Amino Acid Enantioseparation via Chiral Ligand Exchange Phases

The separation of amino acids on chiral ligand exchange columns represents one of the earliest methods for the resolution of these compounds, both free and as derivatives (e.g. dansylated). The original work was performed by Rogozhin and Davankov using resins containing optically active bi- and trifunctional a-amino acids loaded with a metal ion such as copper(II). More recently, more efficient columns have been prepared by bonding chiral amino acid ligands to silica gel. It is also the case that by using a long-chain alkyl-substituted chiral selector such as N-decyl-1-histidine to the mobile phase a 'dynamically coated' CSP can be prepared from a normal reversed-phase column. In such cases it is still necessary to continue to supply a small amount of the chiral selector in the mobile phase to ensure that the ligand leached from the stationary phase is constantly topped up. As with ligand exchangers used as mobile phase additives, the mechanism of retention involves the formation of complexes between the ligand (generally based on l-proline), a metal ion (usually copper(II) and the amino acid itself. Separations are made using reversed-phase types of eluents. Because of the ease of use of ligand exchange chromatography with chiral mobile phases on standard reversed-phase columns, these may be more useful than dedicated stationary phases.

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