Crystal structures of heptakis(2,6-di-O-ethyl)cyclomaltoheptaose [heptakis(2,6-di-O-ethyl)-beta-cyclodextrin]. Solvent-regulated helical assembly of macrocycles

Heptakis(2,6-di-O-ethyl)-beta -cyclodextrin (DE-beta -CD) was crystallized in two forms from hexane and 95% aqueous methanol, respectively: A form I c rystal with the space group P2(1)2(1)2(1) and a form II crystal with the sp ace group P3(1). In both crystals, DE-beta -CD molecules are in a round sha pe with intramolecular O-3-H . . .O-2 hydrogen bonds. In the form I crystal , the DE-beta -CD molecules are arranged along the twofold screw axis to fo rm a helically extended polymeric chain by including the 6-O-ethyl groups o f the adjacent molecule. One hexane molecule with twofold disorder is locat ed in the intermolecular channel along the a-axis. In contrast, the DE-beta -CD molecules in the form II crystal form a helical arrangement along the threefold screw axis. One methanol and one water molecule are included on t he O-6 side of the molecular cavity. The water molecule links the methanol molecule and two ethoxy groups of the adjacent DE-beta -CD molecule with hy drogen bonds. The result suggests the important role of solvent in the form ation of helical arrangement of DE-beta -CD molecules. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.