Migration order of dipeptide and tripeptide enantiomers in the presence of single isomer and randomly sulfated cyclodextrins as a function of pH

The present study was conducted in order to evaluate the cyclodextrin (CD)-mediated chiral separation of peptide enantiomers as uncharged analytes at pH 5.3 using randomly sulfated β-cyclodextrin, heptakis-6-sulfato-β-CD and heptakis-(2,3-diacetyl-6-sulfato -β-CD as chiral selectors. Although less effective compared to stronger acidic conditions, the CDs proved to be suitable chiral selectors for the present set of peptides at pH 5.3. The carrier ability of the negatively charged CDs upon reversal of the applied voltage may also be exploited leading to a reversal of the migration order. In addition, reversal of the enantiomer migration order upon increasing the buffer pH from 2.5 to 5.3 was also observed for Ala-Tyr in the presence of randomly sulfated β-CD, for Ala-Phe, Ala-Tyr, Phe-Phe, Asp-PheNH₂ and Gly-Ala-Phe in the presence of heptakis-6-sulfato-β-CD, and for Phe-Phe and Ala-Leu in the presence of heptakis-(2,3-diacetyl-6-sulfato)-β-CD. The migration behavior could be explained on the basis of the complexation constants and the mobilities of the peptide-CD complexes. While a change in the affinity pattern of the CDs upon increasing the pH was observed for some peptides, complex mobility was the primary factor for other peptide-CD combinations affecting the enantiomer migration order at the two pH values studied.