Hydrogen-bond network in cyclodecaamylose hydrate at 20 K; Neutron diffraction study of novel structural motifs band-flip and kink in α-(1→4)-D-glucoside oligosaccharides

A single-crystal neutron diffraction study of cyclodecaamylose (CA10) was carried out at 20 K. CA10 crystallizes with 27.18 water molecules [(C₆H₁₀O₅)₁₀·27.18H ₂O] in space group C2 with unit-cell constants a = 29.31 (5), b = 9.976 (10), c = 19.34 (2) Å, β = 121.07 (2)°. The asymmetric unit contains a half molecule of CA10 and 13.59 water molecules, the other half being related by a crystallographic twofold rotation axis. All H atoms except two water H atoms could be located from difference neutron-density maps; structure refinement converged at R = 0.635. Two of the five CH₂-O6 groups and one of the 15 O2, O3 hydroxyl groups of CA10 are twofold orientationally disordered. A total of 13.59 water molecules in the asymmetric unit are distributed over 23 positions; 20 of which are in the CA10 cavity, and the other three occupy intermolecular interstices. Of the 123 symmetry-independent hydrogen bonds, 25 (= 20%) are three-centered and 7 (= 6%) are four-centered. Water molecules and O-H groups of CA10 form an extended network with cooperative O-H⋯O-H⋯O-H hydrogen bonds. They are arranged in 11 polygons with three, four, five, six and eight O-H bonds and in homodromic, antidromic and heterodromic arrangements. Nine polygons are located within the cavity and the others are outside.