Stationary phases chiral discrimination

A number of specialised stationary phases have been developed for the separation of chiral compounds. They are known as chiral stationary phases (CSPs) and consist of chiral molecules, usually bonded to microparticulate silica. The mechanism by which such CSPs discriminate between enantiomers (their chiral recognition, or enantioselectivity) is a matter of some debate, but it is known that a number of competing interactions can be involved. Columns packed with CSPs have recently become available commercially. They are some three to five times more expensive than conventional hplc columns, and some types can be used only with a restricted range of mobile phases. Some examples of CSPs are given below ... 

Cavazzini, A. et al.. Study of mechanisms of chiral discrimination of amino acids and their derivatives on a teicoplanin-based chiral stationary phase, J. Chromatogr. A, 1031, 143, 2004. 

When a CSP is applied, the separation mechanism is based on the differences in the interaction between the chiral selector in the stationary phase and the enantiomers of the solute. Depending on the nature of the selector and the type of the solute, the stereoselective interaction can be based on interactions of one or more different types such as inclusion complexation, Tr-jr-interaction, dipole stacking, hydrogen bonding, electrostatic interaction, hydrophobic interaction, and steric interaction [35]. In order to obtain chiral discrimination between the enantiomers, a three-point interaction is required between at least one of the enantiomers and the CSP [36]. The interactions can be of attractive as well as repulsive nature (e.g., steric and electrostatic interactions). 

In the abovementioned hydrocarbons, acting as pheromones of certain moth species, the biologically active stereoisomers could be identified because of the bioassays with pure compounds. Nevertheless, the stereoisomeric composition of the natural products remains unknown. Very unfortunately, today there is no way to unambiguously determine the enantiomeric composition of very small amounts of mono- or dimethylalkanes containing more than 10 carbon atoms. Enantioselective GC, usually the method of choice, will not work in these cases, as chiral discrimination of the known stationary phases is too small. As a result, enantiomers will not be separated. 

Karlsson, A. et al. Enantioselective Ion-Pair Chromatography of Phenolic 2-Dipropylam-inotetrahn Derivatives on Achiral Stationary Phases an Experimental and Theoretical Study of Chiral Discrimination. Acta Chem. Scand. 1993,47,469-481. 

The True Moving Bed. The principle of a true moving bed is schematically illustrated in Figure 21-15 for the separation of a racemate on a chiral stationary phase, being the ideal problem for the separation of a binary mixture. There is countercurrent contact between the solid phase and the eluent which move in opposite directions. The racemate is injected in the middle of the column. Chiral discrimination provided for by the sorbent ... 

S. Schefzig, W. Lindner, K. B. Lipkowitz, and M. Jalaie, Enantiomeric discrimination by a quinine-based chiral stationary phase A computational study. Chirality... 

Roussel, C., Piras, P. and Heitmann, I. (1997). An Approach to Discriminating 25 Commercial Chiral Stationary Phases from Structural Data Sets Extracted from a Molecular Database. Biomedical Chromatography, 11,311-316. 

Enantiomers have identical or nearly identical physical properties unless a reagent or technique is used that is itself chiral. For example, the two enantiomers in Figure 4.2 will have the same boiling point, melting point, refractive index and density since these are bulk effects and cannot discriminate between the two enantiomers. Differences between enantiomers only become apparent when they interact with chiral reagents such as the active sites of enzymes or the chiral stationary phase of a HPLC column. 

Roussel, C., Piras, P., Heitmann, I. An approach to discriminating 25 commercial chiral stationary phases from structural data sets extracted from a molecular database, Biomed. Chromatogr., 1997, 11, 311-316. 

Kaner and coworkers84 85 have recently reported enantiomeric discrimination of amino acids by chiral PAn films. The EB form of PAn doped with (.S )-(+)- or (/ )-(—)-10-camphorsulfonic acid was used to separate racemates of DL-amino acids. The interactions of the chiral PAn films with amino acids suggested that the chiral recognition ability of dedoped PAn is size and shape dependent. These properties may be useful in the employment of chiral PAn as a chiral stationary phase for chromatography. 

---