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241.
Pizarro N Llebaria A Cano S Joglar J Farré M Segura J de la Torre R 《Rapid communications in mass spectrometry : RCM》2003,17(4):330-336
3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) is consumed as the racemate but some metabolic steps are enantioselective. In addition, chiral properties are preserved during MDMA biotransformation. A quantitative analytical methodology using gas chromatography/mass spectrometry (GC/MS) to determine enantioselective disposition in the body of MDMA and its main metabolites including 3,4-methylenedioxyamphetamine (MDA), 4-hydroxy-3-methoxymethamphetamine (HMMA), and 4-hydroxy-3-methoxyamphetamine (HMA) was developed. Plasma and urine samples were collected from a male volunteer. The analysis of MDMA, MDA, and 4-hydroxy-3-methoxy metabolites by GC/MS required a two-step derivatization procedure. The first step consisted of derivatization of the amine with enantiomerically pure Mosher's reagent ((R)-MTPCl). Triethylamine was used as a base to neutralize hydrochloric acid formed during the reaction allowing quantitative derivatization, which resulted in a substantial improvement in the sensitivity of the method compared with other previously described techniques. Further treatment with ammonium hydroxide was required since both amine and hydroxyl groups underwent derivatization in the reaction. Ammonium hydroxide breaks bonds formed with hydroxyl groups without affecting amine derivatives. The second derivatization step using hexamethyldisilazane was needed for metabolites containing phenol residues. This derivatization method permitted the stereochemically specific study of MDMA and its main monohydroxylated metabolites by GC/MS. A detailed study of the chemical reactions involved in the derivatization steps was indispensable to develop a straightforward, sensitive, and reproducible method for the analysis of the parent drug compound and its metabolites. 相似文献
242.
Taking into account the thiophilic properties of iodine, a very simple methodology to achieve 2′,3′-didehydro-2′,3′-dideoxy-β-nucleosides in high yield was performed, using mild, and inexpensive conditions, by means of the treatment of 2′-deoxy-3′,5′-dibenzoyl-2′-iodo-β-nucleoside derivatives with NaHS. The process has shown to be highly dependent of the relative geometry between the iodine atom and the adjacent leaving group. In this way, different essays carried out with pyranose derivatives have concluded in no reaction when the vicinal groups to eliminate do not adopt a trans-diaxial disposition. In addition, the treatment of 2-iodomethyl-cyclopropane-1,1-dicarboxylic acid diethyl ester under the same conditions softly and readily leads to the obtention of a mixture of the expected 2-allyl-malonic acid diethyl ester (as the minor product) and the thiirane derivative 2-thiiranylmethyl-malonic acid diethyl ester (as the major product). In this case, the responsible of the reaction progress are the nucleophilic properties of the sulfur atom rather than the thiophilic character of the iodine atom. 相似文献
243.
Reaction conditions and structure of the starting enamines (cyclic or open-chain) determine greatly the final products of the title reactions. Whereas in benzene and acetonitrile, DMAD and 1 give a mixture of the diastereoisomeric dienamines 5, in methanol they afford pirrolizine 3. Enaminofuranones 2 and 10 furnish the corresponding “Michael adducts” 7a,b,c and 11a,b,c but fail to yield pirrolizines. It has been demonstrated that above b and c adducts differ exclusively on the arrangement of groups around a chiral axis. 相似文献
244.
Spectroscopic characterization of mineral crystals produced by the bacterium Azospirillum brasilense
Kamnev AA Antonyuk LP Chernyshev AV Ignatov VV De Vargas MC 《Analytical and bioanalytical chemistry》1996,355(5-6):739-741
The results of spectroscopic structural and trace elemental analyses of mineral crystals produced by the soil nitrogen-fixing bacterium Azospirillum brasilense cultivated in a synthetic medium are presented and discussed. The mineral formed is shown to have a structure close to struvite (MgNH(4)PO(4) x 6H(2)O; ASTM file No. 15-762) with some differences which may be attributed to the presence of isomorphic admixtures of other cations (struvite is known to have a variety of forms). AAS/AES and ion chromatography analyses for a number of biologically important microelements and their role in the formation of the crystal structure, as well as some questions related to biomineralization are also discussed. 相似文献
245.
Through the sol–gel process, using the so-called neutral amine route, spherical particles of 1:1 zirconia–titania were synthesized from zirconium(IV) and titanium(IV) butoxides as well as 1,12-diaminododecane as precursor species. The obtained product exhibited a hexagonal structure, as determinated by X-ray diffraction data. The obtained material was also characterized by thermogravimetry, differential scanning calorimetry, infrared spectroscopy, scanning electron microscopy, and surface area measurements. Despite the release of template molecules on heating, the spherical morphology was retained up to about 1200°C, at which the disruption of the spheres took place. 相似文献
246.
A fluorimetric assay for cortisol 总被引:2,自引:0,他引:2
Appel D Schmid RD Dragan CA Bureik M Urlacher VB 《Analytical and bioanalytical chemistry》2005,383(2):182-186
A simple, rapid and sensitive fluorimetric assay for the quantitative determination of cortisol is reported. The assay is
based on the formation of a fluorescent dye when cortisol is incubated with a mixture of sulfuric acid and acetic acid. The
fluorescence spectrum recorded for the resulting dye shows a maximum extinction at 475 nm and a maximum emission at 525 nm.
The solvent 2-methyl-4-pentanone was used for extraction and was found to act as a fluorescence amplifier. A limit of detection
of 2.7 μM was achieved, making it possible to forego solvent evaporation. The assay suffers minor interference from 11-deoxycortisol
which exhibits low fluorescence at λ
ex: 460 nm; λ
em: 505 nm. Typical standard deviations were below 4%. We validated the assay using a biotransformation with recombinant Schizosaccharomyces pombe which regioselectively hydroxylates 11-deoxycortisol to cortisol. The method described herein is suitable for preliminary
screening of microorganisms capable of steroid hydroxylation. 相似文献
247.
Polyvinyl chloride samples of various tacticity have been prepared and photochlorinated in CCl4 for 4 hr. The chlorination degree correlates directly with the degree of syndiotacticity. Initial PVC with syndiotacticity about 56 per cent, which can be regarded as alternate sequences of syndiotactic and isotactic diads, is quickly chlorinated. The mechanism of chlorine attack on the macromolecula is directly connected with the stereoregularity. 相似文献
248.
J. G. C. M. van Duijneveldt-van de Rijdt Dr. F. B. van Duijneveldt 《Theoretical chemistry accounts》1970,19(1):83-91
Using perturbation theory of intermolecular forces (including exchange effects) hydrogen-bond energies are calculated for a simple model representing the water dimer. Several orientations of the proton acceptor with respect to the O...O axis are considered and it is found that the optimum orientation depends sensitively on the oxygen lone-pair hybridization assumed. The main orientation-dependent energy term is found to be the classical electrostatic energy between the unperturbed molecules. The relation of the present results to the structures of the ices I and Ic, and to the results of recent SCF studies are discussed briefly.
For some recent CNDO/2 calculations on (H2O)2 which concentrate mainly on rotations of the donor see Ref. [5]. 相似文献
Zusammenfassung Die Energien von Wasserstoffbindungen werden für ein einfaches Modell des dimeren Wassers mit Hilfe der Störungstheorie für intermolekulare Kräfte (mit Austauscheffekten) berechnet. Verschiedene Orientierungen des Protonen-Akzeptors bezüglich der O ... O-Achse werden untersucht; man findet, daß die optimale Orientierung empfindlich von der angenommen Hybridisierung des einsamen Elektronenpaars des Sauerstoffs abhängt. Der hauptsächliche, von der Orientierung abhängige Energieterm ist die klassische elektrostatische Energie zwischen den beiden ungestörten Molekülen. Die Beziehung der vorliegenden Ergebnisse zu den Strukturen von Eis I und Ic sowie zu den Ergebnissen kürzlicher SCF-Rechnungen wird kurz diskutiert.
For some recent CNDO/2 calculations on (H2O)2 which concentrate mainly on rotations of the donor see Ref. [5]. 相似文献
249.
J. L. Q. de Britto A. M. S. Braghirolli A. G. da Silva 《Journal of Radioanalytical and Nuclear Chemistry》1985,96(2):181-186
A new and simplified method was developed for the separation of thallium 201 which will allow the shipment of a “one shot generator” to distant places and easy elution at the destination. Thallium is produced as usually, through the reaction $${}^{nat}Tl/p,xn/^{201} pb\xrightarrow{{\beta ^ + }}{}^{201}Tl$$ with 24-MeV protons from IEN'S CV-28 cyclotron. The separation is based on the properties of a chelating carboxylic acid cation exchange resin column which at pH 4.5 retains lead while thallim is easily eluted. This column can also be used as a regular generator to produced greater quantities of201Tl but at the expense of more elutions. 相似文献
250.
O Ladrón de Guevara C Cortinas de Nava P Padilla J Espinosa M Cebrian L García 《Journal of chromatography. A》1990,528(1):35-41
For the determination of beta-aminoisobutyric acid (BAIBA) in urine samples in which the beta-alanine concentrations are higher than those of BAIBA, the resolution between these two amino acids, separated by reversed-phase liquid chromatography on an octadecylsilane column, was optimized. The chromatographic analysis included precolumn derivatization of amino acids with o-phthalaldehyde, followed by a 15-min isocratic elution and detection at 340 nm. Because of its simplicity, this method should be useful for monitoring urinary excretion of BAIBA. 相似文献