Ionic liquids with amino acids as cations: Novel chiral ligands in chiral ligand-exchange capillary electrophoresis

Ionic liquids (ILs) with l-proline (l-Pro) as cations have been developed for the novel chiral ligands coordinated with Cu(II) in chiral ligand exchange capillary electrophoresis (CLE-CE). Four kinds of amino acid ionic liquids (AAILs), including [l-Pro][CF3COO], [l-Pro][NO3], [l-Pro][BF4] and [l-Pro2][SO4], were successfully synthesized. Among them, [l-Pro][CF3COO] was selected as the model ligand to optimize the separation conditions. The influences of AAIL concentration, pH, and methanol concentration on efficiency of chiral separation were investigated. Then it has been testified that the optimal buffer solution consisted of 25.0 mM Cu(Ac)2, 50.0 mM AAIL and 20% (v/v) methanol at pH 4.0. The interesting thing is well enantioresolution could be observed with [l-Pro][CF3COO] as the new chiral ligand and nine pairs of labeled d,l-AAs were successfully separated with the resolution ranging from 0.93 to 6.72. Meanwhile, the baseline separation of labeled d,l-AAs could be achieved with the other three kinds of AAILs as ligands. The results have demonstrated the good applicability of AAILs with AAs as cations for chiral separation in CLE-CE system. In addition, comparative study was also conducted for exploring the mechanism of the AAILs as new ligands in CLE-CE.