Chiral separation of amino acids by copper(II) complexes of tetradentate diaminodiamido-type ligands added to the eluent in reversed-phase high-performance liquid chromatography: a ligand exchange mechanism

In this paper we report a study on the mechanism of the enantiomeric separation of unmodified D,L-amino acids in RP-HPLC by copper(II) complexes of two tetradentate diaminodiamido ligands, (S,S)-N,N ' -bis(phenylalanyl)ethanediamine (PheNN-2) and (S,S)-N,N ' -bis(methylphenylalanyl)ethane (Me(2)PheNN-2), added to the eluent. The aim is to investigate whether and how a copper(II) complex with no free equatorial positions can perform chiral discrimination of bidentate analytes such as unmodified amino acids. The problem is approached in a systematic way by: (a) varying the different chromatographic parameters (pH, selector concentration, eluent polarity); (b) performing chiral separation with the selector adsorbed on the stationary phase; (c) studying the ternary complex formation of these ligands with D- and L-amino acids in solution by glass electrode potentiometry and electrospray ionization MS. All the experimental data are consistent with a mechanism of chiral recognition, based on ligand exchange, which involves as selectors the species [Cu2L2H-2](2+) and [CuLH-2] and proceeds by displacement of two binding sites from the equatorial positions, giving rise to the ternary species [CuLA](+) and [CuLH(-1)A]. The most important factor responsible for chiral discrimination seems to be the affinity of the diastereomeric ternary complexes for the stationary phase since no enantioselectivity is observed in solution. (C) 2001 Elsevier Science B.V. All rights reserved.