Ligand-exchange phases

V. A. Davankov, Ligand-exchange phases in Chiral Separations by HPLC, A. M. Krstulovic, Ellis Horwood Ltd., Chichester (1989) Chapter 15. 

Ligand binding, in proteins, 20 829-830 Ligand-exchange phases, for chiral separations, 6 82-83 Ligands... 

O Naobumi, H Kitahara, R Kira. Direct separation of enantiomers by high-performance liquid chromatography on a new chiral ligand-exchange phase. J Chromatogr 592 291-296, 1992. 

Davankov VA, Ligand exchange phases, in Chiral Separations by HPLC, Krstulovic A (Ed.), Ellis Horwood, Chichester, p. 447 (1989). 

Schmid et al. [60] demonstrated the enantiomer separation of underivatized amino acids on a monolithic chiral ligand-exchange phase by rod-CEC. The chiral stationary phase was prepared in situ in the capillary by polymerization of methacrylic acid, piperazine diacrylamide, vinylsulfonic acid and /V-(2-hydroxy-3-alloxypropyl)-L-4-hydroxyproline. The monolithic separation bed was covalently linked to the internal capillary wall and thus no frits were required. Fig. 9.13 shows the enantiomer separation of phenylalanine by (A) pure CEC (30 kV), (B) nano-LC (12 bar) and (C) pressure supported CEC (30 kV, 12 bar at the inlet vial). The shortest elution time was clearly obtained by pressure supported CEC, while the highest resolution was found in the pure CEC mode (CEC Rs = 2.11 nano-LC Rs = 0.98 pressure supported CEC Rs= 1.60). 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

Amino acids bonded to silica and loaded with Cu ions can interact in a steroselective manner with amino acids in aqueous solution. The copper ion forms a complex with both the bound and the sample amino acids, as shown in Figure 22.1. Ligand-exchange phases are suited for the separation of amino acids as well as of some )3-amino alcohols and similar molecules because these compounds bear two polar functional groups in adequate spacing. This approach has found limited interest because the column efficiencies are rather low, the detectability of the nonderivatized sample compounds can be a problem and the mobile phase needs to contain copper. 

Chiral stationary phases that are currently available can be classified into those containing cavities (cellulose derivatives, cyclodextrins, synthetic polymers, crown ethers, and chiral imprinted gels), affinity phases (bovine serum albumin, human serum albumin, a-glycoprotein, enzymes), multiple hydrogen-bond phases, Ti-donor and Ti-acceptor phases, and chiral ligand exchange phases. This classification scheme was used in a review that gave numerous pharmaceutical examples of separation by... 

Davankov VA. Ligand-exchange phases. In Krstulovic AM, editor. Chiral separations by HPLC. New York Ellis Horwood Limited 1989. p. 175—93. 

FIGURE 52.14. The structure of silica-based, chiral ligand-exchange phases using L-proline (X=H) or L-4-hydroxyproline (X=OH). 

---