J. Sep. Sci. 2007, 30, 21 – 27 B. Natalini et al. 21 Benedetto Natalini Roccaldo Sardella Antonio Macchiarulo Serena Natalini Roberto Pellicciari Dipartimento di Chimica e Tecnologia del Farmaco, Università degli Studi di Perugia, Perugia, Italy Short Communication (S)-(–)-a,a-Di(2-naphthyl)-2-pyrrolidinemethanol, a useful tool to study the recognition mechanism in chiral ligand-exchange chromatography A dynamic coating of the RP-18 carbon chain layers with the new chiral selector (S)- (–)-a,a-di(2-naphthyl)-2-pyrrolidinemethanol allowed the formation of a mixed chi- ral stationary phase that has been used in the separation of a selected set of amino acid racemates. Both a representative model and classification structure-property relationship studies have been performed in order to study the contribution of hydrophobic, bulky and electron-donating groups in the side chain of the chiral selector to the mechanism of chiral recognition. Keywords: Chiral stationary phase / Classification structure-property relationship / Dynamic coating / Enantioselective separation / Ligand-exchange / Received: June 20, 2006; revised: September 4, 2006; accepted: September 7, 2006 DOI 10.1002/jssc.200600242 1 Introduction During the last few years, a wide variety of molecular sys- tems based on the principle of chiral ligand-exchange chromatography (CLEC), invented by Davankov in the early 1970s [1], were proposed for the enantiomeric HPLC separation of bidentate substrates such as amino acids, amino alcohols, a-hydroxy acids, diamines and peptides. Models developed to describe enantioselectivity in CLEC invoke the presence of multicomponent complexes con- taining a central metal ion (usually Cu 2+ ) chelated by two chiral bifunctional molecules, the selector and the ana- lyte [2]. One or more solvent molecules complete the first solvation sphere of the metal ion. The selector can be pre- sent in solution thus giving rise to a chiral mobile phase (CMP) [3 – 10], or can be covalently bound to the solid sup- port thus generating a chiral stationary phase (CSP) [11 – 16]. The alternate hydrophobic, dynamic coating of the RP-18 surface with a suitable selector provides a mixed chiral stationary phase (MCSP) endowed with a strong lipophilic character, coming through the co-operative action of the carbon chain layers and the hydrophobic residues of the selector, besides the enantioselective reso- lution ability [2, 16 – 24]. With this MCSP, the formation of diastereomeric adsorbates with enantiomers of differ- ent stabilities and different interaction with the mixed stationary phase leads to the separation of the racemate. When the chiral selector is added to the mobile phase (CMP mode), the mechanism of chiral recognition is, in general, quite complex, and it may involve a series of complexation equilibria in the mobile and the stationary phase as well as partition equilibria of the different spe- cies between the two phases [15, 25]. In the particular case of the mixed, chiral coated stationary phase (MCSP mode), the enantiomer recognition process is mainly the result of, to an extent, the peculiar and selective interac- tion ability between the portion of the ternary complex containing the analyte and the stationary phase [2]. While N-substituted amino acids have been largely employed as chiral selectors in CLEC, in view of the strong enantioselectivity effects generated by ligands that contain substituted amino groups with electron donating effects [16, 17], we have recently directed our studies and applications toward selectors with free amino acid moiety, thus demanding of a strongly hydro- phobic side chain the task to interact and discriminate between the enantiomers [2, 24]. Accordingly, S-benzyl- (R)-cysteine (R-SBC) [2] and S-Trityl-(R)-cysteine (R-STC) [24] showed to be new, rather inexpensive chiral selectors endowed with a strong lipophilic character that comes Correspondence: Professor Benedetto Natalini, Dipartimento di Chimica e Tecnologia del Farmaco, Università di Perugia, Via del Liceo, 1, 06123 Perugia, Italy E-mail: natalini@chimfarm.unipg.it Fax: +39-075-5855161 Abbreviations: ACIC, (5R,29R)- and (5S,29S)-5-(2-amino-2-carboxy- ethyl)-4,5-dihydroisoxazole-3-carboxylic acid; AIDA, 1-aminoin- dane-1,5-dicarboxylic acid; CLEC, chiral ligand-exchange chro- matography; CSPR, classification structure-property relation- ship; 3-Matida, 2-(39-methyl-59-carboxythien-29-yl)glycine; 5-Mati- da, 2-(59-methyl-49-carboxythien-29-yl)glycine; MCSP, mixed chiral stationary phase; S-DNPM, (S)-( – )-a,a-di(2-naphthyl)-2-pyrrolidine methanol; SPhg, 4-sulphonophenylglycine; TAla, 3-(29-thienyl)- alanine; TASA, total hydrophobic surface area, THFG, 2-(tetrahy- drofuran-29-yl)glycines i 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com