Crystal form III of β-cyclodextrin-ethanol inclusion complex:: layer-type structure with dimeric motif

The crystal form III of the beta-cyclodextrin (beta-CD)-ethanol inclusion complex [2(C6H10O5)(7)center dot 1.5-C2H5OH center dot 19H(2)O] belongs to the triclinic space group P1 with unit cell constants: a = 15.430(1), b = 15.455(1), c = 17.996(1) angstrom, alpha = 99.30(1)degrees, beta = 113.18(1)degrees, gamma = 103.04(1)degrees. beta-CD forms dimers comprising two identical monomers that adopt a 'round' conformation stabilized by intramolecular, interglucose O-3(n)... O-2(n + 1) hydrogen bonds. The two beta-CD monomers of form III are isostructural to that of form I in the monoclinic space group P2(1) [Steiner, T.; Mason, S. A.; Saenger, W. J. Am. Chem. Soc. 1991, 113, 5676-5687], but exhibit a striking difference from that of form II in the monoclinic space group C2 [Aree, T.; Chaichit, N. Carbohydr. Res. 2003, 338, 1581-1589]. The small guest EtOH molecule orients differently in the large beta-CD cavity. In form III, two disordered EtOH molecules are embedded in the p-CD-dimer cavity. A half occupied EtOH molecule (#1) is located above the O-4 plane of beta-CD #1, whereas another doubly disordered EtOH molecule (#2, #3) is situated at about the middle of the beta-CD-dimer cavity. The three EtOH sites are maintained in positions by making van der Waals contacts to each other and to the surrounding water sites and beta-CD O-3-H group. The EtOH molecules disordered (occupancy 0.3) above the beta-CD O-4 plane in form I and fully occupied beneath the O-4 plane in form II are strongly held in positions by hydrogen bonding with the surrounding water site and beta-CD O-6-H, O-3-H groups. Occurrence of the beta-CD dimer as a structural motif of channel-type packing (form II) and layer-type packing (form III) is attributed to the higher tendency for self aggregation under the moderate acidic conditions. At weak acidic conditions, beta-CD prefers a herringbone mode (form I). (C) 2008 Elsevier Ltd. All rights reserved.