title: Crystal form III of beta-cyclodextrin-ethanol inclusion complex: layer-type structure with dimeric motif.
abstract: The crystal form III of the beta-cyclodextrin (beta-CD)-ethanol inclusion complex [2(C(6)H(10)O(5))(7).1.5C(2)H(5)OH.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)A, 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)cdots, three dots, centeredO-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 beta-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).
authors.affiliation: Department of Chemistry, Faculty of Science, Chulalongkorn University, Phyathai Road, Pathumwan, Bangkok 10330, Thailand. thammarat.aree@gmail.com
authors.forename: Thammarat
authors.initials: T
authors.lastname: Aree
authors.forename: Narongsak
authors.initials: N
authors.lastname: Chaichit
mesh_terms: D020816:Amino Acid Motifs; D001665:Binding Sites; D002236:Carbohydrate Conformation; D003460:Crystallization; D003461:Crystallography / Q000379:methods; D018360:Crystallography, X-Ray / Q000379:methods; D019281:Dimerization; D000431:Ethanol / Q000737:chemistry*; D006860:Hydrogen Bonding; D008968:Molecular Conformation; D015394:Molecular Structure; D012997:Solvents / Q000737:chemistry; D013696:Temperature; D014961:X-Ray Diffraction; D047392:beta-Cyclodextrins / Q000737:chemistry*
chemical_list: D012997:Solvents; D047392:beta-Cyclodextrins; D000431:Ethanol; C031215:betadex