OPTICALLY SELECTIVE ADSORPTION OF o~-AMINO ACIDS ON MONTMORILLONITE-Cu-I-LYSINE COMPLEXES IN HIGH-PRESSURE LIQUID CHROMATOGRAPHY

Optically active cationic complexes adsorbed on montmorillonite can be used for the resolution ofracemic mixtures. Montmorillonite-Cu-lysine systems were used as a solid phase in high-pressure liquid chromatography for the resolution of the optical isomers of a-amino acids. Selectivity constants > 1.5 were measured for phenylalanine and tryptophan. The selectivity constants for the amino acids containing saturated-hydrocarbon side chains were in the range of 1.25-1.44. The montmorillonite-Cu-l-lysine complex displayed a stronger affinity for the 1-isomers of a-amino acids than for the d-isomers at pHs near neutrality. Inasmuch as surface-catalyzed peptide formation on clays has been proposed as a step in chemical evolution, this stronger affinity between the clay-Cu-l-amino acid complex and 1-amino acids might have been significant in prebiotic evolution. The mechanism of optical resolution probably involved ligand exchange. Optimizing the choice of the optically active ligands and of the chelating cation in the chiral agent may improve the resolution of the optical isomers. Key Words--Adsorption, Amino acid, Cu-l-lysine, High-pressure liquid chromatography, Montmorillonite, Optically active ligands. I N T R O D U C T I O N The resolution of racemic a-amino acids by ligand exchange on high-pressure liquid chromatography (HPLC) columns has been carried out using several solid supports, such as organic polymers, reversed phase (n-octadecylsilane or n-octylsilane bound to a silica gel surface; e.g., Lindner et al., 1979), and silica (see, e.g., Davankov, 1981; Kurganov and Davankov, 1981). The complex whose ligand is exchanged must be optically active to bring about resolution between enantiomers. The optically active species used were either covalently bound to the solid phase (e.g., Gubitz et al., 1981) or added to the mobile phase (e.g., Hare and Gil-Av, 1979) in the chromatographic run. Kurganov and Davankov (1981) and Grushka et al. (1983), among others, proposed that sorbed complexes are responsible for the resolution in both the silica and the reversed phase systems, even when the chiral species are applied in the mobile phase. The first partial resolution of optical isomers of metal (III) chelates on small chromatographic columns containing clay was reported by Yamagishi and Soma (1981), Yamagishi (1982), and Yamagishi and Ohnishi (1982). Yamagishi and Ohnishi (1983) were successful in partially resolving the cyclic amino acid proline, as well as other optically active cyclic compounds, but failed in their attempt to resolve acyclic amino acids on similar columns. In the above studies ambient pressure was employed. Although some optical selectivity of a-amino acids on montmoril lonite has been claimed (e.g., Bondy and Harrington, 1979), the resolution of racemic mixtures of acyclic a-amino acids (and of cyCopyright 9 1987, The Clay Minerals Society clic ones with the above-mentioned exception of proline and its derivatives) on montmoril lonite loaded with optically active species has not been reported previously. Some reports indicate that a-amino acids form with transition metal ions (and in particular with Cu 2§ ions) complexes on clay surfaces that differ from the complexes formed in solution (Bodenheimer and Heller, 1967; Yang and Condrate, 1971, 1972; HeUer et al., 1973; Siegel, 1966; Siffert and Kessaissia, 1978). Lysine was reported to adsorb on Cu-montmoril lonite at a Cu : lysine ratio of 1:2 (see, e.g., Bodenheimer and Heller, 1967; Yang and Condrate, 1972), and Bodenheimer and Heller (1967) suggested that the complex of lysine with Cu-montmoril lonite is more stable than the lysine-Cu complex in solution. Most a-amino acids form neutral complexes with Cu 2+. Lysine can form a biligand complex with Cu 2§ in which the a-amino group and a carboxylate oxygen are bound to Cu 2+ and the E-amino group is protonated (Figure 1). The resulting positively charged Cu-lysine complex should adsorb more strongly on the negatively charged clay surface than copper complexes with neutral or acidic amino acids. Bodenheimer and Heller (1967) reported that the adsorption coefficients of glutamic acid (at pH 3.4), glycine (at pH 9.6) and lysine (at pH 9.7) on Cu-montmoril lonite were about 0, 16, and 1600 ml/g, respectively. In the present study, spray-dried bentonite (SP-bentonite; Mingelgrin and Tsvetkov, 1985) loaded with the Cu-l-lysine complex was utilized as a packing material for the HPLC separation of enantiomers of dif-