Selectand/selector complex formation

Information about the stoichiometry of selector-selectand complex is difficult to gain from CE. However, this knowledge is useful in order to characterize the structure of intermolecular complexes as well as for the calculation of the binding constants. Previous research and review papers (3, 4,62,65) summarize the application of this technique to the problems related to chiral CE. As shown in Fig. 4, despite the involvement of different parts of the CL molecule in complex formation, the stoichiometry of CL complexes most likely is the same (1 1) with /3-CD and HDAS-/3-CD (65). 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

Some requirements for complex formation and selector selectand relationships will be briefly discussed. 

Thus, as this example shows, NMR specftometry, which is in general a very powerful technique, may not always be applicable in studies of selector-selectand complexes. One additional example of this kind is shown in Fig. 12 [114]. The data shown in this figure indicate that the continuous variation plot cannot be constructed due to multiple complexations between the DIM and the CM- -CD. Despite this failure, the data shown in [117] are very informative. Besides the aforementioned multiple complex formation, these data indicate that the complexes formed have a different stoichiometry and in addition, the chiral recognition pattern in the complexes with different stoichiometry is opposite to each other. The latter seems to be the unique and the most interesting result of this experiment. 

It should be borne in mind that chiral chromatography is a dynamic process of forming transient noncovalent diastereomeric complexes between chiral solutes and immobilized chiral selectors. Enantioselectivity (a) is a measurement of the thermodynamic stability (A AG) of the two diastereomeric enantiomer/chiral selector complexes. The A AG parameter is the Gibbs free energy difference of the two complex selector-selectand formation. It includes both an enthalpic AAH) and an entropic (TAAS) contributions (Eq. (1)) ... 

The same set of measurements for DNP-Leu as selectand and 0-9-allylcarbamoyl-10,11-dihydroquinidine as selector displayed a considerably different picture. While no indication for H-bond formation between the aromatic a-amino group and the carbamate of the selector could be deduced, the IR spectra of free and respective complexed forms suggested the occurrence of K-K-interactions of the DNP group as evidenced by a shifted C=C stretching vibration from 1594 to 1588 cm in the stronger bound 5-complex (force constant weakened due to delocalized electrons). This type of shift was not found in the weak TJ-complex, but in the corresponding 5-complex of DNP-N-methyl-Leu as well. The spectral data nicely reflect the binding relationships in HPLC where both DNP-Leu and DNP-N-methyl-Leu are well separated with comparable separation factors, but with an elution order that is opposite compared to DNB-Leu. 

One of the main principles for chiral separation used in modern capillary GC is the bonding of the optically active compounds via hydrogen bridges to a stationary-phase material. Feibush and Gil-Av [8] suggested a rapid and reversible formation of association complexes between carbonyl and amide functions of selector and selectand. The formation of diastereomeric associates yields complexes of different stability, depending on the relative configuration. The introduction of dipeptide and diamide phases leads... 

The solvation energy of one enantiomer in the active chiral phase can be described as the contribution of all possible forms of solvent/solute associates. These associates are in equilibrium with fast interconversion rates. Each form contributes to the total free energy according to its particular formation energy and its particular molar fraction [25,26]. These complexes between the selector and selectand should also be as mutually exclusive as possible, to prevent a given interaction from occurring at multiple sites in the diastere-omeric complexes [5]. 

Resolution of optical isomers by chromatography is ascribed to the rapid and reversible formation in the column of diastereomeric complexes between the chiral component of the phase (selector) and the chiral solutes (selectands). If these selector-selectand associates differ sufficiently in their free energy of formation, the resulting differences in partitioning coefficients will lead to the separation of the enantiomers. 

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