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

The pH-dependent reversal of the migration order in cyclodextrin (CD)-media ted capillary electrophoresis (CE) enantioseparations of dipeptides and tri peptides has been studied between pH 2.5 and 3.5 using beta-CD and several of its neutral derivatives. The occurrence of the phenomenon depended on bo th the structure of the CD and the amino acid composition and sequence of t he peptides. While an inversion was observed for several peptides when nati ve beta-CD, dimethyl-beta-cyclodextrin or trimethyl-beta-cyclodextrin were added to the run buffer, no alteration of the order occurred in the presenc e of permethyl-beta-cyclodextrin or hydroxypropyl-beta-cyclodextrin. Most p eptides that displayed a change of the migration behavior upon increasing t he buffer pH contained Phe at the C-terminus. An ionizable carboxyl group i n the peptide structure was a prerequisite. As seen with other uncommon mig ration effects in CE, the pa-dependent reversal of the migration order occu rred in the pH region of the pK(a) values of the peptide carboxyl functions . (C) 2000 Elsevier Science BN. All rights reserved.