β-Cyclodextrin-copper (II) complex as chiral selector in capillary electrophoresis for the enantioseparation of β-blockers

The Copper (II) complex of β-cyclodextrin (β-CD), β-CDCu₂, was prepared and used as a chiral selector for the enantioseparation of 11 β-blockers by capillary electrophoresis. Resolution for 10 pairs of enantiomers was greater than 1.9 and for the four stereoisomers of labetalol (with two chiral centers), the closest two peaks had a resolution of 1.2. The resolution was superior to that obtained by β-CD itself as well as using anionic cyclodextrin, sulfated-β-cyclodextrin (S-β-CD). In order to elucidate the separation mechanism, the conditional binding constants between the enantiomers and the complex chiral selector and the elctrophoretic mobilities of the diastereomeric complexes were calculated by nonlinear regression. It was found that binding difference and mobility difference are two effects that are responsible for enantiomeric separation. When the concentration of selector is low, the differences in binding were dominant in the separation, while at high concentrations, differences in mobility were dominant in the separation. It was possible to achieve changes in migration orders for two of the drugs tested, with the remainder showing robust resolution due to compound effects of mobility and conditional formation constant. The complex chiral selector provides good separation for the enantiomers or stereoisomers of all the drugs tested which cannot be resolved by β-CD itself.