Zirconocene-mediated route to enantiopure 9-oxabicyclononanes functionalized on both carbon bridges

[reaction: see text] A zirconocene-mediated ring contraction of 4-vinylfuranosides generated either from d-arabinose or d-glucose is followed by sequential oxidation to the ketone and alkynyl Grignard addition. The resulting cis-cyclobutanediols are subjected in turn to thermal rearrangement and intramolecular oxymercuration-demercuration. The regiochemistry of the final ring closure is controlled by the nature of R.     

Application of partial least-squares regression to the resolution of highly correlated spectra. Simultaneous spectrofluorimetric determination of Al(3+), Ga(3+) and In(3+)

The simultaneous spectrofluorimetric determination of mixtures of aluminium, gallium and indium as their 8-hydroxyquinoline complexes following extraction into chloroform is studied. The high collinearity of the spectra hindered their resolution by multiple linear regression (MLR) methodology; therefore, experimental data were processed by partial least-squares regression (PLSR) methodology. A previous step in the study of three-dimensional fluorescence spectrum is possible to select the best information to quantify this system with high collinearity. Finally the optimal conditions for quantitation, the best data preprocessing procedure and the most suitable spectral mode for calibration were established. Using an external set allowed the three analytes to be determined simultaineously at concentrations below 1 mug ml(-1) with errors less than 10% for aluminium and indium, and 15% for gallium.     

Following the aggregation of amyloid-forming peptides by computer simulations

There is experimental evidence suggesting that the toxicity of neurodegenerative diseases such as Alzheimer's disease may result from the soluble intermediate oligomers. It is therefore important to characterize extensively the early steps of oligomer formation at atomic level. As these structures are metastable and short lived, experimental data are difficult to obtain and they must be complemented with numerical simulations. In this work, we use the activation-relaxation technique coupled with a coarse-grained energy model to study in detail the mechanisms of aggregation of four lys-phe-phe-glu (KFFE) peptides. This is the shortest peptide known to form amyloid fibrils in vitro. Our simulations indicate that four KFFE peptides adopt a variety of oligomeric states (tetramers, trimers, and dimers) with various orientations of the chains in rapid equilibrium. This conformational distribution is consistent with all-atom molecular-dynamics simulations in explicit solvent and is sequence dependent; as seen experimentally, the lys-pro-gly-glu (KPGE) peptides adopt disordered structures in solution. Our unbiased simulations also indicate that the assembly process is much more complex than previously thought and point to intermediate structures which likely are kinetic traps for longer chains.     

About SnF2 stannous fluoride. IV. Kinetics of the α → γ and β, γ → α transitions

The kinetics of the reconstructive first order phase transitions α → γ (upon heating) and β, γ → α (upon cooling) of SnF₂ were studied by quantitative X-ray powder diffraction. The main feature of the kinetics is that these transformations are incomplete over a wide temperature range. Consequently it is not possible to fit the experimental data using the usual solid state rate expressions. Although these transitions are not reversible (large hysteresis) the empirical use of a kinetic model of a reversible transformation provides a good fit. The α → γ transformation was also investigated by D.T.A.; this transition was found to be greatly influenced by particle size: the transition temperature increases with grain size, and its enthalpy decreases. The X-ray investigation revealed that the process of the α → γ transformation involves an intermediate noncrystalline phase named the “transition phase.” It is suggested that the transition proceeds through a process of fragmentation-reconstruction.     

Lithium diorganocuprate reactions with l-serine derivatives

The lithium diorganocuprate reactions with L-serine derivatives are studied. Retention of configuration, or at least a high enantiomeric excess (>80%) of the formed α-amino esters is observed in all cases. Attempts are made to restrict side-reactions.     

Optimization,validation and comparison of various extraction techniques for the trace determination of polycyclic aromatic hydrocarbons in sewage sludges by liquid chromatography coupled to diode-array and fluorescence detection

There is a need for a better characterization of sludges from wastewater treatment plants which are destined to be spread on agricultural lands. Inorganic pollutants are regularly controlled but organic pollutants have received few attention up to now. On this paper, we have been interested on the analysis of the 16 polycyclic aromatic hydrocarbons (PAHs) listed in the US Environmental Protection Agency (US EPA) priority list and more particularly of the six PAHs listed in the European community list (fluoranthene, benzo[b and k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene). The analysis step consists on liquid chromatography with both fluorescence and UV detections as described in the EPA Method 8310. As for the extraction step, several techniques such as supercritical fluid extraction, pressurized liquid extraction, focused microwave extraction in open vessels, Soxhlet and ultrasonic extractions are compared after optimization of the experimental conditions (solvent nature and quantity, temperature, pressure, duration, ... ) and validation with certified sludges. When optimized, these five extraction techniques are as much efficient with similar relative standard deviation. Whatever the extraction techniques used, the whole analysis protocol permits to quantify PAHs in the range of 0.09 to 0.9 mg/kg of dried sludges.