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

Heptakis(2,6-di-O-ethyl)-β-cyclodextrin (DE-β-CD) was crystallized in two forms from hexane and 95% aqueous methanol, respectively: A form I crystal with the space group P212121 and a form II crystal with the space group P31. In both crystals, DE-β-CD molecules are in a round shape with intramolecular O-3–H⋯O-2 hydrogen bonds. In the form I crystal, the DE-β-CD molecules are arranged along the twofold screw axis to form a helically extended polymeric chain by including the 6-O-ethyl groups of the adjacent molecule. One hexane molecule with twofold disorder is located in the intermolecular channel along the a-axis. In contrast, the DE-β-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 the O-6 side of the molecular cavity. The water molecule links the methanol molecule and two ethoxy groups of the adjacent DE-β-CD molecule with hydrogen bonds. The result suggests the important role of solvent in the formation of helical arrangement of DE-β-CD molecules.