Chiral separation of underivatized amino acids by ligand-exchange capillary electrophoresis using a copper(II)–l-lysine complex as selector

A ligand-exchange capillary electrophoretic method was explored, with l-lysine as the ligand and copper(II) as the central ion. Its applicability was demonstrated with underivatized aromatic amino acids, namely d,l-phenylalanine, d,l-tryptophan, d,l-tyrosine and d,l-β-phenylserine. Optical resolutions of a single pair of amino acid enantiomers, and of mixed amino acids were obtained with a running buffer of 10 mM NH4Ac, 6.67 mM Cu(II) and 13.33 mM l-lysine, pH 7.0. Sodium dodecylsulfate (SDS) was shown to be necessary for simultaneous separation of the mixed amino acids. The resolution was found to increase with the concentration of the copper(II) complex at a copper(II)-to-lysine ratio of 1:2. If the total concentration of copper(II) and lysine was kept at 20 mM, decreasing the ratio of copper(II) to lysine caused a resolution loss of tryptophan, but a slight resolution improvement of the other three amino acids. The pH of buffer is another important factor controlling the separations. For all the studied amino acids, the optimum pH was 6.0. An interesting phenomenon was observed in this study. SDS induces precipitation at a concentration below 32 mM at room temperature (16±2 °C), possibly due to the formation of neutral substance from the SDS monomer and the copper(II)–lysine complex.