INTRINSIC SELECTIVITY IN CAPILLARY ELECTROPHORESIS FOR CHIRAL SEPARATIONS WITH DUAL CYCLODEXTRIN SYSTEMS

Defined as the ratio of the affinity factors of the analytes for a com plexing agent, the intrinsic selectivity is representative of the very nature of the complexing agent, When more than one complexing agent a re present in the background electrolyte, it is possible to define sev eral intrinsic selectivities according to whether complexing agents ar e considered separately or all together, A theoretical model with resp ect to selectivity is presented for separations that involve two compl exing agents, using the concept of apparent constant for complex forma tion, When only independent complexation occurs (absence of mixed comp lexes), then the intrinsic selectivity of a complexing agent X in the presence of a complexing agent Y can be easily related to the intrinsi c selectivity of each complexing agent and to complex formation consta nts. Dual systems of cyclodextrins (CDs), implementing the cationic mo no(6-amino-6-deoxy)-beta-cyclodextrin (beta-CD-NH2) and a neutral CD ( trimethyl-beta-CD (TM-beta-CD) or dimethyl-beta-CD (DM-beta-CD)), were studied to illustrate this model and to offer an alternative to the s eparation of neutral enantiomers when beta-CD-NH2 shows no or insuffic ient stereoselectivity, With a dual beta-CD-NH2/TM-beta-CD system at p H 2.3, arylpropionic acid enantiomers were baseline resolved and benzo in derivatives were partially resolved. For the arylpropionic acids, b eta-CD-NH2, which is not stereoselective, confers on them a nonzero mo bility, while TM-beta-CD allows the chiral recognition. A study of the respective influence of TM-beta-CD and beta-CD-NH2 concentrations was performed to determine the optimal conditions with respect to resolut ion, This theoretical approach allowed characterization of the intrins ic selectivity of neutral CDs for pairs of neutral enantiomers and the refore identification of the potential of neutral chiral agents for ne utral enantiomers.