A. A. Kurganov

Chiral chromatographic separations based on ligand exchange

Taking ligand-exchange chromatographic systems as an example, the effect of the stoichiometry of the solute-chiral selector interaction on the efficiency, selectivity and solute peak profile is discussed. Recent achievements and practical applications of chiral ligand-exchange chromatography are also briefly reviewed.

High-performance ligand-exchange chromatography of α-amino acid enantiomers

Abstract Three packings with the following surface composition were synthesized on LiChrosorb Si 100, d p ue5fb 10 μm: [Cu(II) 3-( L -prolyl)propyl] + (packing I), [Cu(II) 3-( L -hydroxyprolyl)propyl] + (packing II) and [Cu(II) 3-( L -hydroxyprolyl)propyl] + with remaining excess 3-(iodopropyl) groups (packing III). The separation of α-amino acid enantiomers was studied in eluents containing a constant concentration of copper(II) acetate (10 −4 M ). The pH of the buffered eluent was varied in the range 4–6 and the ammonium acetate concentration was varied from 0.001 to 0.1 M , and these were established as dominating parameters in controlling the retention and enantioselectivity. In contrast, the addition of an organic solvent such as methanol, acetonitrile or tetrahydrofuran gave only minor changes. The same was observed for the changes in k′ and α on increasing the methanol content up to 30% (v/v) and by varying the column temperature between 298 and 323°K. The order of elution of α-amino acid enantiomers was generally found to be L - D -; there were, however, a series of exceptions, depending on the type of packing and the eluent composition. Enantioselectivity can be understood in terms of complexation of the enantiomer to a trans -bis(amino acidato)—copper complex and additional interaction through complexation of free ligand sites, hydrophobic interaction between the radical R and the n -propyl spacer of the bonded ligand, hydrogen bonding, etc. The columns permitted the separation of six racemic α-amino acids into the corresponding enantiomers, using common high-performance liquid chromatographic equipment with a UV photometer (254 nm).

Enantioselectivity of complex formation in ligand-exchange chromatographic systems with chiral stationary and/or chiral mobile phases

Abstract Equations are derived describing the retention and separation selectivity of two enantiomeric species in a chromatographic system containing a chiral selector in the stationary and/or in the mobile phase. It is shown that the total column enantioselectivity generally differs from the enantioselectivity of the selector—selectand interaction in solution. In chiral chromatographic systems, there are significant deviations from the principal of reciprocity of mutual chiral selector—selectand recognition.

Polymer-coated reversed-phase packings in high-performance liquid chromatography

The synthesis and properties of polymer-coated RP stationary phases are reviewed. The sorbents are classified according to the method of synthesis. More flexibility in the tailoring of polymer-coated packings is noted. The impact of the polymer coating on the porosity of the oxides to be modified and their chromatographic properties is discussed.

Stereoselectivity in bis(α-amino acid)copper(II) complexes—II[5] spectrophotometric and spectropolarimetric investigation of the solvent effect

Abstract Stereoselectivity phenomena have been studied in the copper(II) bis-complexes of a series of natural bidentate α-amino acids substituted at the nitrogen atom with methyl and benzyl groups in different proportions. The difference in the free energy of formation of complexes with the two ligands of similar and of opposite configurations can be as high as 2·1 ± 0·5 kcal/mole as in the case of a mixed complex of N-benzylproline with N,N-dimethylvaline. Changing the solvent may result not only in a change of the magnitude of stereoselectivity but even in a change of its sign. The efficiencies of spectrophotometry, polarimetric titration and circular dichroism measurements for the quantitative evaluation of stereoselectivity effects are compared.

Characterization of covalently bonded and adsorbed polymer coatings on silica, alumina and zirconia by means of physico-chemical and chromatographic methods

Abstract Polymer coatings on porous silica, alumina and zirconia were prepared by covalent bonding of a copolymer of styrene and vinylsilane on the oxidic surface with or without subsequent cross-linking of the immobilized polymeric layer. Polybutadiene and polychloromethylstyrene were adsorbed on the surface of porous alumina without covalent bonding and were cross-linked after deposition. Analysis of the pore structure of coated oxides by means of nitrogen adsorption, mercury porosimetry and inverse size-exclusion chromatography revealed different polymer distributions across the modified surface and different changes of the porosity of the initial oxides depending on the method of polymer immobilization. By covalent bonding of the copolymer of styrene and vinylsilane to alumina and zirconia with subsequent cross-linking the modifying polymeric layer, highly hydrolytic stable packings were synthesized and their application in separations with aggressive eluents were demonstrated.

Reversed-phase high-performance liquid chromatography of proteins and polypeptides on polystyrene-coated silica supports

Abstract A new type of hydrolytically stable reversed-phase packing material prepared by multi-point covalent binding of polystyrene chains onto the surface of porous silica was examined in the high-performance liquid chromatography of proteins and polypeptides. Whereas wide-pore material was shown to give a rapid and efficient resolution of proteins, packings with smaller pores provided better selectivities for peptides.

Macroporous polymeric monoliths as stationary phases in gas adsorption chromatography

The influence of the conditions of preparation of divinylbenzene-based polymer monoliths on the properties of monolithic capillary columns for use in gas adsorption chromatography was examined. It was found that the polymerization time and the temperature and composition of the polymerization mixture have an effect on the dynamic and chromatographic properties of the columns. The monolithic capillary columns prepared under the optimal synthesis conditions had the height equivalent of a theoretical plate at the level of 20–30 μm, which is an order of magnitude below that of conventional open tubular columns.

Unusual peak distortion in ligand-exchange chromatography of enantiomers under overloaded conditions

Abstract The separation of amino acid enantiomers by means of ligand-exchange chromatography on an adsorbed chiral stationary phase was investigated under overloaded conditions. The selectivity of separation, the limit of mass loading and the peak shapes of enantiomers are strongly affected by the copper(II) concentration in the eluent. Under the optimum conditions it was possible to separate up to 2000 μg of amino acid on a 125 × 4 mm I.D. column. With increase in mass loading the peak shape of the enantiomers changed from symmetrical Gaussian into fronting and subsequently into trapezoidal. During this transformation, peak compression was observed. The complex changes in solute peak shape observed in ligand-exchange chromatography of amino acid enantiomers imply a complex form of adsorption isotherm in these systems.

COPPER(II) COMPLEXES WITH OPTICALLY ACTIVE DIAMINES. V. ENANTIOSELECTIVE EFFECTS IN EQUALLY-PAIRED AND MIXED-LIGAND COPPER(II) COMPLEXES WITH DIAMINES

UV- and circular dichroism (CD) spectra of binary (equally-paired) and ternary (mixed-ligand) copper-(II) complexes with optically active and racemic 1,2-diaminopropane, 2-(aminomethyl)pyrrolidine and their N-benzyl derivatives were studied. For the first time, enantioselective effects in square-planar copperdiamine complexes were observe. n nDifferences in the absorption spectra of labile diastereomeric complexes Cu(R,R) and Cu(R,S), containing two ligands of the same or opposite configurations, albeit sufficient to indicate the non-similarity of the electronic structures of these complexes are nevertheless insignificant. On the contrary, using CD measurements it was possible to evaluate quantitatively the extent of formation of diastereomeric mixed-ligand complexes Cu(ASBS) and Cu(ASBR) on mixing two equally-paired complexes Cu-(ASAS) and Cu(BSBS) or Cu(BRBR), where AS, BS and BR stand for diamine ligands of the corresponding configuration. Differences in the free energy of diastereomeric ternary complexes were found to reach levels as high as 5.5 kJ mol−1. n nProbable structures of diastereomeric complexes are considered. The optical activity of complexes is shown to depend mainly on conformational contributions from chiral chelate rings and chiral distortion of the coordination square-plane, with contributions from the asymmetric carbon and nitrogen atoms playing an ancillary role.