine was attached to the dendrimers at the cysteine thiol group through a disulfide or thioether linkage. The biological activities of the glycopeptide dendrimer conjugates were evaluated in HeLa tumor cells and non-transformed mouse embryonic fibroblasts (MEFs). The concentrations of glycopeptide dendrimer drug conjugates required to achieve inhibition of cell proliferation by interference with the tubulin system were found to be higher (IC50 > 1 microM) compared to the required colchicine concentration. On the other hand, the glycopeptide dendrimer conjugates inhibited the proliferation of HeLa cells 20-100 times more effectively than the proliferation of MEFs. In comparison, non-glycosylated dendrimers and colchicine itself showed a selectivity of 10-fold or less for HeLa cells.     

Spectrométrie de Masse d'Hétérocycles Azotés. VIII–Comportement sous l'Impact Electronique de Nitroarylazoles

The mass spectra of nitrophenylimidazoles and nitrophenylpyrazoles have been examined in order to establish whether neighbouring ortho-ortho′ substituents have an appreciable influence on the fragmentation patterns. For compounds having an ortho nitro group on the heterocycle, specific effects are observed. Isotope effects observed with deuterated derivatives confirm this and establish that in the case of o-nitroimidazoles the 2-H proton adjacent to both nitrogen atoms is involved in the loss of OH. Fragmentation of each compound is specific and study could eventually provide a satisfactory means for structure determination.     

Reactions de cyclisation photochimique D'α-alcoxy cyclohexenones

α-Ketooxetanes and α-alkylidene oxetanols are the main products of the photolysis of 2-alkoxy 2-cyclohexenones; γ-hydrogen abstraction and cyclization of the intermediate biradical are general processes for these enones. Regioselectivity of the reaction and several aspects of the photolysis of a ketooxetanes are discussed. The corresponding 2-alkoxy cross-conjugated dienones rearrange in cyclopropyl ketones when photolyzed in the same conditions.     

Adsorption: an easy and efficient immobilisation of acetylcholinesterase on screen-printed electrodes

An amperometric biosensor based on acetylcholinesterase was constructed by simple adsorption of the enzyme on screen-printed electrodes (SPEs). This sensor was used to detect the inhibitory effects of organophosphorus and carbamate insecticides on acetylcholinesterase, and more particularly of chlorpyrifos ethyl oxon (CP-o). We demonstrate that enzyme adsorption on SPEs allows to obtain stable sensors that present good characteristics and are as efficient as other screen-printed biosensors based on covalent binding or entrapment of acetylcholinesterase (AChE).     

Opening mechanism of G.T/U pairs in DNA and RNA duplexes: a combined study of imino proton exchange and molecular dynamics simulation

The opening pathway of wobble pairs dG.T and rG.U has been investigated in four DNA and two RNA duplexes. Using NMR spectroscopy, we measured the imino proton exchange of both G(H1) and T/U(H3), catalyzed by ammonia, tris, and OH(-), and we calculated the free energy surface related to G.T/U opening by molecular dynamics simulations. Taken together the experimental and theoretical results, we suggest that wobble pairs open through a coupled rotation of the bases toward the major groove where exchange of both imino protons takes place with the surrounding water.     

Synthese regioselective de bases de mannich de cetones dissymetriques

Methods for the regioselective synthesis of Mannich bases starting from unsymmetrical ketones are described. The Mannich base on the more substituted carbon is obtained by reaction with (CH₃)₂N⁺CH₂,CF₃CO₂^- in CF₃CO₂H and its isomer on the less substituted carbon with (iPr)₂N⁺CH₂,ClO₄^- in CH₃CN. In the first case, the orientation corresponds to a non-selective attack of the immonium salt on a mixture of enols in which the more-substituted isomer is predominant. In the second ease, use of a bulky immonium salt induces a selectivity favoring the less-substituted enol.     

Motion of single long DNA molecules through arrays of magnetic columns

We present a videomicroscopy study of T4 DNA (169 kbp) in microfluidic arrays of posts formed by the self-assembly of magnetic beads. We observe DNA moving through an area of 10 000 microm(2), typically containing 100-600 posts. We determine the distribution of the contact times with the posts and the distribution of passage times across the field of view for hundreds of DNA per experiment. The contact time is well approximated by a Poisson process, scaling like the inverse of the field strength, independent of the density of the array. The distribution of passage times allows us to estimate the mean velocity and dispersivity of the DNA during its motion over distances long compared to our field of view. We compare these values with those computed from a lattice Monte Carlo model and geometration theory. We find reasonable quantitative agreement between the lattice Monte Carlo model and experiment, with the error increasing with increasing post density. The deviation between theory and experiment is attributed to the high mobility of DNA after disengaging from the posts, which leads to a difference between the contact time and the total time lost by colliding. Classical geometration theory furnishes surprisingly good agreement for the dispersivity, while geometration theory with a mean free path significantly overestimates the dispersivity.     

Enzyme activity fingerprinting with substrate cocktails

In the postgenomic era, emphasis is shifting from protein identification to protein functional analysis. Enzyme function can be characterized by measuring activity across series of substrates, which generates an activity profile or fingerprint. Activity fingerprinting is particularly useful to differentiate closely related enzymes. Previously reported fingerprinting methods use series of parallel measurements, which are complex and difficult to reproduce. Here we report a new method for fingerprinting enzyme activities based on using mixtures of substrates, or substrate cocktails, in a single reaction that is then analyzed by HPLC. The fingerprints produced are highly reproducible and allow functional differentiation and classification of closely related enzymes, as demonstrated for a series of lipases and esterases. The method is practical, general, and flexible in terms of reaction conditions and can be adapted to any reaction type.