Water Molecule in Hydrophobic Surroundings: Structure of α-Cyclodextrin-Hexahydrate (C6H10O5)6·6H2O

α-CYCLODEXTRIN (cyclohexaamylose) is the smallest member of the family of Schardinger dextrins, a group of cyclic D-glucosyl oligosaccharides each consisting of six to nine α-(1→4) linked glycopyranoses. These torus shaped molecules have voids of approximate diameter 6–9 A and are capable of forming inclusion complexes with a variety of “guest” molecules1,2. In the crystalline state, these complexes can exist as either “cage” or “channel” structures, depending on whether the cyclodextrins align themselves so that the voids form continuous, extended channels. At least eight different, non-isomorphous crystalline modifications of α-cyclodextrin with various different guest molecules are known. Cyclodextrin inclusion complexes can also exist in aqueous solution. As it has been proposed3 that simultaneously with complex formation the included solvent (water) is released, for an understanding of the mechanism of complex formation it is important to determine how many and in what manner solvent molecules are bound to the interior of the cyclodextrin ring. The structure we report here is that of α-cyclodextrin · 6H2O (αCDH2O), the “empty” inclusion complex, and is of the cage type.