Inclusion complexation, optical compounds, cavities

Additives that specifically interact with an analyte component are also very useful in altering the electrophoretic mobility of that component. For example, the addition of copper(II)-L-histidine (12) or copper(II)-aspartame (54) complexes to the buffer system allows racemic mixtures of derivatized amino acids to resolve into its component enantiomers. Similarly, cyclodextrins have proven to be useful additives for improving selectivity. Cyclodextrins are non-ionic cyclic polysaccharides of glucose with a shape like a hollow truncated torus. The cavity is relatively hydrophobic while the external faces are hydrophilic, with one edge of the torus containing chiral secondary hydroxyl groups (55). These substances form inclusion complexes with guest compounds that fit well into their cavity. The use of cyclodextrins has been successfully applied to the separation of isomeric compounds (56), and to the optical resolution of racemic amino acid derivatives (57). 

The retention index calculation of optically active compounds can be considered as a troublesome issue due to complex inclusion complexation retention mechanisms on CD stationary phases if a homologous series, such as the n-alkanes, is used, the hydrocarbons randomly occupy positions in the chiral cavities. As a consequence, n-alkanes can be considered as unsuitable for retention index determinations. Nevertheless, other reference series can be employed on CD stationary phases, such as linear chain FAMEs and FAEEs. However, retention indices are seldom reported for optically active compounds, and publications refer to retention times rather than indices. 

Many types of chiral stationary phase are available. Pirkle columns contain a silica support with bonded aminopropyl groups used to bind a derivative of D-phenyl-glycine. These phases are relatively unstable and the selectivity coefficient is close to one. More recently, chiral separations have been performed on optically active resins or cyclodextrins (oligosaccharides) bonded to silica gel through a small hydrocarbon chain linker (Fig. 3.11). These cyclodextrins possess an internal cavity that is hydro-phobic while the external part is hydrophilic. These molecules allow the selective inclusion of a great variety of compounds that can form diastereoisomers at the surface of the chiral phase leading to reversible complexes. 

Similarly, 6°-metacyclophane forms clathrate complexes with benzene derivatives or alicyclic compounds, where only a little guest selectivity has been found ) The ratio n/m of these crystal lattice (cavity) inclusion compounds varies significantly, e.g., n/m=0.3-0.6 (cyclotricatechylene), 2-6 (tri-O-thymotide). It has been found that tri-O-thymotide resolves optical isomers, e.g., 2-butyl halides, by making host-guest inclusion crystals. These optical resolutions are therefore attributable to the crystal lattice chirality that was induced... 

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