CRYSTAL STRUCTURE OF a-CYCLODEXTRIN-ACETONITRILE-HEXAHYDRATE
Helga Hoier, Thammarat Aree and Wolfram Saenger
Institut für Kristallographie, Freie Universität Berlin,
Takustr.6, D-14195 Berlin, Germany
E-mail: hoier@chemie.fu-berlin.de
Keywords: cyclodextrin, hydrogen bonding,
inclusion complex
a-Cyclodextrin is a cyclic carbohydrate comprised of 6 D-(+)-glucopyranoside units linked by a-( 1-- > 4)-glycosidic bonds. The ring is conformationally stabilized by intramolecular hydrogen bonds formed between secondary hydroxy groups O2-H and O3-H of neighboring glucose units. The cavity of the torus-like macro cycle with 4.5 A in diameter and 6.7 A in depth is large enough to accommodate small hydrophobic as well as hydrophilic guest molecules. Depending on their size, the guest molecules may not fill the whole cavity and are statistically disordered or exhibit high thermal motions [1,2]. Here we report the structure of an a-cyclo-dextrin-acetonitrile hexahydrate complex and analyze the network of hydrogen bonding in the crystal lattice [3].
Crystals of the ternary inclusion complex belong to the orthorhombic space group P212121 with unit cell dimensions a = 9.479(2), b = 14.323(4), c = 37.397(9) A. The a-cyclodextrin forms a regular macro cycle. Within the cavity, one acetonitrile and one water molecule are located with occupancy factors of 0.8 and 0.7, respectively. They are hydrogen bonded to disordered O65B, and acetonitrile is stabilized in position by C5-H…N interactions. The remaining five water molecules link symmetry related cyclodextrin molecules.