![]() ![]() ![]() The reciprocal principle was later extended by Pirkle et al. The converse of this phenomenon also obtains amine enantiomers may have nonequivalent nmr spectra in optically active alcohols and Pirkle and Hoover concluded that the roles of an enantiomeric solute and a CSA may be interchangeable for a given pair of compounds. stated: the enantiomers of secondary and tertiary alcohols may be caused to have nonequivalent nmr spectra through the use of appropriate optically active amines as solvents. The principle of reciprocity by inverting the role of selectand and selector had first been demonstrated via the differentiation of enantiomers by chiral solvating agents (CSA) in NMR spectroscopy. Indeed, it was subsequently demonstrated that P-(+)-hexahelicene-7,7′-dicarboxylic acid disodium salt could be used as the chiral stationary phase (CSP) as π-donor selector for the enantioseparation of N-2,4-dinitrophenyl(DNP)-α-amino acid esters as π-acceptor selectands by HPLC. Thus, in a reciprocal fashion, if the selectands A1 and A2 are chromatographically separated on the selector B1 (or B2), the selectands B1 and B2 are separated on the selector A1 (or A2) as well, whereby the respective pairs A1/A2 and B1/B2 refer to isomers (e.g., stereoisomers) or belong to members of homologous series of compounds ( Figure 2). Origin of the Reciprocal Principle in Chromatographyįollowing the enantioseparation of –helicenes on the optically active charge-transfer agent R-2-(2,4,5,7-tetranitro-9-fluorenylideneamino-oxy)propionic acid (TAPA, Figure 1) or its 2-butyric acid homologue (TABA), Mikeš speculated that the function of the selector and the selectand could be reversed, i.e., optically active helicenes could be used as resolving agents for racemic compounds. Pertinent examples will be described in the following Sections. This excludes proteins, carbohydrates and antibodies, although they represent versatile chiral stationary phases (CSPs). In the special case of chiral recognition, both enantiomers of selectand and selector must be available. Thus the reciprocal recognition principle applies to selective separation system when selectors and selectands change their role and belong to two or more forms of congeners, analogues, isomers etc. The best identified candidate is then used as the optimized lead stationary phase for the target selectands to be separated. One molecule of the selectand is now used as the stationary phase and an array of potential structural related selectors are now employed as analytes in the mobile phase. In order to find an optimized selector for a pair of selectands, the role of selector/selectand can be reversed by the reciprocal principle. ![]() The selector is either present as a stationary phase or as an additive to the liquid mobile phase. In chromatographic partitioning systems, selectors are employed to separate mixtures of selectands which are added to the mobile phase. This terminology extends to all supramolecular interactions such as antibody-antigen, receptor-substrate, metal ion-ligand etc. The new terms were coined in analogy to Ashby’s cybernetic operator-operand terminology. The designations selector and selectand were introduced by Mikeš into chromatography to avoid ambiguities that may arise from the use of the notations solvent-solute, ligand-substrate, or host-guest. Separation approaches in highly selective supramolecular chromatography, with their most refined expression of stereochemical resolution, rely on the fast and reversible non-covalent association equilibrium between functionally and/or structurally complementary partners. ![]()
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