Enantiomers, displacement chromatographic

For exanple, it can be seen in Fig. 3, that the highest possible k values for the phenolic solutes are lower than desirable, even in pure water as eluent. The solubility of these phenolic compounds is also low in pure water. Therefore, as a oenpremise between the opposing requirements of sufficient retention and high solubility, a carrier solution oenpositien of 10 % methanol water was selected for the further studies. Similarly, it can be seen from Fig. 5, that the k values of the Ibuprofen enantiomers are around 10 in the 30 % acetonitrile buffer eluent, therefore this conposition was selected for the carrier solution for the displacement chromatographic studies. 

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. 

Availability. Although commercially available via the degradation of pantothenic acid, (i )-pantolactone is also conveniently prepared by enantioselective reduction of its corresponding keto lactone employing homogeneous catalysis," " or by microbial methods. The (5)-enantiomer has been prepared by inversion of the natural product in 90% yield and 97% ee via triflate activation, acetate displacement, and Lithium Hydroxide hydrolysis. The enantiomers were also prepared by resolution of the race-mate with (R)- and (5)-phenethylamine. A gas chromatographic method exists for ee determination. ... 

The authors concluded that an increase in the bulk of the alcohol enhances the ability of the solute enantiomers to displace the modifier from the CSP that stabilizes the diastereomeric solute/CSP complexes. The enhanced stability of the two solute/CSP complexes magnifies the energy differences between them, resulting in an increase in the observed enan-tioselectivity. A similar effect can be produced by lowering the temperature of the chromatographic system (31). 

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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