Detection and Separation of Free Amino Acid Enantiomers by Capillary Electrophoresis with a Chiral Crown Ether and Indirect Photometric Detection

18-Crown-6 tetracarboxylic acid (18C6H₄) has been successfully used as a chiral selector for capillary electrophoretic (CE), high-performance liquid chromatographic (HPLC), and gas chromatographic (GC) separation of the enantiomers of DL-amino compounds. We have previously used X-ray crystallographic analysis and HPLC with an immobilized 18C6H₄ chiral stationary phase to study chiral recognition by 18C6H₄ of several DL amino acids (DL-AA). In this study CE was used for chiral recognition of several DL-AA in electrolyte solution containing 18C6H₄, in which the analyte (D or L amino acid) interacts freely. Among 14 DL-AA investigated, the enantiomers of nine (Glu, Ile, Met, PheG, Phe, Ser, Tyr, Val, and Thr) were successfully recognized in 4-15 mM 18C6H₄. Indirect photometric detection with a cationic dye, chrysoidine, was used to monitor non-chromophoric DL-AA. Among nine successfully recognized DL-AA, the D forms of Ser, Thr and Met migrated faster than the corresponding L forms. The strengths of interactions predicted from the order of migration of each enantiomer in CE were different from those in HPLC analysis. The different enantiomer recognition probably can be ascribed to the difference between CE in which the selector is not immobilized and HPLC in which the selector is immobilized by means of a spacer.