Cyclodextrin inclusion complexes: Host-guest interactions and hydrogen-bonding networks

An overview is given on the structural characteristics of alpha-, beta- and gamma-cyclodextrins (CDs). These cyclic oligosaccharides consisting of six , seven and eight glucoses form inclusion complexes with guest molecules sm all enough to fit into their central cavities and serve as good model syste ms for non-covalent bonding. Depending on the size and the ionic or molecul ar nature of the guests, the complexes crystallize in cage- or channel-type structures. X-ray and neutron diffraction have been used to study intermol ecular interactions; they provide insight into O-H .. O and C-H ... O hydro gen bonds stabilizing the macrocyclic conformations and the inclusion of th e guest molecules. Throughout the crystal lattices of the CD hydrates, whic h abound with O-H groups, cooperative networks are formed by O--H ... O hyd rogen bonds; in the beta-CD macrocycle, dynamic disorder of the flip-flop t ype is observed, O-(1/2H)...(1/2H)-O, between O(2)-H and O(3)-H groups of a djacent glucoses.