Chiral discrimination of Dns- and unmodified d,l-amino acids by copper(II) complexes of terdentate ligands in high-performance liquid chromatography

The enantiomeric separation of unmodified D,L-amino acids and Dns-amino acids in RP-HPLC by copper(II) complexes of terdentate diaminoamido ligands added to the eluent has been studied. The work is aimed at investigating whether a copper(II) complex with only one free equatorial position can still perform chiral discrimination of bidentate analytes such as unmodified and Dns-amino acids. The problem is approached by determining the nature and the stability of the initial copper(II) binary complexes and of the ternary complexes with amino acids in solution by potentiometry. Different chromatographic parameters were examined (pH, selector concentration, ionic strength, eluent polarity). All experimental data are consistent with a mechanism of chiral discrimination involving formation of diastereomeric complexes in which the amino acid coordinates at the free equatorial position and at the apical position of the initial complex which maintains its original chelate rings. It may still be considered a ligand exchange, implying that coordinated water molecules have been substituted by the analyte. The poor performance of the tricoordinated copper(II) complexes in the chiral discrimination of unmodified amino acids can be ascribed to the low stability of the mixed complexes and to their loose sterical requirements. With Dns-amino acids the most important feature seems to be the higher/lower affinity (match/mismatch) of the enantiomers for the selector adsorbed on the stationary phase.