Separation of Tryptophan Enantiomers by Ligand‐Exchange Chromatography With Novel Chiral Ionic Liquids Ligand

Chiral ionic liquids (CILs) with amino acids as cations have been applied as novel chiral ligands coordinated with Cu 2+ to separate tryptophan enantiomers in ligand exchange chromatography. Four kinds of amino acid ionic liquids, including [L‐Pro][CF 3 COO], [L‐Pro][NO 3 ], [L‐Pro] 2 [SO 4 ], and [L‐Phe][CF 3 COO] were successfully synthesized and used for separation of tryptophan enantiomers. To optimize the separation conditions, [L‐Pro][CF 3 COO] was selected as the model ligand. Some factors influencing the efficiency of chiral separation, such as copper ion concentration, CILs concentration, methanol ratio (methanol/H 2 O, v/v), and pH, were investigated. The obtained optimal separation conditions were as follows: 8.0 mmol/L Cu(OAc) 2 , 4.0 mmol/L [L‐Pro][CF 3 COO] ,and 20% (v/v) methanol at pH 3.6. Under the optimum conditions, acceptable enantioseparation of tryptophan enantiomers could be observed with a resolution of 1.89. The results demonstrate the good applicability of CILs with amino acids as cations for chiral separation. Furthermore, a comparative study was also conducted for exploring the mechanism of the CILs as new ligands in ligand exchange chromatography. Chirality 26:160–165, 2014 . © 2014 Wiley Periodicals, Inc.