Enantioseparation of β2-amino acids on cinchona alkaloid-based zwitterionic chiral stationary phases. Structural and temperature effects

The enantiomers of sixteen unusual β2-amino acids were directly separated on chiral stationary phases containing quinine- or quinidine-based zwitterionic selectors. The effects of the mobile phase composition, the structure of the analyte and temperature on the separations were investigated. Experiments were performed at constant mobile phase compositions in the temperature range −5 to 55 °C in order to study the effects of temperature, and thermodynamic parameters were estimated from plots of ln k or ln α vs. 1/T. Some mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes. It was found that the enantiomeric separations were in most cases enthalpically driven, but entropically driven separation was also observed. The sequence of elution of the enantiomers was determined in some cases.