Chiral stationary phases ligand-exchange

A large number of chiral molecules have been separated with ligand-exchange chiral stationary phases. A few examples with commercially available columns are given in Table 4. 

The first method is based on the conversion of racemic pantothenic acid and its derivatives (panthenol and pantolactone) to DL-pantoic acid. The hydrolysis is carried out in 0.5 M NaOH at 70°C for either 30 min (pantolactone) or 60 min (pantothenic acid and panthenol). Pantoic acid is directly resolved on a ligand-exchange chiral stationary phase MCI gel CRS lOW column (Fig. 12 Table 3). 

Ligand Exchange-Based Chiral Stationary Phases... 

In view of the importance of chiral resolution and the efficiency of liquid chromatographic methods, attempts are made to explain the art of chiral resolution by means of liquid chromatography. This book consists of an introduction followed by Chapters 2 to 8, which discuss resolution chiral stationary phases based on polysaccharides, cyclodextrins, macrocyclic glyco-peptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ethers. The applications of other miscellaneous types of CSP are covered in Chapter 9. However, the use of chiral mobile phase additives in the separation of enantiomers is discussed in Chapter 10. 

The most popular and commonly used chiral stationary phases (CSPs) are polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ether based. The art of the chiral resolution on these CSPs has been discussed in detail in Chapters 2-8, respectively. Apart from these CSPs, the chiral resolutions of some racemic compounds have also been reported on other CSPs containing different chiral molecules and polymers. These other types of CSP are based on the use of chiral molecules such as alkaloids, amides, amines, acids, and synthetic polymers. These CSPs have proved to be very useful for the chiral resolutions due to some specific requirements. Moreover, the chiral resolution can be predicted on the CSPs obtained by the molecular imprinted techniques. The chiral resolution on these miscellaneous CSPs using liquid chromatography is discussed in this chapter. 

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... 

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). 

Ligand exchange chromatography is a very powerful method for separating enantiomers. However, it is limited to enantiomeric compounds that are able to undergo metal complexes with the chiral stationary phase such as amino acids, amino acid derivatives, and amino alcohols. 

Such cases can be encountered in the case of thin-layer chromatographic separation of amino acids, using copper complexes of long chain amino acids as chiral additives via a ligand exchange approach. The copper complexes of alkyl amino acid chiral additives are so strongly adsorbed on the RP stationary phase that they act as a chiral stationary phase [154-156]. 

A chiral stationary phase is used comprising a silica substrate on which is fixed an amino acid. The mechanisms developed are of the "ligand exchange" type. The distinction between two enantiomers is possible due to the formation of mixed diastereoisomerical complexes (chiral solute - transition metal - amino acid fixed on the silica substrate). The transition metal is added to the mobile phase. (Fig. 4)... 

Since enantiomer separation requires the Intervention of some chiral agent, one may utilize either chiral mobile phase additives (CMPA) or chiral stationary phases (CSPs). While the requirement that one add a chiral substance to the mobile phase has obvious limitations for preparative separations, it is not a serious problem for analytical separations. Indeed, for some types of compounds (e.g. amino acids) this approach may be preferred. Quite an extensive literature exists for the use of mobile phases containing chiral bidentate ligands and copper ions for the "ligand exchange" resolution of underlvatized amino acids (1, 2) and for N-dansyl derivatives of amino acids Tartaric acid derivatives have also been used as CMPAs (5). 

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... 

Chiral ligand exchange chromatography can be performed either on an achiral stationary phase with a chiral mobile phase or on a chiral stationary phase IMAC is performed on the metal ion immobilized stationary phase. The latter two stationary phases may be symbolized by the same formula as... 

Only a few chiral stationary phases for TLC enantiomeric separation have been developed. The separation principle most frequently used is based on a ligand-exchange mechanism. The appropriate commercially available precoated plates consist of optimized RP-carriers impregnated with copper salts and chiral selectors based on amino acids. Typical applications of this separation mechanism are mainly amino acids and their derivatives (229,230), as well as hydroxy carboxylic acids (231). Figure 10 shows a concrete example of the application to separation of a racemic mixture of phenylalanine. 

In the CMPAs method, enantiomeric separation is accomplished by the formation of a pair of transient diastereomeric complexes between a racemic analyte and the CMPA. Chiral discrimination is due to the differences in the intetphase distribution ration, solvatation in the mobile phase, or binding of the complexes to the achiral/chiral stationary phase. Ion pairing, ligand exchange, inclusion complexes, and protein interactions represent the major approaches in the formation of diastereomeric complexes. 

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