Chiral Selectors in the Mobile Phase

An alternative to forming covalent derivatives is to employ chiral mobile phase additives that will act as chiral selectors interacting selectivity with the different enantiomers of the amino acids to effect a separation.

For amino acids, separation by chiral ligand exchange has been of considerable importance. In this context a chiral mobile phase can be generated by adding a chiral selector such as l-proline (or another amino acid such as l-arginine, l-histidine or substances such as N,N-di-isopropyl-1-alanine or N-(p-tol-

uenesulfonyl)-l-phenylalanine, etc.) and copper(II) ions to an aqueous/organic solvent. Factors which affect the complex formation include the metal (as indicated above, this is usually copper but zinc, nickel and mercury have also been used albeit with inferior resolution), the metal ion/ligand ratio (usually 2:1), the concentration of the metal/ligand complex and pH. For practical applications the pH of the mobile phase would normally be recommended to be in the range of 7-8 in order to be able to carry out chromatography on conventional reversed-phase columns (this pH preserves the integrity of the columns and higher pH values cause the precipitation of the copper complexes).

As well as chiral ligand exchange, some use has been made of the ability of the cyclodextrins to form inclusion complexes with amino acid derivatives. The cyclodextrins are produced by the partial degradation of starch followed by the enzymatic coupling of the glucose units into crystalline, homogeneous toroidal structures of different molecular size. The three most widely characterized are the a-, ft- and y-cyclodextrins which contain six (cyclohexamylose), seven (cyclo-heptamylose) and eight (cyclo-octamylose) glucose units, respectively. These cyclic, chiral, torus-shaped macromolecules contain the d#)-glucose residues bonded through a-(1p4) glycosidic linkages. The mouth of the torus-shaped cyclodextrin molecule has a larger circumference than at the base and is linked to secondary hydroxyl groups of the C2 and C3 atoms of each glucose unit. The cyclodextrins provide a ubiquitous means of separating enantiomers either as mobile-phase additives or when used to make chiral stationary phases (see below) and an example of this would be the use of ft-cyclodextrin as chiral mobile phase additive for the resolution of dansylated amino acids on a conventional reversed-phase column (C8).

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