2002 Volume 51 Issue 10 Pages 871-876
The enantiomer separation of 5-dimethylamino-1-naphthalene sulfonyl (Dns) derivatives of D,L-phenylalanine and D,L-isoleucine has been studied by capillary zone electrophoresis (CZE) mediated with β-cyclodextrin (β-CD) as a chiral selector. The baseline separation of the respective D and L isomers was observed at the pH region around each isoelectric point, i.e., pH 2.5-4.5, except for the isoelectric point, itself. To understand such phenomena, the solution equilibria of the acid dissociation of Dns-D,L-Phe (designated by HA for the neutral species) and the formation of inclusion complexes with β-CD were studied by CZE at 25°C and an ionic strength of 0.005 M. The electrophoretic mobility (μe) of Dns-D,L-Phe was measured as a function of the pH and of the β-CD concentration, and analyzed by a three-dimensional non-linear least-squares method (SigmaPlot®). The acid-dissociation constants of H2A+ and HA±, the formation constant of the inclusion complexes, HA± · CD and A- · CD, and the electric mobilities of H2A+, A-, and A- · CD were determined and analyzed. The high enantiomer-separation ability observed around the isoelectric point was ascribed to a large difference in the formation between D-HA± · CD and L-HA± · CD.
