Box 24-1 Chiral Phases for Separating Optical Isomers

Optical isomers—also called enantiomers—arc mirror image compounds that cannot be superimposed. For example, the natural amino acid building blocks of proteins are L-amino acids. 

Chromatography with a chiral (optically active) stationary phase is one of the few ways to separate enantiomers. We can estimate ages of fossils up to 500 million years old by measuring the fraction of amino acid that has transformed into the d enantiomer in a fossil.4-5 Amino acids do not have enough vapor pressure for gas chromatography. A volatile derivative suitable for gas chromatography is shown in the figure above.6 

Common chiral stationary phases for gas chromatography have cyclodextrins bonded to a conventional polysiloxane stationary phase.7-8 Cyclodextrins are naturally occurring cyclic sugars. P-Cyclodextrin has a 0.78-nm-diameter opening into a chiral, hydrophobic cavity. The hydroxyls are capped with alkyl groups to decrease the polarity of the faces.9 

Enantiomers have different affinities for the cyclodextrin cavity, so they separate as they travel through the chromatography column. The chromatogram below shows a chiral separation of a by-product found in pesticides. 

Programmed temperature (120 -200°C) chiral separation on a 0.25-mm x 25-m open tubular column with a 0.25-nm-thick stationary phase containing 10 wt% fully methylated p-cyclodextrin chemically bonded to dimethyl polysiloxane. [From W. Vetter and W. Jun, Elucidation of a Polychlorinated Bipyrrole Structure Using Enantioselective GC, Anal. Chem. 3002, 74,4287.] 

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