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

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