Separation of dipeptide and tripeptide enantiomers in capillary electrophoresis using carboxymethyl-beta-cyclodextrin and succinyl-beta-cyclodextrin:Influence of the amino acid sequence, nature of the cyclodextrin and pH

The separation of the LL and oo enantiomers of dipeptides and tripeptides u sing cyclodextrins (CDs) containing carboxyl groups was investigated with r espect to the amino acid sequence of the peptides, the nature of the cyclod extrin and the buffer pH. Compared to succinyl-beta -cyclodextrin, carboxym ethyl-beta -cyclodextrin was the more universal CD for enantioseparations. Reversal of the enantiomer migration,order upon increasing the buffer pH fr om 2.5 to 3.5 was observed in some cases. As shown for Phe-Phe reversal of the migration order also occurred between pH 3.5 and 5.3. Complexation cons tants and complex mobilities change with pH as both, the charge of the pept ide and the charge of the CD vary depending on the pH. The complexation con stants and complex mobilities of the dipeptides Ala-Phe and Phe-Phe were de termined in order to explain the enantiomer migration behavior in the pH ra nge 2.5-5.3. While the complexation constants determined the migration orde r at pH 2.5 and 5.3, complex mobility had a strong influence around pH 3.5- 3.8.