Cyclodextrins, and some analogs, as hosts

Abstract

The α-, β-, and γ-cyclodextrins are oligosaccharides containing 6, 7, and 8 glucose units, respectively, linked between 1 and 4 positions to form macrocyclic rings of toroidal or truncated-cone shape; the secondary hydroxyl side is conventionally defined as the ‘head’ of the cone. The inner and outer surfaces of the toroids are hydrophobic, while the upper and lower faces are hydrophilic. When crystallized from aqueous solutions, the cyclodextrins form inclusion complexes with a large variety of guest molecules ranging from inert gases through polyiodide salts to aromatics. The crystals contain an appreciable number of water molecules, which are hydrogen bonded together and also to the host molecules; the amount of water varies. There are two types of general structure — the clathrates and the tunnel inclusion complexes — and each of these types contains a number of isomorphous or isostructural classes; about two-thirds of the complexes of known structure are of the tunnel type. The guest molecules are generally enclosed within the cavities of the toroids in a variety of packings; there are sometimes water molecules within the cavities in addition to those located between the cyclodextrins, and guests are sometimes found between the host molecules. If the guest is a salt, linear anions such as polyiodides are found within the cavities, while the counter cations are included in the water network. Formation of hydrogen-bonded cyclodextrin dimers is a feature of the tunnel inclusion complexes, and these dimers are mainly head-to-tail in the α-cyclodextrin complexes, predominantly head-to-head in the β-cyclodextrin complexes, and with all three possibilities in the (mostly isomorphous) γ-cyclodextrin complexes so far studied. The catalytic properties of cyclodextrins in organic reactions and their mimicry of enzyme behaviour is ascribed to the manner in which the guest is held in a fixed position and orientation by the host even in solution. Cyclodextrins are used in the pharmaceutical industry because of their water solubility, the innocuous nature of their degradation products, and their ability to encapsulate drug molecules and release these slowly. Analogues to the cyclodextrins are currently being developed.