Macromolecular chiral selectors

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

The first step in method development is selecting an adequate HPLC mode for the particular sample. This choice depends on the character of the sample compounds, which can be either neutral (hydrophilic or lipophilic) or ionic, low-molecular (up to 2000 Da) or macromolecular (biopolymers or synthetic polymers). Many neutral compounds can be separated either by reversed-phase or by normal-phase chromatography, but a reversed-phase system without ionic additives to the aqueous-organic mobile phase is usually the best first choice. Strongly lipophilic samples often can be separated either by non-aqueous reversed-pha.se chromatography or by normal-phase chromatography. Positional isomers are usually better separated by normal-phase than by reversed-phase chromatography and the separation of optical isomers (enantiomers) requires either special chiral columns or addition of a chiral selector to the mobile phase. 

To date, the capability of macromolecular peptides as molecular recognition sites has been poorly characterized. Due to the analytical techniques available, first investigations on host-guest complexes were done in the homogeneous phase [55]. One of the most popular cyclic compounds is the depsipeptide, valinomycin, which can complex potassium ions selectively and carry them through lipid membranes [56], Another interesting receptor is the peptide antibiotic, vancomycin [57], This consists of a complicated tricyclic pep-tidic structure and can be used as chiral selector [58] in capillary electrophoresis and HPLC. 

The separation of enantiomers using macrocyclic antibiotics as chiral selectors is clearly a complex process affected not only by macromolecular and analyfe structure and 3D conformation, but also the pH environment in which the enantioseparations occur (46). 

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