Mixtures mode-selector

Different mechanisms have been devised to achieve chiral resolution by electromigration, but often we have to deal with mixed-mode separations rather than pure processes. In any case, chiral resolution results from stereospecific interactions of a chiral selector, with the enantiomers of the compound giving rise to a difference in migration velocity between the two entities. Chirally selective ligands, such as Cu(II)-L-histidine and Cu(II)-aspartame, have been used for derivatized amino acid mixtures. 

Enantiomeric resolution can be obtained by direct and indirect methods. The derivatization of a mixture of enantiomers with an optical active agent (149,150) into diastereomers is the indirect mode. The diastereomers are separated using achiral HPLC or GC. The direct mode depends on the formation of labile diastereomers (formed by hydrogen bonding, dipole-dipole, jt-jt and/or hydrophobic interactions) between the enantiomers and chiral environment, a chiral stationary phase, with which they interact. HPLC, using chiral stationary phases and derivatization reactions (151-156), has dominated this field, but the increased selectivity and versatility of CE, as well as the ease of incorporating various chiral selectors, make it a viable alternative. The number of chiral selector... 

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