Enantiomer recognition solubility

Another important issue that must be considered in the development of CSPs for preparative separations is the solubility of enantiomers in the mobile phase. For example, the mixtures of hexane and polar solvents such as tetrahydrofuran, ethyl acetate, and 2-propanol typically used for normal-phase HPLC may not dissolve enough compound to overload the column. Since the selectivity of chiral recognition is strongly mobile phase-dependent, the development and optimization of the selector must be carried out in such a solvent that is well suited for the analytes. In contrast to analytical separations, separations on process scale do not require selectivity for a broad variety of racemates, since the unit often separates only a unique mixture of enantiomers. Therefore, a very high key-and-lock type selectivity, well known in the recognition of biosystems, would be most advantageous for the separation of a specific pair of enantiomers in large-scale production. 

The chiral recognition of enantiomers can be of three types (i) desionoselective, (ii) ionoselective, or (iii) duoselective, in which only the non-dissociated, the dissociated or both forms (charged and uncharged), respectively, of the enantiomers selectively interact with the chiral selector. In the case of ionoselective and duoselective interactions, a reversal of the migration order of the enantiomers is theoretically possible by the appropriate selection of CD concentration and the pH of the BGE. The addition of organic modifier to the BGE can also change selectivity by modifying the solubility of the chiral selector and/or of the solute, the complex equilibrium, the conductivity of the BGE and the electroendos-motic flow (EOE) level. Several other factors, such as the temperature, the type and the concentration of the BGE, and the level of the EOE can influence the separation. 

The oldest example of molecular chiral recognition described by Pasteur [1] is the separation of enantiomers based on diastereoisomeric salt formation and subsequent fractionated crystallisation. The principle of the enantiomeric differentation is that one of the salts formed with a chiral reagent is less soluble than the other, and thus precipitates from the solution. This enrichment of one of the enantiomers leads to the optical resolution... 

Even nowadays, particularly in industrial processes, the separation of enantiomers of racemic acids and bases is based on this molecular chiral recognition. The less soluble, i.e. the more stable of these diastereomer salts crystallizes even if the chiral agent in the better soluble salt is replaced by an achiral reagent of similar chemical character, or eventually eliminated, or substituated by a solvent. In this case, a mixture enriched with the more stable diastereomer can be isolated by filtration from the solution of the achiral salt of the enantiomeric mixture or the free enantiomers [2,3]... 

Enantiomers have identical chemical properties in relation to their reactions with achiral reagents. Their physical properties are identical (e.g. solubility, partition coefficients, boiling points, etc.) So why the interest in enantiomer composition This arises from the fact that in a chiral environment enantiomers behave as different compounds. The natural world is constructed of chiral systems that employ structure recognition mechanisms as a regulatory function [1,4,8], The single enantiomers of racemic drugs exhibit differences in their bioavailability, distribution, metabolism, and excretion. It is often the case that one enantiomer is the more active isomer for a given... 

Since first demonstration in 1994 of the potential use of macrocyclic antibiotics as chiral selectors in analysis, glycopeptide antibiotics have been successfully applied for enantiomer separations by liquid chromatography, as recognition components of chiral stationary phases, and by capillary electrophoresis (CE) as soluble chiral selec-tors. Four chiral stationary phases for chromatography with the selectors vancomycin, ristocetin, teicoplanin, and the teicoplanin aglycone are commercialized under the trade name Chirobiotic by Astec and Supelco. Various aspects of analytical applications of glycopeptide antibiotics have been extensively covered in the recent reviews cited above. As an example. Table 2 shows some representative results for CE enantioseparations with vancomycin, ristocetin A, and teicoplanin, which were taken from Ref. 39. 

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