Chiral recognition mechanisms with macrocyclic glycopeptide selectors

Macrocyclic glycopeptide selectors are naturally occurring antibiotics produced by microorganisms. They were found to be excellent chiral selectors for a wide range of enantiomers, including amino acids. Four selectors are commercialized as chiral stationary phases (CSP) for chromatography. They are ristocetin, teicoplanin, vancomycin, and the teicoplanin aglycone (TAG). The key docking interaction for amino acid recognition was established to be a charge-charge interaction between the anionic carboxylate group of the amino acid and a cationic amine group of the macrocyclic peptidic selector basket. The carbohydrate units are responsible for secondary interactions. However, they hinder somewhat the charge-charge docking interaction. The TAG selector is more effective for amino acid enantioseparations than the other CSPs. The "sugar" units are however useful allowing for chiral recognitions of other analytes, e.g., beta-blockers, not possible with the aglycone. Thermodynamic studies established that normal phase and reversed phase enantioseparations were enthalpy-driven. With polar waterless mobile phases used in the polar ionic mode, some separations were enthalpy-driven and others were entropy-driven. The linear solvation energy method was tentatively used to gain knowledge about the chiral recognition mechanism. It appeared to be a viable approach with neutral molecules but it failed with ionizable solutes. With molecular solutes and the teicoplanin CSP, the study showed a significant role of the surface charge-induced dipole interaction and steric effects. The remarkable complementary enantioselectivity effect observed with the four CSPs is discussed.