Enantiomer separation by gas chromatography on cyclodextrin chiral stationary phases

Enantiomeric separation of volatile organics by gas chromatography for the in situ analysis of extraterrestrial materials: kinetics and thermodynamics investigation of various chiral stationary phases

The performances of several commercial chiral capillary columns have been evaluated with the aim of determining the one most suitable for enantiomeric separation in a gas chromatograph onboard a space probe. We compared the GC-MS response of three capillary columns coated with different chiral stationary phases (CSP) using volatile chiral organic molecules which are potential markers of a prebiotic organic chemistry. The three different chiral capillary columns are Chirasil-Val, with an amino acid derivative CSP, ChiralDex-β-PM, with a CSP composed of dissolved permethylated β-cyclodextrins in polysiloxane, and Chirasil-Dex, with a CSP made of modified cyclodextrins chemically bonded to the polysiloxane backbone. Both kinetics and thermodynamics studies have been carried out to evaluate the chiral recognition potential in these different types of columns. The thermodynamic parameters also allow a better understanding of the driving forces affecting the retention and separation of the enantiomers. The Chirasil-Dex-CSP displays the best characteristics for an optimal resolution of the chiral compounds, without preliminary derivatization. This CSP had been chosen to be the only chiral column in the Sample Analysis at Mars (SAM) experiment onboard the current Mars Science Laboratory (MSL) mission, and is also part of the Mars Organic Molecules Analyzer (MOMA) gas chromatograph onboard the next Martian mission ExoMars. The use of this column could also be extended to all space missions aimed at studying chirality in space.

Evaluation of newly synthesized derivative of cyclodextrin for the capillary electrophoretic separation

A highly water-soluble new cyclodextrin (CD) derivative 2-O-acetonyl-2-O-hydroxypropyl-beta-CD (2-AHP-beta-CD) was synthesized and tested as an effective chiral selector for the capillary zone electrophoretic resolution (Rs) of several basic and acidic analytes. The primary purpose of the research was to explore the capability of the 2-AHP-beta-CD as chiral selectors on comparison with the neutral CDs such as beta-CD, DM-beta-CD and HP-beta-CD. Substitution with 2-O-acetonyl-2-O-hydroxypropyl group at the secondary hydroxyl sites of the CD is aimed at influencing the magnitude and selectivity of analyte-CD interactions. The chiral resolution was strongly influenced by the concentration of the CDs and buffer pH. 2-AHP-beta-CD showed the best enantiomer resolution properties among the tested compounds, while the other CDs showed inferior or no performances at all.

Chiral recognition ability of alpha-cyclodextrin with regard to some monoterpenoids under gas-liquid chromatographic conditions

Gas-liquid chromatography was applied to investigate the mechanism of alpha-cyclodextrin (alpha-CD) complexation processes with some chiral monoterpenoids differing from each other in chemical properties and structure. They were chosen from hydrocarbons, alcohols, aldehydes and ketones of acyclic, monocyclic and bicyclic structure. The relationships between the retention factor, k, of a guest solute (G) and alpha-CD concentration were studied. The obtained data enabled the stoichiometry, the stability of individual complexes and the separation factor of enantiomers to be determined. It was found that almost all the investigated monoterpenoids, apart from the acyclic ones, form inclusion complexes with alpha-CD. Straight-line relations (r vs. [alpha-CD]) were observed for monocyclic alcohols and pulegone, without any trace of enantioselectivity. This behaviour indicates that the 1:1 stoichiometry of the G-CD complexes does not lead to chiral recognition. Parabolic relations arising from 1:2 stoichiometry were found for limonene, alpha-phellandrene, some monocycylic ketones and all the investigated bicyclic terpenoids. It appeared that only the second step of complexation displayed marked enantioselectivity. However, a loss of efficiency resulting from slower equilibration is then noticeable. Attempts are made to rationalize the chromatographic results with respect to the structure of the investigated compounds.

Extraordinary chiral discrimination in inclusion gas chromatography. thermodynamics of enantioselectivity between a racemic perfluorodiether and a modified gamma-cyclodextrin

The enantiomers of the perfluorodiether "compound B" [2-(fluoromethoxy)-3-methoxy-1,1,1,3,3-pentafluoropropane], a decomposition product of the inhalational anesthetic sevoflurane [2-(fluoromethoxy)-1,1,1,3,3,3-hexafluoropropane], were separated by gas chromatography on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (Lipodex E), dissolved in polysiloxane PS 255 (30% w/w), with an unexpectedly high separation factor of alpha = 10.6 at 26 degrees C. Using the concept of the retention increment R', non-enantioselective and enantioselective contributions to retention were separated and thus reliable thermodynamic parameters of enantioselectivity, i.e. - deltaS,R(deltaG) = 5.7 (0.05) kJ/mol at 303 K, - deltaS,R(deltaH) = 20.1 (0.64) kJ/mol, deltaS,R(deltaS) = -47.4 (2.0) J/K mol and T(isoenant) = 424 (30) K or approximately 150 degrees C, were determined by temperature-dependent measurements. The enantiomeric bias represents the largest values ever measured in enantioselective gas chromatography. An equation is presented which allows calculation of the non-enantioselective contributions to retention from measurements at two arbitrary concentrations of Lipodex E in polysiloxane. Surprisingly, the enantioselectivity is greatly reduced when employing the beta-cyclodextrin analogue and breaks down completely with the alpha-cyclodextrin analogue of Lipodex E.

Chiral separation of gamma-butyrolactone derivatives by gas chromatography on 2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl-beta-cyclodextrin

The chiral GC separation of 2-alkyl-2-keto-gamma-butyrolactone derivatives and their alcohol analogs using 2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl-beta-cyclodextrin (DIMETBCD) as chiral selector was discussed. The results, supported by the ketone preliminary molecular modelling calculations, suggest that the chiral recognition for DIMETBCD depends more on the geometry than on the polarity of the alkyl substituents on the butyrolactones. Hydrogen bonds and alkyl group steric effects should be an important function of the alcohol chiral recognition for DIMETBCD. Comparison of the retention times of the alcohol derivatives, in achiral and chiral stationary phases, suggests a specific structural effect for the cyclodextrin selctor.

Separation of enantiomers by gas chromatography

The separation of enantiomers by gas chromatography is performed on chiral stationary phases (CSPs) via hydrogen bonding, coordination and inclusion. Thus, typical chiral selectors are amino acid derivatives, terpene-derived metal coordination compounds and modified cyclodextrins. In Chirasil-type stationary phases the chiral selector is anchored to a polysiloxane backbone improving gas chromatographic performance. The present review article describes the state-of-the-art, scope and limitations, applications and mechanistic considerations at the advent of the millennium incorporating 16 figures and 168 references.

Consequences of variable purity of heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin determined by liquid chromatography-mass spectrometry on the enantioselective separation of polychlorinated compounds

The composition of 10 batches of heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (PM-CD) from different suppliers was determined by high-performance liquid chromatography combined with atmospheric pressure chemical ionisation mass spectrometry (MS). Considerable differences were found. Some batches consisted of more than 95% pure PM-CD, whereas others were not completely derivatised or contained a significant amount of by-products. Some suggestions about the structures of these impurities are given though neither nuclear magnetic resonance spectroscopy nor MS-MS investigations could completely reveal their nature. Capillaries for high-resolution gas chromatography were coated with the batches of most differing composition. They demonstrated widely varying column performance and separation properties for selected chiral polychlorinated substances such as chlordane compounds, o,p'-DDT, o,p'-DDD, alpha-hexachlorocyclohexane and atropisomeric polychlorinated biphenyls. The best enantioselectivity was obtained with the purest PM-CD. Compared to separations reported in the literature, a broader enantioselectivity was observed and also trans-heptachlor epoxide and oxychlordane could be resolved into enantiomers.

Dysprosium(III)-diethylenetriaminepentaacetate complexes of aminocyclodextrins as chiral NMR shift reagents

A metal chelating ligand is bonded to alpha-, beta-, and gamma-cyclodextrin by the reaction of diethylenetraminepentaacetic dianhydride with the corresponding 6-mono- and 2-mono(amine)cyclodextrin. Adding Dy(III) to the cyclodextrin derivatives causes shifts in the (1)H-NMR spectra of substrates such as propranolol, tryptophan, aspartame, carbinoxamine, pheniramine, doxylamine, and 1-anilino-8-naphthalenesulfonate. The Dy(III)-induced shifts enhance the enantiomeric resolution in the NMR spectra of several substrates. Enhancements in enantiomeric resolution using cyclodextrin derivatives with the amine tether are compared to previously described compounds in which the chelating ligand is attached through an ethylenediamine tether. In general, the Dy(III) complex of the 6-beta-derivative with the amine tether is a more effective chiral resolving agent than the complex with the ethylenediamine tether. The opposite trend is observed with the 2-beta-derivatives. The presence of the chelating ligand in the 2-beta-derivative hinders certain substrates from entering the cavity. For cationic substrates, evidence suggests that a cooperative association involving inclusion in the cavity and association with the Dy(III) unit occurs. Enhancements in enantiomeric resolution in the spectrum of tryptophan are greater for the secondary alpha- and gamma-derivatives than the beta-derivative.

Enantiomer separation of acidic racemates by capillary electrophoresis using cationic and amphoteric beta-cyclodextrins as chiral selectors

Enantiomer separations of various acidic racemates were performed by capillary electrophoresis (CE) using a commercial cationic beta-cyclodextrin (beta-CD), quaternary ammonium beta-CD (QA-beta-CD), and a commercial amphoteric beta-CD (AM-beta-CD) as chiral selectors. Eleven acidic racemates were successfully separated using QA-beta-CD by changing the CD concentration and the buffer pH. These enantiomer separations were compared with the results using five neutral CD derivatives. Although most racemates were separated with some of the neutral CDs, relatively high CD concentrations were required to obtain baseline separations. In contrast, when QA-beta-CD was employed, the enantiomer separations were successful at low concentrations below 5 mM. Enantiomers of five acidic racemates and ten dansylated amino acids (Dns-amino acids) were separated using AM-beta-CD. Although the baseline separation of racemic 4-chloromandelic acid was not achieved with either QA-beta-CD or five neutral CDs, AM-beta-CD showed complete resolution. Furthermore, the simultaneous enantiomer separation of eight Dns-amino acids was also achieved with AM-beta-CD. Both QA-beta-CD and AM-beta-CD were analyzed by CE and mass spectrometry (MS) in order to identify their compositions because they consisted of a mixture having different degrees of substitution. QA-beta-CD consisted of six components having from one to six quaternary ammonium groups. The composition of AM-beta-CD, however, was very complicated and could not be identified.

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.

Investigation of the conformational behaviour of permethylated cyclodextrins by molecular modelling

Conformations of manually built native and permethylated alpha-, beta-, and gamma-cyclodextrins (CD) were investigated using various computer assisted molecular modelling methods. Calculations were carried by applying the MM+ and the Tripos force field. The influences of atomic charges on the macrocyclic conformations during the optimization procedure were analyzed. The permethylation of hydroxyl groups of cyclodextrins changes bond and torsion angles between the glucose monomers and of the primary substituents. A method to determine the diameters of the cyclodextrin cavity by a modelling approach is described. It is shown that due to permethylation the larger cavity opening is increased and the primary substituents are canted outwards. As a consequence, the torus shape of the molecule changes, which is an important feature for docking and fitting studies.