betadex and alanyltyrosine

A separation selectivity model for capillary electrophoresis enantioseparations of weak bases in the presence of uncharged chiral selectors was described as a function of buffer pH and chiral selector concentration. On the basis of the selectivity at the extreme pH and selector concentration values, 15 principal cases could be distinguished describing the migration behavior of the analytes when increasing either pH or selector concentration. A pH-dependent reversal of the enantiomer migration order can be observed (1) when the complex mobility ratio is reversed due to an enantioselective complexation-induced pK(a)-shift, (2) in the case of an inversion of the ratio of the mobilities of the analyte-selector complex and the free analyte due to significantly weaker complexation of the neutral species, and (3) in the case of opposite chiral recognition of the protonated and uncharged species by the chiral selector. Reversal of enantiomer migration order as a function of the selector concentration at a fixed buffer pH is caused by the opposing effects of complexation constants and complex mobilities at the respective pH value. The model was applied to rationalize the enantioseparations of dipeptides, especially the pH-dependent reversal of the migration order of the LL- and DD-enantiomers of Ala-Tyr in the presence of heptakis-(2,6-di-O-methyl)-beta-cyclodextrin as the chiral selector based on the complexation constants of the protonated and zwitterionic forms of the analytes.

Topics: beta-Cyclodextrins; Dipeptides; Electrophoresis, Capillary; Hydrogen-Ion Concentration; Mathematical Concepts; Stereoisomerism

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.

Topics: beta-Cyclodextrins; Buffers; Cyclodextrins; Dipeptides; Electrophoresis, Capillary; Hydrogen-Ion Concentration; Indicators and Reagents; Mass Spectrometry; Stereoisomerism