Enantioseparation of benzoins and enantiomer migration reversal of hydrobenzoin in capillary zone electrophoresis with dual cyclodextrin systems and borate complexation

Enantioseparations of racemic hydrobenzoin and structurally related compounds, including benzoin and benzoin methyl ether, in capillary zone electrophoresis (CZE) with dual cyclodextrin (CD) systems consisting of S-β-CD (mixed isomers) and a neutral CD, including β-CD and hydroxypropyl-β-CD (HP-β-CD), as chiral selectors in the presence of borate complexation at pH 9.0 were investigated. Effective enantioseparations of hydrobenzoin were achieved with addition of dual CD systems and also with neutral CDs in a borate buffer. The enantioseparation and migration behavior of hydrobenzoin in such an electrophoretic system are primarily governed by the interaction of the borate complex of hydrobenzoin with β-CDs. The CD complexations of both hydrobenzoin and the borate complexes of hydrobenzoin with β-CDs increase in the order S-β-CD<HP-β-CD<β-CD. As a result, enantioseparations of hydrobenzoin with the use of dual CD systems consisting of S-β-CD/β-CD and S-β-CD/HP-β-CD as chiral selectors are more advantageous than that with the use of S-β-CD alone. With these dual CD systems in the presence of borate complexation, the enantiomer migration reversal was observed for hydrobenzoin. The interactions of hydrobenzoin with neutral CDs and with S-β-CD exhibit the same chiral recognition pattern, but opposite effect on the mobility of the enantiomers. The (S,S)-enantiomer of hydrobenzoin was found to interact more strongly than the (R,R)-enantiomer with neutral CDs. For comparison, enantioseparation of hydrobenzoin, together with benzoin and benzoin methyl ether, with dual CD systems in a phosphate background electrolyte at pH 9.0 was also examined. The migration order and enantioselectivity of these three benzoins depend on the degree of CD complexations between benzoins and both S-β-CD and neutral CD in a phosphate background electrolyte. In addition, effective enantioseparations of hydrobenzoin were also achievable with addition of either β-CD at concentrations greater than 1.0 mM or HP-β-CD at concentrations exceeding 2.0 mM in a borate buffer at pH 9.0.