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

Heptakis(2,6-di-O-methyl)-beta-cyclodextrin crystallized from water at 18 d egrees C as dihydrate (DIMEB.2 H2O) in the monoclinic space group P2(1) wit h unit cell dimensions n=15.2415(5), b=10.6391(4), c=23.3239 (8) Angstrom, beta = 101.798(1)degrees. X-ray analysis reveals a torus-like macrocycle st abilized by systematic hydrogen bonds O-3-H .. O-2' and C-6-H .. O-51 betwe en adjacent glucose residues. The O-6-CH3 methoxy groups of three glucose u nits are directed toward but do not close the cavity. Both ends of the cavi ty are closed by adjacent molecules which are related by the characteristic s of 'cage type' crystal packing. One water molecule is included in the apo lar cavity of the torus and has very high anisotropic thermal displacement factor due to lack of hydrogen bonding interactions to DIMEB, while the oth er one is located in an intermolecular void and forms hydrogen bonds to O-3 and O-6 oxygen atoms. (C) 1999 Elsevier Science Ltd. All rights reserved.