Molecular and Crystal Structures of Inulin from Electron Diffraction Data

Abstract

The crystal and molecular structures of inulin, the poly(2→1)-β-d-fructofuranan, hemihydrate and monohydrate, were determined by a constrained linked-atom least-squares (LALS) refinement, utilizing intensities measured from electron diffraction data and stereochemical restraints. Single crystals were grown from dilute aqueous ethanol solutions and their three-dimensional electron diffraction diagrams analyzed. In the hemihydrate form, there is half a molecule of water per fructosyl residue. In this allomorph, which corresponds to air-dried crystals, inulin adopts an orthorhombic P212121 space group with cell parameters a = 16.70 Å, b = 9.65 Å, and c (chain axis) = 14.4 Å. By using the base plane electron diffraction data combined with the upper layer line data, the best molecular model was refined to an R factor of 22.7% and R‘‘ factor of 22.3%. This model has two antiparallel sixfold helices centered on the twofold screw axes intersecting the base plane of the orthorhombic system. The six water molecules are distributed over eight sites in the unit cell. The helical conformation is characterized by φ = 66°, ψ = 154°, and ω = −82°. In the monohydrate form obtained when inulin crystals were kept in a moist environment, there is one molecule of water per fructosyl residue. The unit cell dimensions remained nearly the same (a = 16.70 Å, b = 9.80 Å, and c (chain axis) = 14.7 Å) but substantial differences in the electron diffraction intensities were observed. The best molecular model was refined to an R factor of 18.1% and an R‘‘ factor of 16.5% by using only hk0 reflections of the base plane electron diffraction data. Simulated diffraction patterns of this structure showed that its differences with the hemihydrate could be accounted for only by introducing extra water molecules into the gaps of the crystalline structure without major modification of the inulin conformation.