Capillary electrophoretic separation and computational modeling of inclusion complexes of β-cyclodextrin and 18-crown-6 ether with primaquine and quinocide

The antimalarial drug primaquine (PQ) and its contaminant, the positional isomer quinocide (QC) have been successfully separated using capillary electrophoresis with either β-cyclodextrin (β-CD) or 18-crown-6 ether (18C6) as chiral mobile phase additive. The interactions of the drugs with cyclodextrins and 18C6 were studied by the semiempirical method (Parametric Model 3) PM3. Theoretical calculations for the inclusion complexes of PQ and QC with α-CD, β-CD and 18C6 were performed. Data from the theoretical calculations are correlated and discussed with respect to the electrophoretic migration behavior. More stable complexes are predicted for the PQ-β-CD and PQ-18C6 complexes. The coelution of PQ and QC when α-CD was used as buffer additive can be explained by their comparable stabilities of the inclusion complex formed, while significant differences in the complexation stabilities of the drugs with β-CD is responsible for their separation. The stronger hydrogen bonding in PQ-18C6 system is responsible for the separation between PQ and QC when 18C6 was used as chiral mobile phase additive.