Crystal structure of anhydrous hexakis-(2,3,6-tri-O-methyl)-cyclomaltohexaose (permethyl-α-cyclodextrin) grown from hot water and from cold NaCl solutions

An X-ray diffraction study was carried out on anhydrous permethyl-α-cyclodextrin crystallized from aq solution at 40 °C, space group P 212121 with a = 15.424(9), b = 18.167(7), c = 23.128(8) Å, vol = 6481(5) Å3. The molecule is boat-shaped with the narrow end of the cyclodextrin cavity closed by two diametrically opposing glucose residues. These are strongly tilted towards the molecular axis, so that their primary methoxy groups form van der Waals contacts across the molecular opening. The remaining cavity volume is occupied by the methoxy group of a neighboring molecule (self-inclusion). From solutions containing 1 M NaCl, anhydrous permethyl-α-cyclodextrin crystallizes isomorphously at room temperature.