In many biological processes the activity of one member of an enantiomeric pair can be contrasted with the inactivity or even harmful activity of the other. The successful development of chiral stationary phases (CSP) for HPLC now allows us to monitor the optical purity of bulk drug and its presence in formulations or biological fluids. Further applications can be found within the agrochemical and related industries.
The main types of CSPs are discussed.
Although “brush-type” (Pirkle) chiral selectors are relatively simple molecules, their well defined structure contains three types of functional groups capable of participating in charge transfer (π-π bonding), hydrogen bonding (“dipole stacking” interactions) and steric effects. The monolayer of chiral selector covalently bound to the silica surface usually gives a column of relatively high capacity and efficiency but often with limited chiral discrimination ability. Since the synthesis of the popular D-3,5-dinitrobenzoylphenylglycine phase, significant numbers of these multiple interaction CSPs have been synthesised. Polyaromatic hydrocarbon derivative CSPs are the most recent additions to the range. All “brush-type” phases are typically used with normal-phase eluents.
2. Cellulose and Amylose Bound
Cellulose and amylose are linear polymers of optically active glucose units with molecular weights of 250,000 to
1,000,000. Crosslinked derivatives of these materials coated onto silica give unique chiral selectivity. Their chiral recognition properties depend on the “steric fit” of guest enantiomers into the material’s cavities. Choice of eluent is the key factor affecting chiral recognition. Immobilized polysaccharide based phases are also now available.
3. Cyclodextrin Inclusion
Cyclodextrins are a class of oligosaccharides containing six to twelve optically active glucose units. They are covalently bound to silica to form the corresponding CSP. The physical shape of these molecules is that of a truncated cone, the internal diameter of which is proportional to the number of glucose units. The interior of the cavity is relatively hydrophobic. Secondary hydroxyl groups at the entrance to the cavity contribute to the separation process. The relative stability of the inclusion complexes formed by the enantiomers of the guest molecule at the edge of the cyclodextrin cavity dictates the degree of separation. β-Cyclodextrin and its derivatives are the most commonly used CSPs of this type. Cyclodextrin CSPs are used in reversed-phase and are suitable for preparative separations.
4. Crown Ether
Chiral recognition with crown ether phases is achieved when a complex is formed between the crown ether and an ammonium ion from the analyte. These phases are used for solutes with a primary amino group at or near its chiral centre, such as amino acids and aminoalcohols.
5. Protein Bound
Proteins are high molecular weight polymers containing chiral sub-units. When bound to silica they act as very effective CSPs. The binding or complexation of small enantiomeric molecules is often stereospecific, especially for serum proteins such as α1-acid glycoprotein (AGP) or human serum albumin (HSA). The additional stability of the Ultron ES-OVM and ES-Pepsin columns allow them to be used with high organic content eluents. Immobilised enzymes can similarly be used. Protein immobilised CSPs are typically used in buffered aqueous eluents compatible with many biological samples. They offer good selectivity. Enantiomer retention and stereoselectivity can often be significantly altered by changes in eluent pH or modifier concentration. Their low capacity makes them unsuitable for preparative applications.
6. Network Polymeric
In a network polymeric CSP the chiral selector is anchored into a network polymer by a cross-linking reaction which simultaneously bonds it to the silica. The aim is to combine in one CSP the efficiency and capacity of “brush-type” structure with the chiral recognition power of those based on chiral polymers.
7. Ligand Exchange
Ligand exchange chiral phases are characterised by the attachment of a chiral chelating ligand to the stationary support. In the presence of an appropriate transition metal cation such as copper (ll), a molecular complex is formed with the chiral stationary phase ligand and the analyte. Compounds that are suitable for chiral ligand exchange are α-amino acids, hydroxy acids and small peptides