Efficient and selective photogeneration of cholesterol-derived radicals by intramolecular hydrogen abstraction in model dyads

Three model dyads have been synthesized by esterification of beta- and alpha-cholesterol (Ch) with (S)- and/or (R)-ketoprofen (Kp). The alpha-dyads are efficient photogenerators of the 7-allyl Ch radicals by intramolecular H abstraction. Subsequent cyclization via C-C coupling occurs in a stereoselective way.     

The semi-continuous quadratic mixture design problem: Description and branch-and-bound approach

The semi-continuous quadratic mixture design problem (SCQMDP) is described as a problem with linear, quadratic and semi-continuity constraints. Moreover, a linear cost objective and an integer valued objective are introduced. The goal is to deal with the SCQMD problem from a branch-and-bound perspective generating robust solutions. Therefore, an algorithm is outlined which identifies instances where decision makers tighten requirements such that no ε-robust solution exists. The algorithm is tested on several cases derived from industry.     

New procedure for generating valence and Rydberg orbitals

A previously proposed procedure for generating approximate valence, Rydberg, and continuum orbitals from spectral data is extended to two-valence electron systems. Oscillator strengths and photoionization cross sections are reported for the respective 1s²2s² →1s²2s np and 1s²2s²→1s²2s ep transitions of Be, B⁺, C⁺⁺, N⁺⁺⁺, 0⁺⁴, Ne⁺⁶, Mg⁺⁸, Al⁺⁹, and Si⁺¹⁰. Comparisons show a high level of agreement with Hartree-Fock oscillator strengths, but the accuracy of the cross sections is uncertain.     

Determination of the emission rate of an Am–Be neutron source with a Bonner sphere spectrometer

The Neutron Measurements Laboratory of the Technical University of Madrid (LMN) has an automated panoramic irradiator with a 111 GBq ²⁴¹Am–Be neutron source installed in a bunker-type large room. This facility is going to be used for calibration purposes. Recently, a spectrometry campaign involving four research groups working with different Bonner Sphere Spectrometers (BSS) and using different spectral unfolding codes was carried out. As part of these measurements the emission rate, B(t), was estimated. The application of the generalized fitting method to account for the scattering contribution is difficult due to specific characteristics of the neutron installation. A reduced fitting method, which includes room-return and in-scatter, has instead been used to overcome this problem.Detailed Monte Carlo simulations (with MCNPX code) were also performed to estimate the fluence rate using the measured source strength value. This was performed at different points. Results were then compared with measurements.Finally, the ambient dose equivalent rate measured with a neutron monitor (LB6411) was compared with results using the BSS.     

Intramolecular carbohydrate-aromatic interactions and intermolecular van der Waals interactions enhance the molecular recognition ability of GM1 glycomimetics for cholera toxin

The design and synthesis of two GM1 glycomimetics, 6 and 7, and analysis of their conformation in the free state and when complexed to cholera toxin is described. These compounds, which include an (R)-cyclohexyllactic acid and an (R)-phenyllactic acid fragment, respectively, display significant affinity for cholera toxin. A detailed NMR spectroscopy study of the toxin/glycomimetic complexes, assisted by molecular modeling techniques, has allowed their interactions with the toxin to be explained at the atomic level. It is shown that intramolecular van der Waals and CH-pi carbohydrate-aromatic interactions define the conformational properties of 7, which adopts a three-dimensional structure significantly preorganized for proper interaction with the toxin. The exploitation of this kind of sugar-aromatic interaction, which is very well described in the context of carbohydrate/protein complexes, may open new avenues for the rational design of sugar mimics.     

Robustness of the extraction step when parallel factor analysis (PARAFAC) is used to quantify sulfonamides in kidney by high performance liquid chromatography-diode array detection (HPLC-DAD)

The robustness of a multiresidue method has been analysed for the extraction and quantification of sulfamethoxypyridazine, sulfamethoxazole and sulfadimethoxine in porcine kidney by HPLC-DAD through a Plackett-Burman design. Two experimental responses were examined, the mean recovery from three replicates (accuracy) and their standard deviation (precision). Three factors were tested: the volume of phosphoric acid (pH) added in the extraction step, the time used for passing the sample through the solid-phase extraction cartridge (flow rate) and methanol volume to elute the analytes from the cartridge. Due to the non-specificity of the chromatograms (unknown matrix interferences coelute with each sulfonamide) the PARAFAC model was employed to evaluate the concentration recovered in the experiments of the Plackett-Burman design as well as to identify the spectra of the substances according to the criteria set in the European Decision 2002/657/EC for the analysis of residues. The extraction step was concluded to be robust to the recovery and the standard deviation of all three analytes.     

Comparative study of the separation of methanol–methyl acetate mixtures by pervaporation and vapor permeation using a commercial membrane

In the present study, the permeation behavior of methanol and methyl acetate in the pervaporation (PV) experiments are compared with those in vapor permeation (VP) experiments using a PVA-based composite membrane. Experiments have been carried out to study the selectivity and mass transport flux of the systems under varying operations conditions of feed temperature (40–60 °C) and feed methanol concentrations (2–34 wt%). The selected membrane was found to be methanol selective. Results show higher permeation flux but a similar separation factor for methanol in PV than in VP. For PV operation, the resulting separation factor at 60 °C shows a monotonous decrease (6.4–4.1) as the alcohol concentration in the feed mixture increases (2.3–34 wt%), whereas the total flux increases from 0.97 to 7.9 kg m⁻² h⁻¹. Based on the solution-diffusion theory, a mathematical model that describes satisfactorily the permeation fluxes of methanol and methyl acetate in both the PV and VP processes has been applied. The fluxes of both permeants can be explained by the solution-diffusion model with variable diffusion coefficients dependent on MeOH concentration in the membrane. Both PV and VP processes can be described with the same model but using different fitting parameters.     

Effect of Chemical Crosslinking on the Swelling and Shrinking Properties of Thermal and pH‐Responsive Chitosan Hydrogels

The ability to form a gel through the physical or chemical crosslinking of chitosan has been well documented. In an attempt to mimic biological systems, thermal and pH‐sensitive chitosan cylindrical hydrogels were produced by a combination of physical and chemical crosslinking processes. To this end, chitosan hydrogels prepared from alkali chitin were molded in cylinders and, once washed, were further crosslinked with glutaraldehyde at stoichiometric ratios, R (= [? CH?O]/[? NH₂]), of 1.61 and 3.22 × 10^?2. Variation in swelling as a result of stepwise changes in temperature between 40 and 2 °C at pH values of 7.0, 7.6, and 8.0 revealed that the system responds in markedly different manners dependent upon the pH. At pH 7.0, cooling from 40 to 2 °C results in contraction of the gel network structure. While raising the temperature from 2 to 40 °C leads to a rapid swelling response (i.e., ca. a twofold increase in the amount of solvent uptake). Subsequent cooling to 2 °C is accompanied by a new contraction cycle. At pH ≥ 7.6 the temperature dependence of the swelling–contraction behavior is exactly the opposite of that observed at pH 7.0. Very similar trends were observed for the gels at both degrees of crosslinking. The swelling–shrinking behavior observed in gels of pH ≥ 7.6, is similar in kind to that of uncrosslinked gels and is interpreted in terms of a lower critical solution temperature (LCST) volume phase transition, driven by hydrophobic association, presumably involving residual acetyl groups in the chitin. The results at pH 7.0 suggest that the slight ionization of the ? NH groups leads to destruction of the hydrophobic hydration thus effectively reversing the negative thermal shrinking.

Evolution of the swelling ratio, S, as a function of time and temperature for crosslinked chitosan hydrogels. Circles represent S values recorded at pH 7.0 and triangles those at pH 7.6.     

Highly diastereoselective formation of C2-symmetric bis-thioglycoside Pd(II) complexes: the role of the exo anomeric effect

Treatment of several C(2) symmetric bis-thioglycosides with Pd(CH(3)CN)(2)Cl(2) always leads to a single diastereomeric Pd(II) complex as a consequence of the exo-anomeric effect.     

Synthesis and characterization of optically active polyamides derived from carbohydrate-based monomers

The synthesis of three new stereoregular AB-type polyamides based on D -ribono-1,4-lactone, L -arabinose, and D -xylose has been carried out by the active ester polycondensation method. These polyamides were characterized by elemental analysis, IR and NMR spectroscopies, and powder X-ray diffraction. They displayed optical activity and had a pronounced affinity to water, although they were not soluble in this solvent. The polyamide obtained from D -ribono-1,4-lactone was highly crystalline and yielded films with spherulitic texture. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3645–3653, 1997