High-performance liquid chromatographic enantioseparation of glycyl di- and tripeptides on native cyclodextrin bonded phases. Mechanistic considerations.

Di- and tripeptides containing at least one glycine moiety were separated into enantiomers on alpha-, beta- and gamma-cyclodextrin bonded phases after their precolumn derivatization with 9-fluorenylmethyl chloroformate (FMOC-Cl). It is shown that the choice of a suitable cyclodextrin bonded phase used with a nonaqueous polar mobile phase offers a wide range of possibilities to optimize enantioselectivity. Use of the FMOC derivative greatly enhances sensitivity, stability and enantioselectivity. It is postulated that under reversed-phase conditions the inclusion complex formation between the hydrophobic aromatic part of the FMOC-functionalized peptides and non-polar cyclodextrin cavity interior is the major factor contributing to retention. However, under these conditions there seems to be insufficient interaction between the hydrophilic peptide chain and cyclodextrin hydroxyls. Hence no chiral recognition and enantiomeric separation is observed. In systems operating with polar organic mobile phases the inclusion complex is suppressed as the cyclodextrin cavity is largely occupied by the mobile phase. The enantioselectivity observed is caused by hydrogen bonding between peptide chain and the hydroxyl groups at the mouth of the cyclodextrin. The stereospecific interactions depend very strongly on the cyclodextrin size, the length of the peptide chain and the mobile phase composition.