Enantiomeric separations in capillary electrochromatography with crown ether-capped β-cyclodextrin-bonded silica particles as chiral stationary phase

Abstract

Two novel types of crown ether capped β-cyclodextrin (β-CD) bonded silica, namely, 4'-aminobenzo-X-crown-Y (X=15, 18 and Y=5, 6, resp.) capped [3-(2-O-β-cyclodextrin)-2-hydroxypropoxyl propylsilyl-appended silica, have been prepared and used as stationary phases in capillary electrochromatography (CEC) to separate chiral compounds. The two stationary phases have a chiral selector with two recognition sites: crown ether and β-CD. They exhibit excellent enantioselectivity in CEC for a wide range of compounds. After inclusion of metal ions (Na- or K-) from the running buffer into the crown ether units, the stationary phases become positively charged and can provide extra electrostatic interaction with ionizable solutes and enhance the dipolar interaction with polar neutral solutes. This enhances the host-guest interaction with the solute and improves chiral recognition and enantioselectivity. Due to the cooperation of the anchored β-CD and the crown ether, this kind of crown ether capped β-CD bonded phase shows better enantioselectivity than either β-CD-or crown ether bonded phases only. These new types of stationary phases have good potential for fast chiral separation with CEC.