Mechanistic investigations of cinchona alkaloid-based zwitterionic chiral stationary phases

Novel zwitterionic cinchona alkaloid-based chiral selectors (SOs) were synthesized and immobilized on silica gel. The corresponding brush-type chiral stationary phases (CSPs) were characterized as zwitterionic ion-exchange-type materials and exhibited remarkable enantioselectivity for their zwitterionic target analytes, viz. underivatized amino acids and aminosulfonic acids. We rationally designed structural modifications on the strong cation exchange (SCX) subunit of the zwitterionic SO and investigated the influence on chiral recognition power for amphoteric solutes. SOs with chiral isopropyl- or cyclohexyl-moieties in vicinity to the SCX site showed broadest application range by baseline resolving 39 out of 53 test compounds, including α-, β-, and γ-amino acids with different substitution patterns. Furthermore, we introduced two pseudoenantiomeric zwitterionic CSPs which combined the unique features of providing comparable enantioselectivities but reversed enantiomer elution orders. By application of slightly acidic polar organic mobile phases as preferred elution mode, we found that certain amounts of aprotic acetonitrile in protic methanol substantially increased enantioselectivity and resolution of amino acids in a structure-dependent manner.

► Zwitterion exchange-type CSPs were prepared and evaluated in HPLC. ► CSPs were operated in polar organic mode for enantioseparation of free amino acids. ► Two CSPs provided possibility for systematically switching elution orders. ► Methanol and acetonitrile mixtures provided best overall enantioseparation performance.