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 small enough to fit into their central cavities and serve as good model systems for non-covalent bonding. Depending on the size and the ionic or molecular nature of the guests, the complexes crystallize in cage- or channel-type structures. X-ray and neutron diffraction have been used to study intermolecular interactions; they provide insight into O-H .. O and C-H ... O hydrogen bonds stabilizing the macrocyclic conformations and the inclusion of the guest molecules. Throughout the crystal lattices of the CD hydrates, which abound with O-H groups, cooperative networks are formed by O--H ... O hydrogen bonds; in the beta-CD macrocycle, dynamic disorder of the flip-flop type is observed, O-(1/2H)...(1/2H)-O, between O(2)-H and O(3)-H groups of adjacent glucoses.