Enantiomeric separation of a tetrapeptide with cyclodextrin extension of the model for chiral capillary electrophoresis by complex formation of one enantiomer molecule with more than one chiral selector molecules

In this paper the enantiomeric separation by capillary electrophoresis (CE) of the tetrapeptide LEF553 (Tyr-D-Arg-Phe-Phe-HN₂) and its diastereoisomers is presented. The run buffer consisted of 10 mmol l^-1 heptakis(2,6-di-O-methyl)-β-cyclodextrin in 0.1 mol l^-1 phosphoric acid adjusted to pH 3.0 with triethanolamine. Large differences were found between the different steric conformations of the peptide. The relationships between the apparent mobility differences of the enantiomers versus the cyclodextrin concentration were investigated. For some of the conformations of the peptide, the curve forms that were obtained could not be explained by Wren's model, which assumes a 1:1 interaction between enantiomer and chiral selector. A possible explanation could be that each peptide molecule could interact with two or three cyclodextrin molecules. An extended model in which the enantiomer complexes with more than one chiral selector molecule is presented. Using this extended model, curves for the mobility difference versus the chiral selector concentration can be obtained that have the same shape as the experimental curves.