Chiral recognition techniques

Chiral recognition techniques have been used in photoinduced electron transfer reactions and the differential formation of homochiral against heterochiral exciplexes. In the systems studied excited states of lanthanide complexes are quenched by transition metal complexes. The complexes with trigonal dihedral (D3) symmetry which can exist as enantiomers with either left handed (a) or right-handed (A) configurational chirality are chosen. The system consists of a mixture of racemic lanthanide complex and optically resolved... 

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. 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

Macaudiere and co-workers performed a comparison of LC and SFC on a polymer based-CSP (Chiralpak OT) [64]. The chromatographic behavior of this CSP seemed to be quite different in SFC than in LC, although satisfactory separations were achieved with both techniques. The chiral recognition mechanisms may be altered by the nature (hexane-based or CO,-based) of the eluent. 

Compared to other chiral separation techniques, the practical importance of CLEC is relatively low because of its complexity. Nevertheless, CLEC is one of the best-investigated techniques from the theoretical point of view. Many theoretical concepts developed in CLEC were of general interest for the explanation and prediction of chiral recognition in all chromatographic systems. 

In summary, molecular modeling when used in combination with instrumental techniques, especially 1D-ROESY and X-ray crystallography, may significantly contribute to understanding the nature of the intermolecular forces responsible for chiral drug-cyclodextrin interactions and chiral recognition. 

A selection of the most successful CSPs, chiral particles and chiral additive techniques used for analytical and preparative enantioseparation by LC is discussed in the following sections with respect to molecular recognition and experimental application. As additional sources of background information recent books and review articles2-16, which contain numerous relevant references and examine the most important aspects of the field of liquid chromatographic enantioseparation, should be consulted. 

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