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V Schurig

Publications and source records attributed to V Schurig.

At least 55 records · Page 3Linked to original sources
PubMed1998

Dual chiral recognition system involving cyclodextrin derivatives in capillary electrophoresis II. Enhancement of enantioselectivity.

The enantiomer separation of hexobarbital was investigated by open tubular electrochromatography (OTEC) using the chiral stationary phase (CSP) CHIRASIL-DEX (a permethylated beta-cyclodextrin covalently linked to a dimethylpolysiloxane) and by cyclodextrin-electrokinetic chromatogaphy (CD-EKC) using anionic beta-cyclodextrin-sulfo-n-propyl ether (SPE-beta-CD) and cationic beta-cyclodextrin-2-hydroxy-3-trimethylammoniumpropyl ether chloride (HTAP-beta-CD) added to the running buffer. By employing two chiral selectors, the enantiomer separation of hexobarbital was then studied simultaneously by OTEC with CHIRASIL-DEX and by CD-EKC with either SPE-beta-CD or HTAP-beta-CD in the dual chiral recognition mode. In conjunction with CHIRASIL-DEX, anionic SPE-beta-CD decreased the chiral separation factor alpha due to compensation of enantioselectivity whereas the cationic additive HTAP-beta-CD increased the chiral separation factor alpha due to enhancement of enantioselectivity. It is concluded that CHIRASIL-DEX imparts an opposite enantioselectivity to the enantiomers of hexobarbital as compared to the charged CDs SPE-beta-CD and HTAP-beta-CD. Unusual peak broadening phenomena are observed in the dual chiral recognition system comprised of CHIRASIL-DEX and HTAP-beta-CD. The possible consequences of accidental dual chiral recognition systems caused by wall stacking effects of the mobile phase additives onto the inner surface of the capillary column are discussed.

Acrylic Resins↗
PubMed1997

Direct binding of low molecular weight haptens to ELISA plates.

Immobilization of low molecular weight haptens in ELISA usually involves their coupling to protein molecules or covalent binding to the solid phase. In this study we demonstrate that it is possible to directly bind the hapten p-aminophenylalanine to gamma-irradiated polystyrene microtiter plates for the detection of antibodies that stereospecifically recognize the chiral center of alpha-amino acids. Simple incubation of the hapten in aqueous buffer, without additional activation, results in a stable coating that is suited for use in ELISA.

Amino Acids↗
PubMed1997

Chiral discrimination of inhalation anesthetics and methyl propionates by thickness shear mode resonators: new insights into the mechanisms of enantioselectivity by cyclodextrins.

The discrimination of the enantiomers of methyl lactate, methyl 2-chloropropionate, and the inhalation anesthetics enflurane, isoflurane, and desflurane in the gas phase has been performed using thickness shear mode resonators. The selective coating was a modified perpentylated gamma-cyclodextrin derivative dissolved in a polysiloxane matrix. A new model for the sorption of the chiral compounds into the cyclodextrin cavities and into the polymer matrix was established for the purpose of characterizing the sensor responses. This characterization included the fitting of the sensor responses (preferential and nonpreferential sorption) according to the model and extracting the characteristic parameters. In particular we attempted to explain the observed variation of the chiral discrimination factor alpha with changing analyte or cyclodextrin concentrations and search for an invariable parameter, characteristic for a certain analyte-cyclodextrin combination. The process of chiral or "molecular" recognition was thoroughly investigated.

Anesthetics, Inhalation↗
PubMed1997

Enantioselective determination of chiral organochlorine compounds in biota by gas chromatography on modified cyclodextrins.

Approaches to the gas chromatographic enantiomer separation of chiral organchlorines (alpha-hexachlorocyclohexane, cis- and trans-chlordane, heptachlor, heptachlorepoxide, oxychlordane, o,p'-DDT, compounds of technical toxaphene and stable atropisomeric polychlorinated biphenyls) are reviewed. Chiral stationary phases based on cyclodextrin derivatives and used for the gas chromatographic enantiomer separation of the chiral organochlorines are described. Enantiomeric ratios of chiral organochlorines in technical mixtures and biological samples are reported and discussed.

Animals↗
PubMed1997

Preparative enantiomer separation of the inhalation anesthetics enflurane, isoflurane and desflurane by gas chromatography on a derivatized gamma-cyclodextrin stationary phase.

The preparative enantiomeric separation of the inhalation anesthetics enflurane (1) and isoflurane (2) in very high chemical (> 99.5%) and enantiomeric excess (ee > 99%) by gas chromatography (GC) on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (4), dissolved in the apolar polysiloxane SE-54 and coated on Chromosorb P AW DMCS, is described. Up to 1 g of each enantiomer of 1-2 can been obtained per diem. The enantiomers of the highly volatile desflurane (3) can also be separated, albeit with diminished ee. The enantiomeric excess of 1-3 was checked by analytical GC on 4 and the absolute configuration of 2 and 3 has been determined via anomalous X-ray diffraction.

Anesthetics, Inhalation↗
PubMed1997

Approach to the thermodynamics of enantiomer separation by gas chromatography. Enantioselectivity between the chiral inhalation anesthetics enflurane, isoflurane and desflurane and a diluted gamma-cyclodextrin derivative.

The thermodynamics of enantioselectivity, -delta D,L(delta G), -delta D,L(delta H), delta D,L(delta S) and Tiso, have been determined by gas chromatography employing the concept of the retention increment R' for the inhalation anesthetics enflurane (1), isoflurane (2) and desflurane (3) and the selector octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (4) in the polysiloxane SE-54. It is shown that the separation factor alpha is concentration-dependent. Therefore, the separation factor alpha should not be employed as a criterion for enantioselectivity in diluted systems. The -delta DL(delta G) data for 1 and 4 are corroborated by 1H NMR spectroscopic measurements.

Anesthetics, Inhalation↗
PubMed1997

Electrokinetic chromatography employing an anionic and a cationic beta-cyclodextrin derivative.

beta-Cyclodextrin, 1, can act as a chiral buffer additive in capillary zone electrophoresis (CZE) owing to its ability to form diastereomeric complexes with the enantiomers of ionic compounds. Reaction of 1 with 2,3-epoxy-propyltrimethylammonium chloride gave preponderantly 6-O-(2-hydroxy-3-trimethylammoniopropyl) derivatives (2) of varying degrees of substitution (d.s.). Analogous reaction of 1 with 1,3-propanesultone yielded 6-O-(sulfo-n-propyl) derivatives (SPE-beta-CD, 3) of varying d.s. The purity and the d.s. of 2 and 3 were determined by 1H NMR and electrospray ionization mass spectrometry (ESI-MS). These charged beta-cyclodextrin derivatives were used for the chromatographic enantiomer separation of a series of neutral barbiturates, of chlorthalidone, terbutaline, warfarin, salbutamol and brompheniramine by cyclodextrin electrokinetic chromatography (CD-EKC). During the separation of chlorthalidone with 3 as chiral additive, a reversible interconversion of the enantiomers (enantiomerization) was observed at temperatures below 20 degrees C.

Anions↗
PubMed1997

Stereoselective pharmacokinetic analysis of valnoctamide in healthy subjects and in patients with epilepsy.

OBJECTIVE: To investigate the pharmacokinetics of the four stereoisomers of valnoctamide, a mild tranquilizer endowed with anticonvulsant properties. METHODS: Racemic valnoctamide, 400 mg, was administered orally to seven healthy subjects and to six patients with epilepsy stabilized with long-term carbamazepine therapy. In the patients with epilepsy, valnoctamide kinetics was also reassessed after 8-day oral dosing at a dosage of 600 mg daily. Plasma samples were assayed by gas chromatography-mass spectrometry with use of a capillary column coated with chiral stationary phase that enabled baseline resolution of the four stereoisomers, designated hereafter as A, B, C, and D (where A and C, together with B and D, represent enantiomeric pairs). RESULTS: In healthy subjects, stereoisomers A, C, and D showed similar kinetics, with an apparent oral clearance (CL/F) of about 4 1/2 L/hr, a half-life (t1/2) of about 10 hours, and an apparent volume of distribution (VSS/F) of about 65 L. However, stereoisomer B showed a much higher clearance (8.7 +/- 0.9 L/hr) and a shorter t1/2 (5.8 hours). For all stereoisomers, CL/F values in patients with epilepsy were about tenfold higher than those found in healthy subjects. Compared with healthy subjects, patients with epilepsy also showed shorter t1/2 values and higher VSS/F values for each of the stereoisomers. After 7-day dosing, CL/F values at steady state were lower than those determined in the same patients after a single dose. CONCLUSIONS: Valnoctamide exhibits enantioselectivity and diastereoselectivity, an observation that may have important practical implications if pharmacodynamic differences between stereoisomers are also found. The observed pharmacokinetic differences between healthy subjects and patients with epilepsy are likely to be related to induction of metabolism of valnoctamide stereoisomers by carbamazepine.

Administration, Oral↗
PubMed1997

Minimum alveolar anesthetic concentration values for the enantiomers of isoflurane differ minimally.

Results of in vivo and in vitro studies of the anesthetic potencies of the enantiomers (optical isomers) of isoflurane provide various results ranging from no difference to differences of nearly two fold. A finding of a difference in anesthetic requirement in the whole animal has particular relevance to theories of anesthetic mechanisms of action because it suggests that anesthesia may result from a specific anesthetic-receptor interaction. This led to our decision to redetermine the minimum alveolar anesthetic concentration (MAC) of (+)-S and (-)-R enantiomers of isoflurane in 12 Sprague-Dawley rats (six per group). The (+)-S enantiomer gave a MAC of 0.0144 +/- 0.0012 atm (i.e., 1.44% +/- 0.12% at 1 atm pressure; mean +/- SD) and the (-)-R enantiomer gave a MAC of 0.0169 +/- 0.0020 atm. Although the 17% greater value for the (-)-R enantiomer is qualitatively consistent with previous results the difference is not significant (P = 0.06), and the absolute difference is smaller than that found by a previous study. However, given the small sample size, our power to define a small significant difference is limited. Regardless of statistical significance, our results do not confirm the conclusion that interaction with a specific receptor is important to the mechanism of action of inhaled anesthetics.

Anesthetics, Inhalation↗
PubMed1996

Analysis of charged cyclomalto-oligosaccharides (cyclodextrin) derivatives by ion-spray, matrix-assisted laser-desorption/ionization time-of-flight and fast-atom bombardment mass spectrometry, and by capillary electrophoresis.

The use of electrospray-ionization mass spectrometry (ESIMS), matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry (MALDITOFMS), and fast-atom bombardment mass spectrometry (FABMS) for the rapid determination of molecular weight, degree of substitution (ds), and purity is demonstrated for charged derivatives of cyclomalto-heptaose (beta CD) and -octaose (gamma CD). The access to anionic sulfoalkyl ethers (alkyl: ethyl, n-propyl, and n-butyl) and to beta CD-2-hydroxy-3-trimethylammoniumpropyl ether chlorides (HTAP-beta CD) leads only to mixtures of products, the compositions of which can be determined directly from ESI and MALDITOF mass spectra. All charged derivatives consist of a mixture of unreacted and higher substituted compounds. The substitution patterns obtained by MS are in good agreement with the results of experiments on the separation of beta CD-sulfoalkyl ethers by capillary electrophoresis (CE).

Capillary Action↗
PubMed1995

Unified enantioselective capillary chromatography on a Chirasil-DEX stationary phase. Advantages of column miniaturization.

Immobilized Chirasil-DEX (mono-6-O-octamethylenepermethyl-beta-cyclodextrin chemically linked to dimethylpolysiloxane) can be employed as a versatile chiral stationary phase in chromatography. The chiral polymer has a long lifetime and is configurationally and thermally stable. The concept of unified enantioselective chromatography has been demonstrated for the enantiomer separation of hexobarbital by gas chromatography, supercritical fluid chromatography, liquid chromatography and capillary electrochromatography on a single open-tubular column (1 m x 50 microns I.D.) coated with Chirasil-DEX. The advantages of miniaturization in contemporary chromatographic enantiomer separation are demonstrated. Chirasil-DEX coated on porous silica is also useful for enantiomer separation in high-performance liquid chromatography.

Carbohydrate Sequence↗
PubMed1995

Enantioselective determination of chiral 2,2',3,3',4,6'-hexachlorobiphenyl (PCB 132) in human milk samples by multidimensional gas chromatography/electron capture detection and by mass spectrometry.

The enantiomeric ratio (ER) of the chiral polychlorinated biphenyl (PCB) 132 (2,2',3,3',4,6'-hexachlorobiphenyl) was determined in human milk samples. The enantiomers were separated by multidimensional gas chromatography (MDGC), using different achiral columns for pre-separation and a Chirasil-Dex column for enantiomer separation, and quantified by electron-capture detection (ECD) and by mass spectrometry (MS/SIM). The determined enantiomeric ratios varied between 0.45-0.85 (enantiomeric excess: 38-8%).

Chromatography, Gas↗
PubMed1994

Enantiomer separation using mobile and immobile cyclodextrin derivatives with electromigration.

The principle of electrochromatography is applied to enantiomer separation with wall-coated capillaries of 50 microns (ID). As chiral stationary phase an immobilized polysiloxane containing chemically bonded monokis-6-O-octamethylene-permethyl-cyclodextrin (beta- or gamma-Chirasil-Dex) was used. The results are compared with the conventional method of adding cyclodextrin derivatives as chiral pseudostationary phase to the running buffer.

Buffers↗
PubMed1994

Enantioselective degradation of alpha-hexachlorocyclohexane and cyclodiene insecticides in roe-deer liver samples from different regions of Germany.

Remarkably high concentrations of alpha-hexachlorocyclohexane (alpha-HCH), cis-heptachlorepoxide and oxychlordane were found in roe-deer liver samples both from the northern and southern German states Schleswig-Holstein and Baden-Württemberg, respectively. The data revealed no significant regional differences, but they showed some common characteristics: a preferential degradation of (+)-alpha-HCH, and a preferential enrichment of (+)-oxychlordane and of (+)-cis-heptachlorepoxide as determined by chiral capillary gas chromatography using modified cyclodextrin phases. Calculation of the spearman rank correlation coefficients rS supported the assumption that higher concentrations of alpha-HCH may result in a stronger decomposition of the (+)-enantiomer, while higher levels of cis-heptachlorepoxide and oxychlordane appear to lead to a faster decomposition of the respective (-)-enantiomer or a preferential formation of the respective (+)-enantiomer.

Animals↗
PubMed1992

Enantio- and regioselectivity in the epoxide-hydrolase-catalyzed ring opening of simple aliphatic oxiranes: Part I: Monoalkylsubstituted oxiranes.

The in vitro conversion of chiral aliphatic monoalkylsubstituted oxiranes into 1,2-diols catalyzed by epoxide hydrolase of rat liver microsomes occurs with substrate enantioselectivity and regioselectivity. Substrate enantioselectivity is generally low, and has the same sense, for methyloxirane, vinyloxirane, epichloro-, and epibromohydrin. In the hydrolysis of t-butyloxirane inhibitory effects are involved leading to a complex pattern of enantioselectivity. All investigated monosubstituted aliphatic oxiranes are hydrolyzed with high regioselectivity by nucleophilic attack of water at the unsubstituted ring carbon atom. The enantiomeric excess of the unreacted oxirane substrates and the diol metabolites formed were determined by complexation and inclusion gas chromatography.

Animals↗
PubMed1992

Enantio- and regioselectivity in the epoxide-hydrolase-catalyzed ring opening of aliphatic oxiranes: Part II: Dialkyl- and trialkylsubstituted oxiranes.

The extent of substrate enantioselectivity and regioselectivity of a series of aliphatic 2,3-dialkyl- and trialkylsubstituted oxiranes in their in vitro epoxide-hydrolase-catalyzed hydrolysis depends on the size of the alkyl residues and on the substitution pattern of the oxirane ring. The enzyme-catalyzed hydrolysis of cis-oxiranes, containing at least one methyl substituent, shows complete or nearly complete substrate enantioselectivity and regioselectivity with nucleophilic attack by water occurring with inversion of configuration at the methylsubstituted ring carbon atom of (S)-configuration. In the hydrolysis of the isomeric trans-oxiranes, both enantiomers are metabolized with a higher rate for the (2S;3S)-enantiomer. The conversion of trimethyloxirane occurs with high substrate enantioselectivity in favor of the (S)-enantiomer and with complete regioselectivity at the monomethylsubstituted ring carbon atom. The differentiation of the enantiotopic ring carbon atoms (product enantioselectivity) in the smallest aliphatic meso-oxirane, cis-2,3-dimethyloxirane, leads to (2R;3R)-butane-2,3-diol with ee = 86%. cis-2-Ethyl-3-propyloxirane, possessing alkyl residues larger than methyl, represents an extremely poor substrate in the epoxide-hydrolase-catalyzed hydrolysis process.

Animals↗