In preliminary communications, we reported the diastereoselective synthesis of cularine and sarcocapnine via the intramolecular ring closure of nitrenium and oxenium ions, a new highly diastereoselective reductive methylation with (+)-8-phenylmenthyl chloroacetate followed by reduction with sodium borohydride, and a facile entry to the isoquinoline precursors by aza-Wittig electrocyclic ring closure. We now report the full details of the syntheses of (+)-O-demethylcularine, (+)-cularine, (+)-sarcocapnidine, (+)-sarcocapnine, and (+)-crassifoline and describe different methods of synthesis of their precursors. 相似文献
Microcin J25 forms stable monolayers at the air-water interface showing a collapse at a surface pressure of 5 mN/m, 220 mV of surface potential, and 6 fV per squared centimeter of surface potential per unit of molecular surface density. The adsorption of microcin J25 from the subphase at clean interfaces leads to a rise of 10 mN/m in surface pressure and a surface potential of 220 mV. From these data microcin appears to be a poor surfactant per se. Nevertheless, the interaction with the lipid monolayer further increase the stability of the peptide at the interface depending on the mode in which the monolayer is formed. Spreading with egg PC leads to nonideal mixing up to 7 mN/m, with hyperpolarization and expansion of components at the interface, with a small excess free energy of mixing caused by favorable contributions to entropy due to molecular area expansion compensating for the unfavorable enthalpy changes arising from repulsive dipolar interactions. Above 7 mN/m microcin is squeezed out, leaving a film of pure phospholipid. Nevertheless, the presence of lipid at 10 and 20 mN/m stabilize further microcin at the interface and adsorption from the subphase proceeds up to 30 mN/m, equivalent to surface pressure in bilayers. 相似文献
The amount of counterions in layer-by-layer (LBL) films of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) has been determined with X-ray photoelectron spectroscopy (XPS) for films prepared from solutions with various NaCl concentrations. Sodium and chloride counterions are present in LBL films produced from salt solutions, which are located at the surface and in the bulk of the films. The percentage of bulk counterions increases with the ionic strength of the polyelectrolyte before reaching a constant value. The bulk sodium/sulfur percentage ratios tend to 0.8 for samples washed with pure water and for samples washed with NaCl aqueous solutions, while the bulk chlorine/nitrogen percentage ratios tend to 0.5 for the same samples. The ratio between the percentages of polyelectrolyte ionic groups lies close to unity for all samples, indicating that counterions do not contribute to charge compensation in the polyelectrolyte during the adsorption process. The presence of counterions in LBL films is explained by Manning condensation near the polyelectrolyte ionic groups, leading to inter-polyelectrolyte ionic bondings via ionic networks. It is believed that condensation leads to the formation of NaCl crystallites in these LBL films, which was confirmed by X-ray diffraction measurements. 相似文献
Rosiglitazone (RSG) is marketed as a racemic mixture although the antidiabetic activity is essentially related to the (S)-enantiomer. The chiral center has an adjacent carbonyl group; therefore, the (R)-enantiomer could be transformed to the (S)-enantiomer or vice versa by keto-enolic tautomerism. The literature indicates that this racemization is slow enough to allow the evaluation of the properties of the isolated enantiomers. However, there is no information about the enantioselective kinetic disposition and metabolism of RSG. Additionally, there are no studies on the racemization of its metabolites. Considering these facts, a chiral HPLC method was developed and used for the first time to study the racemization of RSG and its main metabolites. Different conditions, including those used to evaluate the in vitro enantioselective metabolism, were employed. The simultaneous chiral separation of RSG and metabolites was achieved on a Chiralcel OJ-H column by employing methanol/ethanol (90:10, v/v) as mobile phase. The racemization studies showed that the half-life of RSG decreased more than 30 times when the temperature increased from 4 to 37 °C. It was also observed that the half-life of RSG changed from approximately 20 h at pH 3.5 to approximately 2 h at pH 7.4. The same profile was observed for its metabolites. Organic solvents and UV light did not present influence on the racemization process. In addition, a Complete Factorial Design was conducted to evaluate the influence of some parameters that can be changed during an in vitro metabolism study. The results obtained showed that the racemization occurs under in vitro metabolism conditions.
A colorimetric method based on silver nanoparticles was developed for the determination of melamine in milk. Silver nanoparticles were synthesized without any stabilizer, using sodium borohydride as the reducing agent. Optimization of the variables for the formation of the nanoparticles was performed by factorial design, resulting in stable colloidal silver nanoparticles with a mean diameter of 14.0?±?2.7?nm. Spectrophotometric measurements performed at 475?nm showed a linear range from 0.033 to 1.50?mg?L?1 of melamine with limits of detection and quantification of 0.009 and 0.031?mg?L?1, respectively. The method provided highly sensitive determination of melamine in milk. 相似文献
Recent developments in wastewater treatment have led to a renewed interest to obtain elemental sulphur (S°) as a by-product from bioreactors. However, practical studies are limited by the gap of adequate analytical techniques for its determination. This paper provides a statistical study and matrix effect evaluation of an adapted spectrophotometric method for routine S° analyses in aqueous samples, based on a methodology previously described by Hart (1961). Four complex matrices were tested: domestic sewage and effluent samples from three different bioreactors. Tested performance criteria included linearity, matrix effect, limit of detection and quantification and S° recovery. Results were linear (R2 = 0.99994) in the studied range (5 to 100 mg S° L?1) and no matrix effect was observed. The accuracy was based on recovery values that varied from 100% to 106%. The colloidal S° separation and extraction protocol was also considered suitable for aqueous samples, reaching more than 99.0% of S° recovery. 相似文献
Enones (Z)-3-methyl-(Z)-3-chloromethyl- and (Z)-3-bromomethyl-4-R-3-buten-2-one (R = n-pentyl, phenyl, 2′- and 4′-chlorophenyl, 3′- and 4′-nitrophenyl, 4′-methoxyphenyl) were synthesized and subjected to reduction by the microorganisms Saccharomyces cerevisiae andGeotrichum candidum. Whereas the bioreduction of 3-methy-4-R-3-buten-2-ones afforded the corresponding (S)-4-R-3-methybutan-2-ones, the bioreduction of 3-chloromethyl- and 3-bromomethyl-4-R-3-buten-2-ones afforded the corresponding (R)-4-R-3-methybutan-2-ones. 相似文献