High enantiomeric excess of the flavor relevant 4-alkyl-branched Fatty acids in milk fat and subcutaneous adipose tissue of sheep and goat

Volatile 4-alkyl-branched fatty acids are characteristic flavor compounds of sheep and goat. Due to the methyl branch, the carbon C-4 represents a stereogenic center with the possible presence of one or both enantiomers in the respective samples. In this study, we used enantioselective gas chromatography to study the enantiomeric composition of 4-methyloctanoic acid (4-Me-8:0) and 4-ethyloctanoic acid (4-Et-8:0) in milk and dairy products from sheep and goat as well as in goat subcutaneous tissue. Different columns coated with modified cyclodextrins were tested to resolve racemic 4-alkyl-branched fatty acid methyl ester standards. The best enantiomer resolution was obtained on 25% octakis(2,3,6-tri-O-ethyl)-γ-cyclodextrin (γ-TECD) diluted in OV-1701. For analysis of the food samples, the lipids were extracted and fatty acids in the extracts were converted into fatty acid methyl esters. Non-aqueous reversed-phase high-performance liquid chromatography was used to fractionate the samples in order to gain one solution enriched in 4-Me-8:0 methyl ester and one solution enriched with 4-Et-8:0 methyl ester. Subsequent analysis by enantioselective gas chromatography with mass spectrometry allowed only the detection of one enantiomer of 4-Me-8:0 and 4-Et-8:0 in the samples. By means of a non-racemic standard of 4-Me-8:0, it was found that the predominant enantiomer was (R)-4-Me-8:0.

Supramolecular systems-based extraction-separation techniques coupled to mass spectrometry

The combination of supramolecular chemistry and MS has not only been fruitful in the field of gas-phase fundamental studies of host-guest complexes and supramolecular assemblies. Mass spectrometric analysis has also benefited from the ability of supramolecular systems to behave as pseudophases in which solutes partition from the bulk solvent phase. Supramolecular systems-based extraction and concentration schemes and separation techniques have been widely used in different fields of analytical chemistry and are ideally suited for coupling with MS. This review describes the present status of the application of supramolecular chemistry in mass spectrometric analysis and includes topics such as the use of coacervative liquid-liquid extraction and hemimicelle/admicelle-based SPE of organic compounds prior to chromatography and electrophoresis. It also discusses the recent advances in enantioselective analysis using CD in electrophoresis- and chromatography-MS. The potential and analytical challenges of these approaches in environmental and bioanalytical chemistry, where one can expect significant developments in the future, are outlined.

Thermodynamic study of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated β-cyclodextrin stationary phase

Thermodynamic studies were performed on 12 pairs of N-trifluoroacetyl-O-alkyl nipecotic acid ester enantiomers on diluted permethylated beta-cyclodextrin stationary phase (CP Chirasil-Dex CB). The influence of ester alkyl group structure on interaction with permethylated beta-cyclodextrin (Me-CD) and enantioselectivity was studied. The types of alkyl groups studied included n-alkyl (C1-C5) and groups containing branching at differing locations relative to the chiral center of the molecule. The results show that for a given molecular weight, the n-alkyl esters have stronger interactions with Me-CD than esters containing branched alkyl groups. However, although having weaker interactions with Me-CD, esters containing alpha-branched alkyl groups exhibit higher enantioselectivity than the corresponding n-alkyl or beta-branched isobutyl esters. From the retention data, thermodynamic parameters were estimated using the retention increment method and enthalpy-entropy compensation plots (ln R' versus deltaH) were constructed. The results suggest that ester enantiomers with branching at the alpha-carbon of the ester alkyl group have additional and/or different types of enantioselective interactions with Me-CD than the C1-C5 n-alkyl esters or beta-branched isobutyl ester. In order to obtain a qualitative sense of the interaction with Me-CD, structures of the diastereomeric complexes formed between Me-CD and some of the ester enantiomers were modeled using simulated annealing molecular dynamics.

Probability rule for chiral recognition

Molecular Chirality is of central interest in biological studies because enantiomeric compounds, while indistinguishable by most inanimate systems, show profoundly different properties in biochemical environments. Enantioselective separation methods, based on the differential recognition of two optical isomers by a chiral selector, have been amply documented. Also, great effort has been directed towards a theoretical understanding of the fundamental mechanisms underlying the chiral recognition process. Here we report a comprehensive data examination of enantio separation measurements for over 72000 chiral selector-select and pairs from the chiral selection compendium CHIRBASE. The distribution of alpha = k'(D)/k'(L) values was found to follow a power law, equivalent to an exponential decay for chiral differential free energies. This observation is experimentally relevant in terms of the number of different individual or combinatorial selectors that need to be screened in order to observe alpha values higher than a preset minimum. A string model for enantiorecognition (SMED) formalism is proposed to account for this observation on the basis of an extended Ogston three-point interaction model. Partially overlapping molecular interaction domains are analyzed in terms of a string complementarity model for ligand-receptor complementarity. The results suggest that chiral selection statistics may be interpreted in terms of more general concepts related to biomolecular recognition.

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.

Chiral gas chromatographic separation of 2-oxabicyclo[3.3.0]octane derivatives and their synthetic precursors

Chiral GC separation of (+/-)-2-allyl-2-carboethoxycyclopentanone (9) and the alcohols (+/-)-3-(hydroxymethyl)-5-carboethoxy-2-oxabicyclo[3.3.0]octane (7), (+/-)-2-allyl-2-carboethoxycyclopentanol (8), and their acetylated and trifluoroacetylated derivatives were investigated on three derivatized beta-cyclodextrins (CDs) diluted in SE-54 or 1701-OH: 2,3,6-tri-O-methyl-beta-CD (PMCD); 2,3-di-O-methyl-6-O-(tert-butyldimethylsilyl)-beta-CD (DIMETBCD); 2,3-di-O-acetyl-6-O-(tert-butyldimethylsilyl)-beta-CD (DIACTBCD). The understanding of these chiral separations is extremely relevant, since cyclopentanic and bicyclic cyclopentanic rings are common structural features of many important natural products and new pharmaceutical drugs. In general DIMETBCD diluted in SE-54 showed the best chiral resolution to alcohols 7 and 8 and only DIACTBCD showed enantioselectivity to 9. Hydrogen bonds prediction and dipole moments data were obtained by molecular modeling calculations for 7ab and 8ab and Ac and TFA derivatives. Comparison of these data with the chromatographic parameters for the related compounds were used to explain the differences of their elution orders and diastereo- and enantiomeric separations on the above chiral stationary phases (CSPs). The results suggest that the CSPs enantioselectivities are not affected by the carboethoxy-functionalized cyclopentanic and bicyclic cyclopentanic rings themselves but mainly by the functional group on the other stereogenic center.

Structure elucidation, enantioselective analysis, and biogenesis of nerol oxide in Pelargonium species

A novel enantioselective synthesis of nerol oxide (3, 6-dihydro-4-methyl-2-(2-methyl-1-propenyl)-2H-pyran) was used for the determination of the absolute configuration at C-2. The order of elution of the enantiomers on octakis-(2, 3-di-O-butyryl-6-O-tert-butyldimethylsilyl)-gamma-cyclodextrin in OV 1701-vi as the chiral stationary phase in enantioselective GC was determined as (2R) before (2S). Enantioselective multidimensional GC/MS (enantio-MDGC/MS) was used for the determination of the enantiomeric ratios of nerol oxide in different geranium oils. As a result, in all investigated oils nerol oxide occurs as a racemate. The biogenesis of nerol oxide in Pelargonium species was investigated by feeding experiments using deuterium-labeled neryl glucoside as the precursor. The Pelargonium plants were able to convert the fed precursor into racemic nerol oxide, which has to be considered as a "natural racemate".

Gas chromatography

This review of the fundamental developments in gas chromatography (GC) includes articles published from 1996 and 1997 and an occasional citation prior to 1996. The literature was reviewed principally using CA Selects for Gas Chromatography from Chemical Abstracts Service, and some significant articles from late 1997 may be missing from the review. In addition, the online SciSearch Database (Institute for Scientific Information) capability was used to abstract review articles or books. As with the prior recent reviews, emphasis has been given to the identification and discussion of selected developments, rather than a presentation of a comprehensive literature search, now available widely through computer-based resources. During the last two years, several themes emerged from a review of the literature. Multidimensional gas chromatography has undergone transformation encompassing a broad range of activity, including attempts to establish methods using chromatographic principles rather than a totally empirical approach. Another trend noted was a comparatively large effort in chromatographic theory through modeling efforts; these presumably became resurgent with inexpensive and powerful computing tools. Finally, an impressive level of activity was noted through the themes highlighted in this review, and this was particularly true with detectors and field instruments.

Modified cyclodextrins as chiral selectors: molecular modelling investigations on the enantioselective binding properties of heptakis(2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl)-beta-cyclodextri n

Molecular modelling methods have been used to investigate the enantioselective binding properties of chiral dihydrofuranones on heptakis(2,3-di-O-methyl-6-O-tert.-butyldimethylsilyl)-beta-cyclod extrin in capillary gas chromatography. A conformational analysis of the modified beta-cyclodextrin was performed using annealed molecular dynamics. With the program GRID the molecular interaction potential for each of the received energetically reasonable structures of the beta-cyclodextrin and the dihydrofuranones was evaluated using different probe groups. The results of these computations have been used as starting points for constructing geometrically reasonable host-guest complexes between the beta-cyclodextrin and the dihydrofuranones. The subsequently performed molecular dynamics simulations yielded different complex states reflecting the conformational flexibility of the diastereomeric complexes. Considering the evaluated interaction energy between the beta-cyclodextrin and the dihydrofuranones as a measure of complex stability the results are in close agreement with the experimentally determined elution sequences. The methodology for the construction of the interaction model used in this study is capable of simulating the experimental data. We believe that it may serve as a basis for predictions of hitherto unknown elution sequences at modified cyclodextrins.

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.

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.