Introduction

Cyclodextrins are cyclic nonreducing oligosaccharides containing from six to twelve glycose units in a C-1 chair conformation, bonded through a-(1,4) linkages (Figure 1). The glycopyranose units are arranged in the shape of a hollow truncated cone. The larger opening of the molecule is surrounded by the secondary (C-2 and C-3) hydroxyl groups, while the primary (C-6) hydroxyl groups constitute the smaller end of the cone.

Since all the primary and secondary hydroxyl groups are located at the outside of the molecule, the exterior faces are hydrophilic. The interior cavity, essentially comprised of methylene linkages and glycosidic oxygen bridges, is relatively hydrophobic in comparison with polar solvents such as water. Furthermore, the glycosidic oxygen bridges produce a high electron density, giving the interior of the cavity a slightly Lewis-base character.

The three smallest cyclodextrin homologues are readily commercial available:

• a-cyclodextrin (cyclohexamylose);

• ft-cyclodextrin (cycloheptamylose);

• y-cyclodextrin (cyclooctamylose);

The basic property of cyclodextrins is their ability to form selective inclusion complexes with a broad variety of organic and inorganic molecules.

The formation of inclusion complexes is in general determined by the ability of the guest molecule to closely fit the cavity of the cyclodextrin. However, the polarity of the guest molecule also plays an important role.

Inclusion complexes are usually formed in the presence of water or in water mixed with organic modifiers.

Figure 1 Numbering of carbon atoms in the cyclodextrin ring structure.

One of the first effective uses of cyclodextrins in chromatography was as mobile phase additive in thin-layer chromatography. In the mid-1980's a process was developed to produce stable cyclodextrin high performance liquid chromatographic (HPLC) phases. Nowadays, native a-, ft- and y-cyclodextrin, as well as a variety of derivatized cyclodextrin HPLC columns are commercially available. Also many cyc-lodextrin-based capillary gas chromatography (GC) columns are on the market. With the growing importance of capillary electrophoresis in chiral separations, the use of native cyclodextrin and cyclodextrin derivatives as an electrolyte additive steadily increases.

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