Polysaccharides as chiral selectors

H Nishi. Enantiomer separation of basic drugs by capillary electrophoresis using ionic and neutral polysaccharides as chiral selectors. J Chromatogr A 735 345-351, 1996. 

A review of enantiomeric separations by CE using polysaccharides as chiral selectors has been reported (217). Ionic and neutral polysaccharides (e.g., heparin, chondroitin sulfate, dextrin, and maltodextrins) have been used to resolved enantiomers. Racemic acidic drugs were resolved with maltodextrins (218), while heparins and cyclodextrins were used to resolve oxamigue (219). A large number of drugs have been found to bind enantioselectively to proteins. 

Gotti R, Pomponio R, Cavrini V. Linear, neutral polysaccharides as chiral selectors in enantioresolution of basic drug racemates by capillary electrophoresis. Chromatographia 2000 52 273-277. 

Nishi et al. [110] used dextran and dextrin as chiral selectors in capillary-zone electrophoresis. Polysaccharides such as dextrins, which are mixtures of linear a-(l,4)-linked D-glucose polymers, and dextrans, which are polymers of D-glucose units linked predominantly by a-(l,6) bonds, have been employed as chiral selectors in the capillary electrophoretic separation of enantiomers. Because these polymers are electrically neutral, the method is applicable to ionic compounds. The enantiomers of basic or cationic drugs such as primaquine were successfully separated under acidic conditions. The effects of molecular mass and polysaccharide concentration on enantioselectivity were investigated. 

The chiral SO is coated to the capillary wall. This technique is known as open-tubular electrochromatography (OT-EC) )5(X)1. Enantioselective OT-EC methods have been described for CDs 1277), proteins ).501), polysaccharides )5021 and terguride [503] as chiral selectors. The main disadvantage is the low loading capacity, and therefore this technique has not gained high popularity. 

Together with CDs, other chiral selectors such as non-cyclic oligosaccharides and polysaccharides [118, 119], chiral surfactants [120-123], macrocyclic antibiotics [124, 125], proteins, peptides [126, 127], peptide libraries [128, 129], ligand exchange materials [130], and synthetic macrocyclic compound [131] can be used as chiral selectors in EKC. 

The type of CSPs used have to fulfil the same requirements (resistance, loadabil-ity) as do classical chiral HPLC separations at preparative level [99], although different particle size silica supports are sometimes needed [10]. Again, to date the polysaccharide-derived CSPs have been the most studied in SMB systems, and a large number of racemic compounds have been successfully resolved in this way [95-98, 100-108]. Nevertheless, some applications can also be found with CSPs derived from polyacrylamides [11], Pirkle-type chiral selectors [10] and cyclodextrin derivatives [109]. A system to evaporate the collected fractions and to recover and recycle solvent is sometimes coupled to the SMB. In this context the application of the technique to gas can be advantageous in some cases because this part of the process can be omitted [109]. 

Mourier s report was quickly followed by successful enantiomeric resolutions on stationary phases bearing other types of chiral selectors, including native and deriva-tized cyclodextrins and derivatized polysaccharides. Many chiral compounds of pharmaceutical interest have now been resolved by packed column SFC, including antimalarials, (3-blockers, and antivirals. A summary is provided in Table 12-2. Most of the applications have utilized modified CO, as the eluent. 

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. 

Based on the theory, the separation of enantiomers requires a chiral additive to the CE separation buffer, while diastereomers can also be separated without the chiral selector. The majority of chiral CE separations are based on simple or chemically modified cyclodextrins. However, also other additives such as chiral crown ethers, linear oligo- and polysaccharides, macrocyclic antibiotics, chiral calixarenes, chiral ion-pairing agents, and chiral surfactants can be used. Eew non-chiral separation examples for the separation of diastereomers can be found. 

As discussed in Section 3.1.6.1., natural biopolymers are useful chiral selectors, some of which are readily available they are constructed from chiral subunits (monomers), for instance, from L-amino acids or D-glucose. If synthetic chiral polymers of similar type are to be synthesized, appropriate chiral starting materials and subunits, respectively, must be found. Chiral polymers with, for example, a helical structure as the chiral element, are built using a chiral catalyst as chirality inducing agent in the polymerization step. If the chirality is based on a chiral subunit, the chirality of the polymer is inherent, whereas if the polymer is constructed from chiral starting materials, chiral subunits are formed which lead to chirally substituted synthetic polymers that in addition may order or fold themselves to a supramolecular structure (cf. polysaccharides). 

In contrast, CSPs have achieved great repute in the chiral separation of enantiomers by chromatography and, today, are the tools of the choice of almost all analytical, biochemical, pharmaceutical, and pharmacological institutions and industries. The most important and useful CSPs are available in the form of open and tubular columns. However, some chiral capillaries and thin layer plates are also available for use in capillary electrophoresis and thin-layer chromatography. The chiral columns and capillaries are packed with several chiral selectors such as polysaccharides, cyclodextrins, antibiotics, Pirkle type, ligand exchangers, and crown ethers. 

The chiral recognition mechanisms in NLC and NCE devices are similar to conventional liquid chromatography and capillary electrophoresis with chiral mobile phase additives. It is important to note here that, to date, no chiral stationary phase has been developed in microfluidic devices. As discussed above polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, proteins, crown ethers, ligand exchangers, and Pirkle s type molecules are the most commonly used chiral selectors. These compounds... 

A CSP consists of a chiral selector, which either alone constitutes the stationary phase or which has been immobilised to a solid phase. The chiral selector is a low molecular weight compound or a polymer, either synthetic or natural. A broad range of CSPs has been developed. Examples of CSPs that have been used successfully include polysaccharides, such as cellulose and its derivatives [6] and cyclodex-trins [7], and proteins, e.g. bovine serum albumin, aj-acid glycoprotein, cellulase, trypsin and a-chymotrypsin [8]. Several different synthetic polymers have also proven to be useful CSPs, for example the Blaschke-type CSPs (polyacrylamides and polymethacrylamides) [9] and the Pirkle-type CSPs [10]. 

As in analytical chiral LC, Daicel derivatised polysaccharide CSPs are the most frequently used materials in preparative scale chiral separations. Recently CSPs have been prepared in which derivatised polysaccharides have been covalently bonded to the solid support rather than coated on as in the Diacel materials. The rationale for this is that it is advisable to reduce the chance of the chiral selector leeching off the column in trace amounts to contaminate samples of chiral dmgs isolated by production scale LC. However, the extent to which the Daicel coated CSPs are now used in production scale chiral LC would tend to suggest that such a problem, if it exists, is not a very significant risk. 

The chiral selectors applied to CE include native cyclodextrins as well as neutral and charged derivatives, oligo- and polysaccharides, chiral crown ethers. 

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