Crystal structure of heptakis(2,6-di-O-methyl)-β-cyclodextrin dihydrate: a water molecule in an apolar cavity

Heptakis(2,6-di-O-methyl)-β-cyclodextrin crystallized from water at 18 °C as dihydrate (DIMEB·2 H2O) in the monoclinic space group P21 with unit cell dimensions a=15.2415(5), b=10.6391(4), c=23.3239 (8) Å, β=101.798(1)°. X-ray analysis reveals a torus-like macrocycle stabilized by systematic hydrogen bonds O-3H··O-2′ and C-6H··O-5′ between adjacent glucose residues. The O-6CH3 methoxy groups of three glucose units are directed toward but do not close the cavity. Both ends of the cavity are closed by adjacent molecules which are related by the characteristics of ‘cage type’ crystal packing. One water molecule is included in the apolar cavity of the torus and has very high anisotropic thermal displacement factor due to lack of hydrogen bonding interactions to DIMEB, while the other one is located in an intermolecular void and forms hydrogen bonds to O-3 and O-6 oxygen atoms.