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Estimation of the pH‐independent binding constants of alanylphenylalanine and leucylphenylalanine stereoisomers with β‐cyclodextrin in the presence of urea

The separation of stereoisomers, particularly enantiomers, is important when their physiological activity differs. We have resolved the four stereoisomers each of alanylphenylalanine (Ala‐Phe) and of leucylphenylalanine (Leu‐Phe) by capillary electrophoresis using β‐cyclodextrin as a buffer additive and urea to enhance its solubility. A study of the influence of pH and β‐cyclodextrin concentration on the separations showed that weak inclusion complexes were formed between the dipeptides and chiral selector. It was found that pH could alter the migration order of enantiomers L‐Ala‐L‐Phe and D‐Ala‐D‐Phe, as well as L‐Leu‐L‐Phe and D‐Leu‐D‐Phe; however, there was no change in order for the other pairs of optical isomers. Electrophoretic mobility data were used to estimate the acid dissociation constants of the dipeptide isomers at pH < 7 with no chiral selector present. By varying the concentration of β‐cyclodextrin, the chiral selector, the binding constants of Ala‐Phe and Leu‐Phe optical isomers in their fully protonated and zwitterionic forms were estimated. For the four Ala‐Phe stereoisomers, K = 42–66 M–1 and 4–41 M–1 for the cationic and zwitterionic forms, respectively. For the four Leu‐Phe stereoisomers, K = 43–94 M–1 and 1–28 M–1 for the cationic and zwitterionic forms, respectively.