Chiral separation mechanisms

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 Ki for HSA binding to racemic warfarin has been reported for 3-6 pM by various techniques, including frontal analysis and equilibrium dialysis, and is temperature- and pH-dependent. See Loun, B., Hage, D.S. Chiral separation mechanisms in protein-based HPLC columns. 1. Thermodynamic studies of (R)- and (S)-warfarin binding to immobilized human serum albumin. Anal. Chem. 1994, 66, 3814-3822. 

TABLE 3 Chiral Separation Mechanisms, Analyte Requirements, and Modes of Operation for Different Chiral Stationary-Phase Types... 

CSP type Chiral separation mechanism Analyte requirements Mode of operation... 

Loun B, Hage DS. Chiral separation mechanisms in protein-based HPLC columns, n. Kinetic studies of R- and S-warfarin binding to immobilized human serum albumin. Anal Chem 1996 68 1218-25. 

Chemically modiHed soibents] soibents and precoated layers for ion-exchange chromatography, 115-119 Chiral separation mechanism direct separation of enantiomers on TLC plates coated with chiral compounds, 635 enantiomeric separation on adsorbents with chiral cavities, 622-626... 

The degree of substitution in derivatized CDs may affect the chiral separation mechanism [21], The changes in enantioselectivity of a derivatized CD were attributed to the loss of hydrogen bonding groups in the or-position to the chiral centers of the CD molecule, or to a gain of additional interaction sites around the base of the derivatized CD molecule [27]. 

TLC and PC also are LC techniques. Same stationary phases are used in both LC and TLC [10]. The chiral separation mechanism that governs separation in LC has been extensively studied and is applicable to PC. Thus, it will be discussed here. 

Today, a more pertinent classification should be proposed in accordance with the great number of chiral selectors reported in LC (about 1500 CSPs have been collected in ChirBase database) and our new knowledge of chiral separation mechanisms. For instance, chromatographic results are clearly suggesting that bimodal and supramolecular systems should be gathered quite separately. 

Chiral separation mechanisms are often well explained, and models may offer reliable prediction of the elution orders. Figure 7.2 shows a LEC recognition model adapted from Davankov et al. [35]. 

Booth and Wainer [50] have demonstrated the role of multiple hydrogenbonding interactions in the stability of the diastereomeric complex occurring throughout conformational changes of both ATPC chiral selector and solute. Authors pointed out in this study that chiral recognition does not fit a three-point interaction mechanism. Other aspects of chiral separation mechanisms on polysaccharide triphenylcarbamate derivatives have been recently reviewed by Yamamoto andOkamoto [51],... 

The first part of the book examines the fundamental principles of chirality and TLC. It describes the necessary materials, laboratory equipment, procedures, and strategies for the separation, quantification, isolation, and analysis of chiral compounds. The second part evaluates the reaUworld enantioseparations and densitometric analyses. Emphasizing pharmaceutical applications, the book examines the chiral separation mechanisms and methods for analyzing the chiral purity of diastereoisomers, amino acids, beta-blockers, and NSAIDS. Topics also include commercial stationary phases and chiral modifiers of mobile phases. 

---