Crystal structure of a new α-cyclodextrin hydrate form. Molecular geometry and packing features: disordered solvent contribution

The crystallographic study of a new hydrated form of α-cyclodextrin (cyclohexaamylose) is reported. C36H60O30·11H2O; space group P212121 with cell constants a=13.839(3), b=15.398(3), c=24.209(7) Å; final discrepancy index R=0.057 for the 5182 observed reflections and 632 refined parameters. Besides four ordered water molecules placed outside α-cyclodextrins, the structure shows regions of severely disordered solvent mainly confined in the oligosaccharide cavities. The contribution of the observed disorder has been computed via Fourier inversions of the residual electron density and incorporated into the structure factors in further refinements of the ordered part. The α-cyclodextrin molecule assumes a relaxed round shape stabilised by a ring sequence of all the six possible O2⋯O3 intramolecular hydrogen bonds. The four ordered water molecules take part in an extensive network of hydrogen bonds (infinite chains and loops) without modifying the scheme of intramolecular H-bonds or the (−)gauche conformations of O-6–H hydroxyl groups. The structure shows a new molecular arrangement, for an “empty” hydrated α-cyclodextrin, like that “brick-type” observed for α-CD in the iodoanilide trihydrate complex crystallising in an isomorphous cell.