pH-dependence of complexion constants and complex mobility in capillary electrophoresis separations of dipeptide enantiomers

The chiral separation of the LL- and DD-enantiomers of the dipeptides Ala-Tyr, Phe-Phe, and Asp-PheOMe has been investigated at pH 2.5 and pH 3.5 using beta-cyclodextrin (beta-CD), heptakis-(2,6-di-O-methyl)-beta-cyclodextrin, and heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin as chiral selectors. According to electrospray mass spectrometry, heptakis-(2,6-di-O-methyl)-beta-cyclodextrin was a mixture of six isomers. Reversal of the enantiomer migration order upon increasing the buffer pH from 2.5 to 3.5 was observed for all peptides with beta-cyclodextrin, for Ala-Tyr and Phe-Phe in the presence of heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin, and for Ala-Tyr using heptakis-(2,6-di-O-methyl)-beta-cyclodextrin. The migration behavior could be explained on the basis of the complexation constants and the mobilities of the peptide-cyclodextrin complexes. Both, the binding constants and complex mobilities decreased with increasing pH as the overall-charge of the peptides decreased. While the complexation constants primarily determined the migration order at pH 2.5, complex mobility dominated in most cases at pH 3.5.