Liquid Chromatography - Cyclodextrin 2
The mouth of the torus-shaped cyclodextrin molecule has a larger circumference than at the base and is linked to secondary hydroxyl groups of the C2 and C3 atoms of each glucose unit (see figure 55). The primary hydroxyl groups are located at the base of the torus on the C6 atoms. As these hydroxyl groups are free to rotate, they partially block the base aperture. The size of the cavity increases with increasing number of glucose units (figure 54). The secondary hydroxyl groups can be reacted with appropriate reagents to introduce further interactive character to the cyclodextrin molecule. The very effective chiral characteristics of the cyclodextrins structures arise from the many chiral centers they contain, for example, b-cyclodextrin has 35 stereogenic centers.
Courtesy of Supelco
Figure 55 A Molecular Model of Cyclodextrin
When the a, b, or g cyclodextrins are derivatized, the hydroxyl group on the 2-position reacts first. However, the derivative is still size selective and interaction will be determined by the size and functional groups contained by the interacting molecule. Derivatizing the 6-hydroxyl position has little or no effect on chiral selectivity but does enhance the loading capacity of the stationary phase. This position is used for anchoring the cyclodextrins to silica gel in the preparation of LC stationary phases.