Reversed determination of the formation constants of 1-allyl terguride with mandelic acid optical isomers using capillary electrophoresis

Abstract

A method is presented for the accurate determination of equilibrium constants of complex formation between mandelic acid enantiomers and 1-allyl terguride. In earlier studies this ergot alkaloid has proven to be a potential chiral selector for racemic acidic compounds. In these studies, the formation constant was determined by measuring the effective mobility of some racemic acidic analytes at varying concentrations of ergot alkaloid. For these experiments, the assumption was made that the complex mobility was zero. In order to validate these data, the experimental set-up was reversed. In this new set-up, the cationic chiral selector is injected as sample, while the background electrolyte (BGE) contained either of the two optically pure mandelic acid enantiomers, in varying concentrations. For accurate determination of the effective mobility, tetrabutylammonium was used as a mobility reference and the ionic strength of the BGE was kept constant. By performing these experiments at two different pH values, it was possible to determine complex-formation constants, and chiral selectivity towards both the dissociated and the non-dissociated mandelic acid enantiomers. Results show that only the dissociated acid interacts selectively with the ergot alkaloid confirming our earlier results. In our earlier experiments we made the assumption that the non-dissociated acid does not interact with the ergot alkaloid. The experimental data obtained by the current method however, show that, although this interaction is not enantioselective, it cannot be neglected. Optically pure mandelic acid proved to be a suitable chiral selector for the separation of terguride enantiomers.