Enantioseparation of aromatic α-hydroxycarboxylic acids: The application of a dinuclear Cu2(II)-β-cyclodextrin complex as a chiral selector in high speed counter-current chromatography compared with native β-cyclodextrin

The use of a dinuclear Cu-2(II)-beta-cyclodextrin complex (Cu-2-beta-CyD) as a chiral selector (CS) was successfully applied to chiral high speed counter-current chromatography (HSCCC) for the enantioseparation of aromatic alpha-hydroxycarboxylic acids. alpha-cyclohexylmandelic acid (CHMA) as a case was studied to optimize the separation temperature, the pH value of the aqueous phase, and the concentration of the chiral selector, which were comparatively studied between Cu-2-beta-CyD and native beta-cyclodextrin (beta-CyD) as CSs. The thermodynamic parameters of the formation of the complex between the enantiomers and the CSs were obtained, and the complex formation constants between the enantiomers and Cu-2-beta-CyD or beta-CyD were determined using induced circular dichroism technology. The results indicate that Cu-2-beta-CyD can remarkably improve the enantioseparation compared with beta-CyD, mainly due to the ion-pairing interactions between the deprotonation of carboxyl group of CHMA and the Cu(II) ion of Cu-2-beta-CyD. Five of other aromatic alpha-hydroxycarboxylic acids were also enantioseparated successfully by HSCCC using Cu-2-beta-CyD as a CS. (C) 2014 Elsevier B.V. All rights reserved.