Chiral recognition mechanisms involved

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. 

Nonaqueous enantioseparations in CE have been reported since 1994 [62]. Nonaqueous buffers offer certain advantages compared to aqueous buffers from the viewpoints of alternative chiral recognition mechanisms involved in the separation [63], lower electric current and Joule heat generation, higher solubility and stability of certain analytes and chiral selectors [64-67], easier online coupling to a mass spectrometer, etc. 

The chiral recognition mechanism for these types of phases was attributed primarily to hydrogen bonding and dipole—dipole interactions between the analyte and the chiral selector in the stationary phase. It was postulated that chiral recognition involved the formation of transient five- and seven-membered association complexes between the analyte and the chiral selector (117). 

It is true that the unambiguous elucidation of chiral recognition mechanisms on various protein-based CSPs is challenging and often difficult since precise information about the tertiary and quaternary stmctures of proteins is not always available. Multiple stereo-specific sites may be involved in chiral recognition process. However, it is encouraging to see the progresses that have been made in this field in recent years [17, 95—102]. 

As discussed in Sect. 3.5, the interactions involved in the chiral recognition on Pirkle-type CSPs are mainly attractive forces, such as k-jt, hydrogen-bonding, and dipole-dipole interactions. Although bonded Pirkle-type CSPs have been used in reversed phase and polar nonaqueous mobile phase, most of the applications were found in normal-phase mode. With the introduction of SFC for the resolution of enantiomers [185], bonded Pirkle-type CSPs were among the most studied CSPs in the early application of chiral SFC [172, 175, 181, 186], Comparable enantioselectivity and the same elution order of enantiomers were usually observed for the enantioseparations of many compounds. Accordingly, similar chiral recognition mechanisms were believed to operate in both LC and SFC conditions [186]. However, when the enantioseparations of jt-acidic compounds on the n-acidic... 

One of the most recently reported CCC chiral separations where chiral recognition occurs in the aqueous phase involves the separation of gemifloxacin enantiomers using (-(-)-(18-crown-6)-tetracarboxylic acid (I8C6H4) as CS [37]. This CS has been previously used in CE to resolve the enantiomers of chiral primary amines. The macrocyclic polyether ring in ISCeHq structure forms stable inclusion complexes with protonated primary amines. This interaction is the basis of the chiral recognition mechanism. 

A number of specialised stationary phases have been developed for the separation of chiral compounds. They are known as chiral stationary phases (CSPs) and consist of chiral molecules, usually bonded to microparticulate silica. The mechanism by which such CSPs discriminate between enantiomers (their chiral recognition, or enantioselectivity) is a matter of some debate, but it is known that a number of competing interactions can be involved. Columns packed with CSPs have recently become available commercially. They are some three to five times more expensive than conventional hplc columns, and some types can be used only with a restricted range of mobile phases. Some examples of CSPs are given below ... 

More than 100 CSPs are commercially available nowadays, which should make the separation of any pair of enantiomers feasible. However, the enantiorecognition mechanisms involved in the chiral recognition between the analytes and the CSPs are complex and therefore the selection of the appropriate CSPs, depending on the structure of the analyte, is a difficult task. A common approach to develop a new enantioseparation is the stepwise trial-and-error approach based on detailed consideration of the enantiorecognition mechanisms between the chiral selector and the analyte, or on the analyst s experience, or on the consultation of literature or databases. However, this approach is time-consuming and often unsuccessful owing to the fact that achieving enantioresolution is often purely empirical... 

The last column of Table 7 contains the chiral selector and the third column describes the type of interaction between the selector and the substrate, i.e., the mechanism of the chiral recognition involved. 

Figure 14. Model of two competing mechanisms for chiral recognition involving intermolecular rc-n interactions and dipole-dipole stacking and/or hydrogen bonding. Reprinted with permission from...

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