I

Fig. 2.13 Schematic view of elliptical polarized electromagnetic radiation including parameters deducing the four Stokes parameters. The orthogonal coordinate system is formed by r and l, the distances a, a', and b are given with the angles P and y equipped with the required enantioselective instrumentation for resolving mirror image isomers. We distinguish between indirect and direct resolution of enantiomers. For indirect resolution, the enantiomer analytes (R-analyte and S-analyte) are transformed by a chemical reaction with an enantiopure reactant (e.g. R-reactant) into diastereoisomers (R,R-derivative and S,R-derivative) which can be resolved with classical achiral stationary phases. We have seen that diastereoisomers have distinguishable physico-chemical properties such as boiling points, etc. Therefore they are susceptible to separation on achiral stationary phases. For direct resolution of mirror image analytes, a chiral stationary phase is required that contains an enantiopure selector (e.g. R-selector). Here, the stereogenic centers of the chiral analyte and the chiral stationary phase have to be brought together in such a way that there is an interaction between them. Thereby, separable diastereoisomeric associates are formed (R,R-associate and S,R-associate) from inseparable enantiomers. Chromatographic separation is based on a difference in partition coefficients of the analyte between a stationary phase and a mobile phase. In enantioselective gas chromatography, the stationary phase contains enantiomerically pure compounds such as the amino acid valine or cyclodextrin molecules.

It is in particular the development of wall-coated open-tubular (WCOT) capillary columns treated with chiral selectors that allows the highly efficient resolution of enantiomers. Different chiral selectors were developed and are still in use today. I will briefly present the most representative chiral selectors for enantioselective gas chromatography, because we will need them in Chap. 9 for the profound understanding of the "chirality-module" onboard the ROSETTA Lander which is now on the way to land on the surface of a comet where it will perform an important experiment on the origin of life's asymmetry. In practice a dazzling number of different chiral stationary phases has been applied for the separation of enantiomers (Gubitz and Schmid, 2004; Meierhenrich 2004). Here, Chirasil-Val phases, cy-clodextrin phases, chemically bonded cyclodextrin phases, and acyclodextrin phases will be briefly described and illustrated.

Chirasil-Val phases: Quite often, the classical, thermally stable Chirasil-Val phases (Frank et al. 1978) are still in use. They are characterized by a high viscosity and low volatility of the organic polymeric siloxane (Schurig 1986). These "elastic" or "movable" dimethyl polysiloxanes possess covalently bonded diamide functions as one can see in Fig. 2.14, which are able to perform hydrogen bridge, dipole-dipole, as well as van der Waals bonds with the chiral analyte. One or two "arms" of the polymer are able to form an associative complex with the specific

Fig. 2.14 Chemical structure of the Chirasil-Val polymer used as liquid film attached to the inner wall of a capillary column. The amino acid valine incorporated into the diamide chiral arm (side chain) of the dimethyl polysiloxane can be used in its l- or d-configuration resulting in Chirasil-l-Val and Chirasil-d-Val phases, respectively. Chirasil-l-Val and Chirasil-d-Val phases are commercially available10

Fig. 2.14 Chemical structure of the Chirasil-Val polymer used as liquid film attached to the inner wall of a capillary column. The amino acid valine incorporated into the diamide chiral arm (side chain) of the dimethyl polysiloxane can be used in its l- or d-configuration resulting in Chirasil-l-Val and Chirasil-d-Val phases, respectively. Chirasil-l-Val and Chirasil-d-Val phases are commercially available10

10 In enantioselective chromatography often both l-Val and d-Val phases are used in order to change the elution order of enantiomers. It is noteworthy that the l-Val phase is on the market with a layer thickness of 0.12 |m, whereas the commercially available d-Val phase shows a layer thickness of 0.08 |m only. As a consequence, for specific chiral analytes different separation factors and resolutions are observed.

enantiomer (Bayer 1983) in order to hold back one enantiomer stronger (and longer) than the other.

Chiral stationary phases with polymeric Chirasil-Val layers are often used for the resolution of amino acid enantiomers because amino acids interact perfectly with the chiral diamide side chain. Figure 2.15 gives an example of the amino acid leucine, base-line separated into its enantiomers. Consequently, Chirasil-Val phases are often applied in experiments for the study of various amino acid pools considered as "prebiotic".

Cyclodextrin phases: The classical and most commonly used chiral selectors are cyclodextrins (CDs), i.e., namely, cyclic oligosaccharides, which have been applied for 50 years for separating racemates. Since then, these selectors have made possible more than 1 000 chiral resolutions (Armstrong and Jin 1990; Armstrong et al. 1990; Schurig and Jakubetz 1997).

CDs are glycosidially bonded cyclic oligosaccharides of 6,7 or 8 D-glucose-units called a-CD, P-CD or y-CD, respectively.11 The ring size of the CD molecule can be adapted for optimal resolution of chiral analytes. Typically, the space within an a-CD can accommodate a C6 arene ring, a P-CD can admit molecules as big as naphthalene, while y-CD can include anthracene or phenanthrene molecules. Furthermore, CDs can be chemically modified at the big entrance site in carbon position number 2 and 3 and alternatively at the small entrance site in position 6. Fig. 2.16 depicts the chemical structure of a P-cyclodextrin molecule.

Fig. 2.15 Typical baseline resolution of the d- and l-leucine enantiomers in form of ethoxy carbonyl ethyl ester (ECEE) derivatives (Abe et al. 1996) on 25 m Chirasil-l-Val installed in an Agilent 6890/5973 GC-MSD system. Three overlaid chromatograms given in blue, red, and black color are shown. Splitless injection, oven temperature programmed to 3 min at 70°C, heated by 5°C/min to 155°C where it was kept constant for 5 min. Helium was used as a carrier gas with a constant flow of 1.5 mL/min

11 Properties of S-CD structures with 9 d-glucose-units have not yet been described in the literature. A retrosynthesis of these molecules would provide additional insight into chiral host-guest complexes and might challenge organic chemists.

Fig. 2.16 Chemical structure of a P-cyclodextrin ring typically used for enantioselective gas chro-matography. Here, cyclodextrins are dispersion-like embedded in the stationary phase. Highly enantioselective cyclodextrins can be created by changing the chemical function of the side chain. R = Methyl, permethylated cyclodextrin; R = S-2-hydroxypropyl cyclodextrin;commercial Astec Chiraldex B-PH cyclodextrin phase of Sigma-Aldrich. Alternatively, the Astec Chiraldex G-TA phase incorporates a 2,6-di-O-pentyl-3-trifluoroacetyl derivative of y-cyclodextrin and the B-DA Chiraldex consists of a 2,6-di-O-pentyl-3-methoxy derivative of P-cyclodextrin. These columns are commercially available at the Sigma-Aldrich Co. who recently purchased Astec Inc.

Fig. 2.16 Chemical structure of a P-cyclodextrin ring typically used for enantioselective gas chro-matography. Here, cyclodextrins are dispersion-like embedded in the stationary phase. Highly enantioselective cyclodextrins can be created by changing the chemical function of the side chain. R = Methyl, permethylated cyclodextrin; R = S-2-hydroxypropyl cyclodextrin;commercial Astec Chiraldex B-PH cyclodextrin phase of Sigma-Aldrich. Alternatively, the Astec Chiraldex G-TA phase incorporates a 2,6-di-O-pentyl-3-trifluoroacetyl derivative of y-cyclodextrin and the B-DA Chiraldex consists of a 2,6-di-O-pentyl-3-methoxy derivative of P-cyclodextrin. These columns are commercially available at the Sigma-Aldrich Co. who recently purchased Astec Inc.

Current research still tries to discover suitable substituents in order to modify CDs in these three positions (see e.g. Tisse et al. 2006). In an innovative manner, the research team of Jean-Philippe Bouillon at the University of Rouen actually seeks to create a hybrid between a Chirasil-Val phase and a cyclodextrin phase by attaching an amino acid to the CD molecule in position 6 in order to obtain highest enantiomeric resolution for a wide range of chiral analytes. Results are not published yet.

CDs are produced by partial degradation of natural starch. Consequently, they exist only in one-handed form. Synthetic chemistry has not yet been able to ret-rosynthesize the optical antipode for implementation into a stationary phase even if it would be very useful for specific kinds of experimental applications.

Free CDs are not suitable as chiral stationary phases in capillary gas chromato-graphy: They are too polar and their melting points are too high. The introduction of specific hydrophobic groups leads to reduced melting points and better thermal stability. Stationary CD molecules are chiral and have been thought - as indicated above - to form associative complexes by inclusion (Schurig and Nowotny 1988) or other spatial arrangements (Lutz 1988) to interact with the chiral analyte.

In general, an optimum GC-separation of the enantiomers can be achieved when the stationary CD phase is able to include the chiral analytes into its cavities. By this inclusion, additional interactions between the CD molecule ("host") and the enan-tiomer ("guest") could become active that could not be formed by the conventional diamide substituted polymeric siloxane Chirasil-Val phases. The result of these additional interactions of the CD phases is - for a wide range of analytes such as amines, alcohols, and diols - a much better chiral recognition.

Chemically bonded cyclodextrin phases can be considered as the 3rd generation of chiral GC selectors. They contain permethylated P-cyclodextrin (P-CD) molecules, cyclic oligosaccharides composed of 7 D-glucose-units, chemically bonded to a polysiloxane film by octamethylene spacers which renders the column inert and robust against chemical and physical damage (Schurig et al. 1994; Jakubetz et al. 1997; Schurig and Jakubetz 1997). The chemical structure of the CD molecule anchored to the polysiloxane film is depicted in Fig. 2.17.

Linear oligodextrin phases: As outlined before, it has long been focussed on the cavities of cyclodextrin molecules contributing to the enantiomer separation of chi-ral analytes forming inclusion complexes dominated by non-polar interactions. The alkoxy groups located at the outside of the cyclodextrin torus were often ignored. These groups would respond to polar interactions with more polar analytes (Schurig et al. 1989). Surprisingly, linear oligodextrins, also entitled "acyclodextrins", were recently shown to have considerable potential as chiral selectors as well. In this case, host-guest interactions by inclusion can be excluded. Nevertheless, this 4th and most modern generation of chiral GC selectors provides good resolutions of chiral

Fig. 2.17 Permethylated P-cyclodextrin chemically bonded to the polysiloxane phase. These polymers provide excellent separation factors for chiral hydrocarbon analytes. Trade name: Chirasil-Dex CB developed by Chrompack and now commercially available via Varian Inc.

Was this article helpful?

0 0

Post a comment