Determination of association constants between enantiomers of orciprenaline and methyl-beta-cyclodextrin as chiral selector by capillary zone electrophoresis using a partial filling technique

The principles for the determination of conditional association constants o f enantiomers by capillary zone electrophoresis employing a partial filling technique (PFT) using methyl-beta-cyclodextrin as chiral selector is prese nted. Orciprenaline was used as a model compound. Partial filling is a sepa ration technique, where different lengths of the chiral selector solution a re introduced into the capillary to a final zone length shorter than the ef fective length of the capillary, prior to application of the solutes. Lengt hening of the separation zone results in improving enantioresolution in add ition to decreasing electrophoretic mobility of the enantiomers, because of longer interaction time between the solute and chiral selector. The degree of the reduction in electromobility depends on the affinity of the solute to the chiral selector, i.e. strength of the complex formed between the sol ute and cyclodextrin. The decrease in the electrophoretic mobility with inc reasing length of the separation zone is used for determination of the asso ciation constant. The association constants of the enantiomers of orciprena line and the chiral selector were evaluated from the slope of the plot, obs erved electrophoretic mobility versus the ratio between the length of the s eparation zone and the effective length of the capillary. It was found that the association constants were independent of the chiral selector concentr ation. The mean values were 110 M-1 and 160 M-1 for respective enantiomer. Constants obtained by a conventional GE technique were in good agreement wi th those from the PFT experiments. The highest enantioselectivityy was obta ined when about 50% of the solute was distributed to the selector phase.