Comparison of the separation performances of cinchona alkaloid-based zwitterionic stationary phases in the enantioseparation of β2- and β3-amino acids.

The enantiomers of twelve unusual β2- and β3-homoamino acids containing the same side-chains were separated on chiral stationary phases containing a quinine- or quinidine-based zwitterionic ion-exchanger as chiral selector. The effects of the mobile phase composition, the nature and concentration of the acid and base additives and temperature on the separations were investigated. The changes in standard enthalpy, ∆(∆H°), entropy, ∆(∆S°), and free energy, ∆(∆G°), were calculated from the linear van't Hoff plots derived from the ln α vs. 1/T curves in the studied temperature range (10-50 °C). The values of the thermodynamic parameters depended on the nature of the selectors, the structures of the analytes, and the positions of the substituents on the analytes. A comparison of the zwitterionic stationary phases revealed that the quinidine-based ZWIX(-)™ column exhibited much better selectivity for both β2- and β3-amino acids than the quinine-based ZWIX(+)™ column, and the separation performances of both the ZWIX(+)™ and ZWIX(-)™ columns were better for β2-amino acids. The elution sequence was determined in some cases and was observed to be R < S and S < R on the ZWIX(+)™ and ZWIX(-)™ columns, respectively.