Direct chiral resolution of aliphatic α-hydroxy acids using 2-hydroxypropyl-β-cyclodextrin in capillary electrophoresis

Abstract

Chiral resolution of α-hydroxy acids (lactic acid, 2-hydroxybutyric acid, 2-hydroxy-3-methylbutyric acid, 2-hydroxyisocaproic acid) without derivatization was performed by capillary electrophoresis using 2-hydroxypropyl-β-cyclodextrin (2HP-β-CD). An increase in the bulkiness of the alkyl group in these acids enhanced the resolution. The association constants for complexes of 2HP-β-CD with the α-hydroxy acids were determined by spectrophotometric and electrophoretic methods. Good agreement was found between the two methods. An increase in the bulkiness of the alkyl group in these acids brought about an increase in the association constants. When cyclohexanol that was included in 2HP-β-CD was added to the background electrolyte, chiral separation of these compounds was completely prevented. These results suggest that α-hydroxy acids having a short alkyl chain, and also chiral compounds having aromatic rings, could be included in 2HP-β-CD to be chiroptically separated.