X-ray structures of hexakis(2,6-di-O-methyl)-α-cyclodextrin in two crystal forms

Abstract Hexakis(2,6-di-O-methyl)-α-cyclodextrin (DMαCD) crystallizes in the space group P21 from 10% NaCl aqueous solution at room temperature and in the space group P212121 by the slow evaporation of water at ca. 90 °C. The P21 crystal shows a typical cage-type packing structure and a water molecule is included within its macrocyclic cavity. In the P212121 crystal, DMαCD molecules are arranged in a helically extended polymeric chain formed by the inclusion of an O-6CH3 methoxyl group of the adjacent molecule related by the twofold screw axis. In spite of the difference in the packing structure, the macrocyclic conformation of DMαCD molecules in the two crystals is nearly identical. The round structure of the macrocycle is maintained by the intramolecular O-3H—O-2 hydrogen bonds formed between adjacent residues. The rootmean-square distance between equivalent two atoms calculated after the least-squares superposition of the two molecules is 0.36 A for non-hydrogen atoms except O-6 and methyl carbon atoms.