Influence of the structure of cyclodextrins and amino acid sequence of dipeptides and tripeptides on the pH-dependent reversal of the migration order in capillary electrophoresis

The pH-dependent reversal of the migration order in cyclodextrin (CD)-mediated capillary electrophoresis (CE) enantioseparations of dipeptides and tripeptides has been studied between pH 2.5 and 3.5 using β-CD and several of its neutral derivatives. The occurrence of the phenomenon depended on both the structure of the CD and the amino acid composition and sequence of the peptides. While an inversion was observed for several peptides when native β-CD, dimethyl-β-cyclodextrin or trimethyl-β-cyclodextrin were added to the run buffer, no alteration of the order occurred in the presence of permethyl-β-cyclodextrin or hydroxypropyl-β-cyclodextrin. Most peptides that displayed a change of the migration behavior upon increasing the buffer pH contained Phe at the C-terminus. An ionizable carboxyl group in the peptide structure was a prerequisite. As seen with other uncommon migration effects in CE, the pH-dependent reversal of the migration order occurred in the pH region of the pKa values of the peptide carboxyl functions.