TOPOGRAPHY OF CYCLODEXTRIN INCLUSION COMPLEXES .26. STRUCTURE OF THE GAMMA-CYCLODEXTRIN 1-PROPANOL-17H2O INCLUSION COMPLEX

The crystal structure of the hydrated 1-propanol inclusion complex of gamma-cyclodextrin (gamma-CD), C48H80O40.C3H8O.17.0H2O, was determined from two sets of X-ray diffraction data collected from two crystals grown in different batches; the results are practically identical except for a few occupancy factors of water molecules and hydrogen-bonding interactions involving O(6) and water molecules. Both crystals have tetragonal space group P42(1)2 with cell dimensions: a = b = 23.840 (5), c = 23.227 (5) angstrom, and a = b = 23.8093 (5), c = 23.2069 (6) angstrom, respectively, Z = 6. The structures were refined by the blocked full-matrix least-squares method to R factors of 0.067 for 5029 unique observed reflections [I greater-than-or-equal-to 3-sigma(I)] and 0.082 for 4303 unique observed reflections, respectively. The gamma-CD molecules are stacked on top of each other, their axes coinciding with the crystal fourfold rotation axis. Within the same stack, three independent gamma-CD molecules are in tail-to-tail, head-to-head and head-to-tail arrangements, and hydrogen bonds are formed between the adjacent gamma-CD molecules with the O(2), O(3) and O(6) hydroxyls. The cavities of the gamma-CD molecules form endless 'major' channels. They accommodate 3.1 water molecules distributed over eight sites and the 1-propanol molecules which, however, could not be traced unambiguously owing to crystallographic disorder around the fourfold axis. The 'minor' interstitial channels between the stacks of gamma-CD molecules are filled with 9.7 water molecules distributed over 12 sites which are involved in a complicated hydrogen-bonding network to primary and secondary hydroxyls of gamma-CD molecules. There are 12.8 H2O in the asymmetric unit, or 12.8 x 8/6 = 17.0 H2O per gamma-CD molecule, as given in the title. All glucose residues are in the usual 4C1 chair conformation with minor distortion. In the series alpha-, beta-, gamma-CD, the C(4)-O(4)-C(1)' interglucose bond angles reduce from 119.0 to 117.7 to 116.8-degrees. The average O(2)...O(3)' intramolecular hydrogen-bonding distances between adjacent glucoses for gamma-CD is 2.83 angstrom, much shorter than for alpha-CD (about 3.00 angstrom), but nearly the same as for beta-CD, indicating a tighter ring of hydrogen bonds around the O(2), O(3) rims in beta- and gamma-CD than in alpha-CD. The O(6) hydroxyl groups of glucoses 1 to 5 are twofold disordered with the major occupation sites pointing 'away' from the molecular cavity, while the minor occupation sites point 'inward'.