Cyclodextrins are oligosaccharides formed from the bonding of six to twelve glucopyranose (sugar) units into a cyclic, cone shaped structure. The number of sugar units varies from 6 to 12. Three sizes are commonly used in the manufacture of commercially available GC phases: alpha, beta & gamma corresponding to 6, 7 and 8 glucopyranose units respectively.
The cyclodextrin molecule forms a truncated conical cavity, the diameter of which depends on the number of glucopyranose units. For chiral resolution to occur, a portion of the molecule must enter the cyclodextrin cone cavity.
The selectivity of a cyclodextrin phase is dependent on the size of the analyte. Alpha-cyclodextrin will include single phenyl groups or napthyl groups endon.
Beta-cyclodextrin will accept napthyl groups and heavily substituted phenyl groups. Gamma-cyclodextrin is useful for bulky steriod-type molecules.
Overall, the beta form has been found to have the widest application.
DR-Cyclodextrin columns: Phase composition
Extensive substitution of the cyclodextrin backbone has yielded many different commercially available phases dot-red cyclodextrin columns are based on substituted beta- and gamma- cyclodextrin.
|Phase composition||Phase code|
Resulting columns offer useful differences in efficiency, sample capacity and, most importantly, enantiomeric selectivity.