Methods to Study Mechanisms

Information on chiral recognition mechanisms is mostly obtained by studying differences between binding energies of enantiomers and a chiral selector. Table 2 lists the different methods. 

Spectroscopic methods can work with the chiral selector associated with the ligand either in solid state or in solution. The chiroptical spectroscopies, circular dichroism, and optical rotatory dispersion, represent an important means for evaluating structural properties of selector-ligand adducts [14]. NMR can specifically investigate proton or carbon atom positions and differentiate one from the other. X-ray crystallography is a powerful technique to investigate the absolute configuration of diastereoisomeric complexes but in the solid state only. Fluorescence anisotropy is a polarization-based technique that is a measure, in solution, of the rotational motion of a fluorescent molecule or a molecule + selector complex [15]. 

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. 

is a perfect gas constant, T is the absolute temperature (°C + 273 in Kelvin) and 4 is the column phase ratio (ratio of the stationary phase volume over the mobile phase volume). 

Comparing the selectivity values a (ratio of the two retention factors k over fe) for the two enantiomers gives information on the enantioselective part of the interaction [16]. 

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