Chiral recognition Enantiomer 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 ... 

When optically inactive polystyrene was used as adsorbent, no difference in the relative peak intensity at m/z 288 to 286 was detected. Moreover, in the resolution of (RS)-1,1 -bi-2-naphthol and (if5)-l,l,-bi-2-naphthol-rf2 on the CSP, no isotope effect was observed. These findings indicate that the difference in EI-MS spectra is due to the difference in desorption between the enantiomers from the chiral adsorbent tris(5-f uoro-2-methylphenylcarbamate). This method can be used to discriminate the chirality of other enantiomers of small molecules if they show peaks in their EI-MS spectra in the presence of chiral polymers. Similar chiral recognition was detected by negative ion fast-atom bombardment mass spectrometry [34],... 

Fig.4 The principle of chiral recognition by gas sensors chiral discrimination by preferential sorption of the enantiomers of N-TFA-alanine methyl ester (N-TFA-Ala-OMe) into enantioselective (R)- and (S)-octyl-Chirasil-Val polymers. The chemical information is transformed into optical signals by the respective transducers...

A significant ability to discriminate between chiral amines based on the quenching of S-di-2-naphthylprolinol fluorescence emission was reported by Diamond et al. [32], fl-Phenylethylamine (PEA) was seen to have a much greater efficiency as a quencher than the S-enantiomer. l- and D-norephedrine, which have structural conformation similarities to PEA, were also observed to have an enantiomeric selectivity. The mechanism of chiral recognition is proposed to be a combination of hydrogen bonding and 3D chirally restricted space. 

The discrimination between enantiomers ofZ-phenylalaninate was observed only using receptors 32 and 33 for the complexation. For 34, no chiral recognition of enantiomers was detected. The same transport ratios and slow transport of Z-phenylalaninate by this receptor can be explained by formation of an external complex (Z-phenylalaninate is not bound between the amino acid chains of the receptor 34 and no sandwich complex is formed). 

Lipkowitz [63] used molecular dynamics simulations to answer the following questions a) What are the intermolecular forces responsible for analyte binding to the CSP b) Where on or in the host does the analyte bind c) What are the differential interactions giving rise to chiral discrimination d) What differences do R and S-enantiomers experience in the CD cavity e) Are existing chiral recognition mechanisms valid His computational work was based on experimental separations... 

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