Evaluation of an optically active crown ether for the chiral separation of di- and tripeptides

The direct optical resolution of a number of di- and tripeptides was achieved by capillary zone electrophoresis using an enantioselective crown ether as buffer additive. The protonated primary amines form inclusion complexes with the crown ether. Chiral resolution is based on different stability constants of the diastereomeric complexes thereby changing the electrophoretic mobilities of the enantiomers. Enantioselectivity is strongly affected by the distance between the amine functionality and the chiral carbon atom. This effect was studied using di- and tripeptides especially synthesized for this purpose. In general, baseline resolution was obtained for those peptides with the amine group located as far as four bonds from the stereogenic center. Additionally, tripeptides possessing two chiral centers were separated to investigate the potential of the chiral selector for the analysis of complex analytes with related structures. Experimental factors such as crown ether concentration, buffer pH and temperature also show a strong influence on the resolution. These factors can be successfully employed for method optimization.