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The reversal of the elution order of cyclic alpha-amino acid enantiomers as a function of the temperature on a copper(II)-N,S-dioctyl-D-penicillamine ligand-exchange column is described. The thermodynamic parameters accounting for the retention and the separation of analytes were determined by means of van't Hoff plots. The influence of different chromatographic conditions on these parameters was investigated, showing little effect of the Cu(II) concentration 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 determining factor for the retention of the analytes. Based on these findings, a separation mechanism is postulated comprising the importance of complex formation for primary docking at the stationary phase, while hydrophobic interactions are crucial for chiral discrimination.

High-performance liquid chromatographic (HPLC) separation of stereoisomeric cyclic beta-substituted alpha-quaternary alpha-amino acids was performed by ligand-exchange on a copper(II)-D-penicillamine chiral stationary phase. The investigated amino acids are the 1-amino-2-methylcyclohexanecarboxylic acids, the 1-amino-2-hydroxycyclohexanecarboxylic acids, the 1-amino-2-methylcyclopentanecarboxylic acids and the trans-configured 1,2-diaminocyclohexanecarboxylic acids. The effects of the mobile phase composition (copper(II) concentration, type and content of organic modifier, pH) and the temperature on the enantio- and diastereoselectivity were studied and the conditions were optimised to resolve the four stereoisomers of each of the said amino acids in single chromatographic runs. A reversal of the elution order occurred for enantiomers of some of the amino acids in dependence on the acetonitrile content of the eluent. This phenomenon is explained by at least two different copper(II) complexes of the tridentate ligand penicillamine.

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