Mechanisms of Chiral Resolution

As discussed above, most chiral resolutions of environmental pollutants have been carried out on CDs and their derivative-based CSPs in GC. 

Therefore, in this section, the mechanisms of chiral resolution of environmental pollutants on CD-based CSP will be discussed. The mechanisms of chiral resolution in gas and liquid chromatography are more or less similar. There are very few reports dealing with chiral recognition mechanisms in GC however, some attempts have been made to describe chiral recognition mechanisms in liquid chromatography. Therefore, the approaches used for liquid chromatography may be utilized to explain chiral recognition mechanisms in GC. 

In spite of the development of more advanced techniques such as HPLC and CE, GC still maintains its status in the chiral resolution of environmental pollutants due to its various advantages. GC is considered to be the 

Groger, E. Katharina and A. Woyke, Cyclodextrins, Science Forum an der Universitat Siegen, Siegen, 2001. 

Stalcup, in G. Subramanian, ed., A Practical Approach to Chiral Separations by Liquid Chromatography, VCH, Weinheim, 1994, p. 95. 

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FIGURE 27 Graphical representation of the mechanism of chiral resolution on a polysaccharide-based CSP. 

The cellulose derivatives used for chiral TLC are trisphenylcarbamate, 2,3-dichlorophenylcarbamate, 2,4 -dichlorophenylcarbamate, 2,6 -dichlorophenylcar-bamate, 3,4-dichlorophenylcarbamate, 3,5-dichlorophenylcarbamate, 2,3-dimethyl-phenylcarbamate, and 3,5-dimethylphenylcarbamate. Aboul-Enein et al. [178] have reviewed the chiral resolution of racemates on polysaccharide chiral TLC plates. They discussed the role of the substituents of polysaccharide derivatives on chiral resolution. The effects of the substituents of cellulose derivatives and the mechanisms of chiral resolution on these plates are similar to what is found for HPLC CSPs. 

Kremer et al. [123] observed the hydrophobic pockets as the binding site on AGP protein. However, more than one binding site was reported. Haupt et al. [124] presented a retention model for the chiral resolution of uncharged solutes, felodipine, on AGP and the model has assumed the presence of two different stereoselective sites for different enantiomers. In another study, Waters et al. [125] carried out certain thermodynamic experiments for the determination of the mechanism of chiral resolution on AGP protein. The authors reported the two... 

CL Cooper, JB Davis, MJ Sepaniak. Mechanisms of enantiomeric resolution in cyclodextrin-modified capillary electrophoretic separations of binaphthyl compounds. Chirality 7 401-408, 1995. 

For applications and development of chiral resolution methods, it is essential to have knowledge of the chiral resolution mechanism on polysaccharide-based CSPs. At the molecular level, this mechanism is still unclear because of difficulties associated with spectroscopic studies, as discussed earlier. Nevertheless, some experimental efforts have been made, and chiral resolution reportedly has been achieved through different types of bonding on the chiral grooves of polysaccharide-based CSPs. 

As a further test of the etched open tubular approach for the analysis of optical isomers, another column was fabricated based on the selector naphthylethylamine that had been attached to porous silica by the silanization/hydrosilation method for use in HPLC [70]. As in the HPLC experiments, this column was best suited for the resolution of the optical isomers of dinitrobenzoyl methyl esters of amino acids. The best separation (a = 1.14) was obtained for the alanine derivative. In addition, the peak symmetry and efficiency for the naphthylethylamine column was significantly better than that obtained on the cyclodextrin column. However, as shown in HPLC experiments, changes in the amino acid moiety (replacing alanine with valine, etc.) often results in a loss of chiral resolution. In the case of optical isomers, the separation mechanism in HPLC and CEC modes is identical since only interaction between the solute and the bonded phase can result in resolution of the enantiomers. 

The mechanism of chiral recognition of the enantiomers in series of clathrochelates was studied in research reported in Ref. 311, in which [Sb2(cZ-tart)2]2- anion was employed as a chiral eluent. It was established that in the majority of cases, A-enantiomer is eluted first, and that bulky alkyl capping groups increase the degree of resolution. 

The many data which have accumulated through chromatographic studies of resolution permit a) to single out systems which seem suitable for more detailed investigations of the intimate molecular mechanism of chiral recognition by methods such as X-ray crystallography and b) to correlate the influence of structural factors of selectors and selectands on stereoselectivity. 

The dependence of chiral recognition on the formation of the diastereomeric complex imposes constraints on the proximity of the metal binding sites, usually either an hydroxy or an amine a to a carboxyHc acid, in the analyte. Principal advantages of this technique include the abiHty to assign configuration in the absence of standards, enantioresolve non aromatic analytes, use aqueous mobile phases, acquire a stationary phase with the opposite enantioselectivity, and predict the likelihood of successful chiral resolution for a given analyte based on a weU-understood chiral recognition mechanism. 

The first successful chiral resolutions through enantioselective membranes have been published recently, but few cases are applicable to the preparative scale, mainly due to mechanical and technical limitations. Low flow rates, saturation of the chiral selectors and loss of enantioselectivity with time are some of the common problems encountered and that should be solved in the near future. 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

In another study, the authors reported a comparative study of the enantiomeric resolution of miconazole and the other two chiral drugs by high performance liquid chromatography on various cellulose chiral columns in the normal phase mode [79], The chiral resolution of the three drugs on the columns containing different cellulose derivatives namely Chiralcel OD, OJ, OB, OK, OC, and OE in normal phase mode was described. The mobile phase used was hexane-isopropanol-diethylamine (425 74 1). The flow rates of the mobile phase used were 0.5, 1, and 1.5 mL/min. The values of the separation factor (a) of the resolved enantiomers of econazole, miconazole, and sulconazole on chiral phases were ranged from 1.07 to 2.5 while the values of resolution factors (Rs) varied from 0.17 to 3.9. The chiral recognition mechanisms between the analytes and the chiral selectors are discussed. 

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