Crystal structure of hexakis(2,6-di-O-methyl)-alpha-cyclodextrin-acetonitrile dihydrate: a channel formed by methyl groups harbors a chain of five partially occupied water sites

Hexakis(2,6-di-O-methyl)-alpha-cyclodextrin (DIMEA) crystallizes from 1:1 w ater-acetonitrile as DIMEA-acetonitrile-dihydrate in the orthorhombic space group P2(1)2(1)2(1), unit cell dimensions a = 14.2775(5), b = 15.7312(5), c = 31.1494(11) Angstrom. Refinement of the structure against 5540 X-ray di ffraction data converged at an R factor of 0.083. The macrocycle exhibits a 'round' conformation and is stabilized by intramolecular, interglucose O-3 -H(n)... O-2(n + 1) and C-6-H(n)... O-5(n + 1) hydrogen bonds. Acetonitrile is included in the central cavity of DIMEA and held in position by C-5-H . .. N interactions. The two water molecules in the asymmetric units are dist ributed over six sites. One is fully occupied due to hydrogen bonding to O- 3 groups of two symmetry-related DIMEA molecules, whereas the five remainin g sites show occupancies between 0.15 and 0.25. These sites are in hydrogen bonding contact with O ... O distances between 2.59 and 3.50 Angstrom and are located in infinite, hydrophobic channels parallel to the a-axis, which are coated with methyl groups of symmetry-related DIMEA. (C) 1999 Elsevier Science Ltd. All rights reserved.