A thermodynamic study of the temperature-dependent elution order of cyclicalpha-amino acid enantiomers on a copper(II)-D-penicillamine chiral stationary phase

The reversal of the elution order of cyclic a-amino acid enantiomers as a f unction of the temperature on a copper(II)-N,S-diodyl-D-penicillamine ligan d-exchange column is described. The thermodynamic parameters accounting for the retention and the separation of analytes were determined by means of v an't Hoff plots. The influence of different chromatographic conditions on t hese parameters was investigated, showing little effect of the Cu(II) conce ntration in the eluent but strong influence of the organic modifier content on the separation. Further, the pH of the mobile phase was found to be a d etermining factor for the retention of the analytes. Based on these finding s, a separation mechanism is postulated comprising the importance of comple x formation for primary docking at the stationary phase, while hydrophobic interactions are crucial for chiral discrimination.