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

The crystal form III of the β-cyclodextrin (β-CD)–ethanol inclusion complex [2(C6H10O5)7·1.5C2H5OH·19H2O] belongs to the triclinic space group P1 with unit cell constants: a = 15.430(1), b = 15.455(1), c = 17.996(1) Å, α = 99.30(1)°, β = 113.18(1)°, γ = 103.04(1)°. β-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 β-CD monomers of form III are isostructural to that of form I in the monoclinic space group P21 [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 β-CD cavity. In form III, two disordered EtOH molecules are embedded in the β-CD-dimer cavity. A half occupied EtOH molecule (#1) is located above the O-4 plane of β-CD #1, whereas another doubly disordered EtOH molecule (#2, #3) is situated at about the middle of the β-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 β-CD O-3–H group. The EtOH molecules disordered (occupancy 0.3) above the β-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 β-CD O-6–H, O-3–H groups. Occurrence of the β-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, β-CD prefers a herringbone mode (form I).