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a-. (3-. and 7-cvclodextrins
Derivatized cyclodextrins Aqueous, organic silica support
Crown ethers Aqueous
Macrobiotics (e.g., vancomycin V) Aqueous
Amino acia-cu /\queous immoDiiizeci onro silica or
Charge-transfer phases Nonpolar organic polymer resins
Chiral 7r-electron acceptors Alcohols Immobilized onto silica
Chiral 77-electron donators Materials themselves
Chiral polymers (swollen microcrystalline)
Cellulose esters 'Nonpolar organic Coated (or bonded) onto
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Amylose carbamates silica
Chiral-derivatized polyacrylamides^ ę Chemically bonded to silica
Proteins \queous (or physically adsorbed)
Serum albumins (BSA, HSA)
_________crown ethers, and chiral macrocylic antibiotics are frequently used selectors in this
group. All of these offer a variety of different interactions (hydrophobic, hydrogen-bonding, dipolar interactions, and in some cases electrostatic). These stationary phases _________are operated predominantly with mixed aqueous-organic mobile phases.______________________
b. Chelate complexation involving an optically pure chiral selector, a central metal ion (most often Cu2+) and the chiral analyte in the form of a ternary complex [65,66]:
[chiral selector ligand-metal-chiral analyte].
Chiral discrimination is achieved if the two diastereomeric complexes
differ in their Gibbs energy of complex formation. Here, different binding strengths
_________(retention factors) of the analyte enantiomers with the immobilized selector are ob-
served. These stationary phases are operated with aqueous mobile phases.
c.—Charge-transfer complexes, in which charge-transfer interactions of various types are decisive for stereorecognition. These include tt-tt interactions between activated (electron-rich) and deactivated (electron-poor) aromatic rigs, hydrogen-bonding, and
---------acid-base interactions [67,68]. These interactions are emphasized in nonpolar eluents.
Therefore, the mentioned separations are carried out in nonpolar organic mobile phases, which contain a few percent of a more polar modifier to tune retention.
2. Chiral recognition using chiral polymers. These polymers are usually carbohydrate-, polyacrylate-, or polyacryamid-based, and are coated (and fixed) onto the surface of the
Retention and Selectivity
Figure 9 Schematic representation of interactions between an optically pure immobilized selector and enantiomeric analytes. Better correspondence of interaction sites results in stronger retention. Enantiorecognition requires at least three interactions with one enantiomer and a different strength of at least one interaction comparing the enantiomers.
support materials [69-71]. With some chiral or helical polymers, a surrounding of the analyte by the network, in a manner similar to an inclusion mechanism, is envisaged. In other cases, some kind of 1:1 complex between analyte molecules and the monomer units of the polymer appears to be the key to stereorecognition. Charge-transfer interactions are of significant importance, with most of these polymers, especially with those phases using substituted celluloses and amyloses. Eluents are predominantly nonpolar.
3. Chiral recognition by interactions with receptor-type sites of proteins [72,73]. Aqueous eluents are used for this.
A more detailed discussion of separation mechanisms, structures of selectors, mobile phases, separation conditions and optimization strategies is given elsewhere (see Chap. 21). The use of chiral selectors as mobile-phase additives; that is, the formation of diastereomeric complexes of the various types discussed in the foregoing (host-guest, chelate, and charge-transfer complexes) between the analytes and a sufficiently soluble selector in the mobile phase is briefly discussed in Sec. V.