Chiral reciprocity principle

In order to generally categorize the reaction schemes mentioned previously and the following ones in the course of indirect enantioseparation techniques, it has to be emphasized again, that the reciprocity principle should always be applicable. This means that if a chiral acid as the CDA can be used successfully to resolve the enantiomers of a chiral amine, then this optically pure amine as the CDA will equally well separate the enantiomers of the acid by the indirect method. The OPA reaction (see Figure 4) is therefore equally well suited for analyzing the optical purity of thiols, amines or amino acids. 

The notion of reciprocity in chiral recognition has played an important role in the design of chiral selectors. In principle, if a single molecule of a chiral selector has different affinities for the enantiomers of another substance, then a single enantiomer of the latter will have different affinities for the enantiomers of the initial selector. In an effort to design a chiral stationary phase capable of separating naproxen, Pirkle et al. [97] first designed two stationary phases in which the carboxyl function of naproxen was linked to a silica matrix... 

Many of the chiral stationary phases have been developed by systematically applying the principle of reciprocity to the enantiomer binding interactions. A number of diverse racemates are analyzed on a chiral stationary phase containing as chiral selector the immobilized target molecule for the enantiomers of which a new selector is desired. The racemate showing the highest enantioselectivity in this system is selected, and... 

Chiral stationary phases (CSPs) developed on the basis of the strategy devised by Pirkle et al. (Pirkle 1980, 1984, 1988, 1992 Wolf 2002) depend on an explicit recognition of the three-dimensional fit between the CSP molecule and the enantiomers of the analyte. This strategy is based on Piride s Principle of Reciprocity (Figure 4.14), namely, that a single enantiomer of a racemate which separates well on one CSP will, when used to produce a second CSP, usually afford separation of the enantiomers of analytes that are structurally similar to the chiral selector of the first CSP. Figure 4.14 also shows two chiral selectors that do exhibit reciprocity, as well as a representation of how the specific interaction between the (S)-form of the first CSP can select for one enantiomer of the second. This is a typical example of the Pirkle-type chiral selectivity. 

In earlier papers, enantioseparations of charged analytes with neutral chiral selectors were attributed to CZE and the enantioseparations of neutral analytes with charged chiral selectors to EKC. However, from the mechanistic point of view, there is no principal difference whether an analyte or a chiral selector is charged. Actually, it is the subject of convention which counterpart of chiral recognition process will be named selectand and which one chiral selector. The reciprocal chiral recognition strategy for a design of effective chiral selectors proposed by Pirkle and co-workers in HPLC is based on this philosophy [9] and that principle certainly applies for enantioseparations in EKC also [10]. 

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