Crystal Packing Patterns of Cyclodextrin Inclusion Complexes

Abstract

Cyclodextrin inclusion complexes crystallize in two basically different patterns, the cage and the channel type. The cage type occurs when cyclodextrins are packed crosswise (fishbone) or, if they are packed side-by-side, in layers and adjacent layers are displaced by about one half molecule. In each case, the internal cavity of one cyclodextrin is closed on both sides by neighbouring cyclodextrins. On the other hand, channel complexes are formed if cyclodextrins are stacked like coins in a roll so that cavities line up to produce long channels. In these crystal structures, cyclodextrins can be arranged in head-to-head or head-to-tail mode. In the smaller α-cyclodextrin, cage type structures are formed with small, molecular guests whereas long molecular guests and ionic guest molecules induce channel type structures. The latter are generally preferred with the γ- and -γ-cyclo-dextrin series which is probably due to the higher tendency for self aggregation in these two members of the cyclodextrin family.