Enantiomers, displacement chromatographic separations

Since the chloroanilines are sufficiently retained (k >5) in a 10 % v/v methanol water eluent, and the Ibuprofen enantiomers are sufficiently retained in a 30 % v/v acetonitrile buffer eluent, these solvents were selected as carrier solvents for the displacement chromatographic separations. Also, these solvents were used to determine the adsorption isotherms of p-nitrophenol and 4-t-butylcyclohexanol on beta-cyclodextrin silica. The isotherms were determined from frontal chromatographic measurements as described in (56). The isotherms are shown in Figs. 7 and 8. Since both isotherms are downwardly convex, p-nitrophenol and 4-t-butylcyclohexanol might prove useful displacers for our test solutes, provided that they are more strongly adsorbed that the solutes. 

Similar displacement chromatographic separations have been obtained for other samples as well including naphthol iscmers (67), nitroaniline iscmers (67), nitrophenol iscmers (67), the cis- and trans-iscmers of 3-hexen-l-ol (68), and the enantiomers of mephobarbital (69), hexobarbital (69), dansyl leucine (69) and dansyl valine (69). Sample loadings on the 4.6 mn ID. analytical columns varied between 0.1 milligram and 60 milligram the concentration of the separated solutes in the collected fractions ranged from 0.1 nM to 10 nM. 

When substances that themselves bind to specific sites on SA are added to the mobile phase, competitive displacements, that is, a lowering of k and a, are not the only possibilities. There is also the potential for an allosteric interaction to occur in which the affinity of the protein for the solute is increased by the addition of the modifier. For example, the addition of 10 jiM (S)-WAR to the mobile phase increased the k of the S-enantiomers of lorazepam and lorazepam hemisuccinate by 4 and 72%, respectively (113). The k s of the R-enantiomers were not affected and, therefore, the observed a s increased by 5 and 76%, respectively. These results not only increased the chromatographic separation of the respective enantiomers, but also indicated that there was an allosteric interaction between WAR and S)-Iorazepam and (S)-lorazepam hemisuccinate. 

The chromatographic separation of enantiomers, often referred to as enantioseparation, has received a great deal of attention in recent years. Both liquid (LC) and gas (GC) chromatographic procedures are used. The former is extremely useful for enantioseparations because of the available variations in scale, mechanism, and technique. It has been used in enantioseparations from analytical to preparative in scale, taking advantage of various modes of diastereoisomeric interactions andusing elution and displacement techniques. All the chromatographic methods involve diastereoisomeric interactions between the enantiomers of interest and... 

Long-lived diastereomers are generated by chemical derivatization of the enantiomers with a chiral reagent. They may be separated subsequently by achiral means. Their formation energies have no relevance to their chromatographic separation it is, rather, due to the difference in their solvation energies. Differences in their shape, size, or polarity will affect the energy needed to displace solvent molecules from the stationary phase [1]. 

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