Chiral mechanisms

Working at different temperatures allows one to perform thermodynamic studies which, in some cases, can provide information on the chiral mechanism. Chromatographic methods give the enantiomer retention factors, k. It is relatively easy to measure the k factors at different temperatures. The slope and intercept of the Van t Hoff plots (Ln k versus T) contain, respectively, the enthalpy. A//, and entropy, A5, variations of each enantiomer-selector global (chiral + achiral) interaction. 

Bi-Langmuir adsorption isotherms of enantiomeric pairs and CSPs were determined to gain information on chiral mechanisms. In the few cases fully studied, it was found that the two isomers interacted with type I nonselective sites as well as with type II enantioselective sites [18]. The bi-Langmuir equation is expressed as ... 

The quest for chiral selectors can be arbitrarily separated in two paths the synthetic route and the natural route. The synthetic route studies the chiral molecule evaluating possible interactions (Table 1) and designs a selector that will interact differently with an enantiomeric form than with its mirror image. The natural route follows Pasteur and uses the fact that the living world is made of countless chiral selectors and produces pure enantiomers. Once a natural chiral selector has been selected, it is tested with its natural chiral target(s) and with many other enantiomers. The observation of the results allows estimating a posteriori possible chiral mechanisms. 

It was found that polar enantiomers could be separated with CDs in nonaqueous polar medium (e.g., 99% acetonitrile with 1% methanol). In this situation, inclusion complexation is unlikely, the solvent molecules occupying the CD cavity. The chiral mechanism involves H-bonds with the spatially oriented hydroxyl groups at the rims of the cavity and other interactions with the numerous asymmetric carbons of the glucopyranose units [36]. Polar organic mobile phases were tried with other CSPs and greatly extended their usefulness enhancing the role of H-bond interactions that were screened by water molecules. 

In drawing this preface to a close, while all authors presented their unique point of view on chiral mechanisms in enantiomeric separations, they would like to impress upon the readers that we are still a very long way from fiill understanding of the enantiomer-chiral selector interactions leading to chiral separation. For instance, solvents are used. Solvent effects are very important and yet very difficult to predict accurately. The different author approaches should give an idea to the reader on the complexity of the chiral separation problem. 

Miller, KE. Rich, D.H. Molecular Mechanics Calculations of Cyclosporin A Analogues. Effect of Chirality and Degree of Substitution on the Side-Chain Conformations of (2s, 3r, 4r, 6e)-3-Hydroxy-4-methyl-2-(methylamino)-6-octenoic Acid and Related Derivatives. 

Two mechanisms for chiral separations using chiral mobile-phase additives, analogous to models developed for ion-pair chromatography, have been... 

Most chiral chromatographic separations are accompHshed using chromatographic stationary phases that incorporate a chiral selector. The chiral separation mechanisms are generally thought to involve the formation of transient diastereomeric complexes between the enantiomers and the stationary phase chiral ligand. Differences in the stabiHties of these complexes account for the differences in the retention observed for the two enantiomers. Often, the use of a... 

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. 

Chiral separations present special problems for vaUdation. Typically, in the absence of spectroscopic confirmation (eg, mass spectral or infrared data), conventional separations are vaUdated by analysing "pure" samples under identical chromatographic conditions. Often, two or more chromatographic stationary phases, which are known to interact with the analyte through different retention mechanisms, are used. If the pure sample and the unknown have identical retention times under each set of conditions, the identity of the unknown is assumed to be the same as the pure sample. However, often the chiral separation that is obtained with one type of column may not be achievable with any other type of chiral stationary phase. In addition, "pure" enantiomers are generally not available. 

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). 

Although the chiral recognition mechanism of these cyclodexttin-based phases is not entirely understood, thermodynamic and column capacity studies indicate that the analytes may interact with the functionalized cyclodextrins by either associating with the outside or mouth of the cyclodextrin, or by forming a more traditional inclusion complex with the cyclodextrin (122). As in the case of the metal-complex chiral stationary phase, configuration assignment is generally not possible in the absence of pure chiral standards. 

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