Molecular Packing and Intermolecular Interactions in Chitosan/Hydrogen Halides Complexes

Instrumentation Analysis Center, Tokyo University of Agriculture and Technology* Faculty of Technology, Tokyo University of Agriculture & Technology** Graduate School of Science, Osaka University*** Research Institute for Advanced Science and Technology, Osaka Prefecture University****
â—‹Keiichi Noguchi* Masakazu Kanenari** Kenji Okuyama*** Kozo Ogawa****

In order to elucidate the intermolecular interactions involved in complex formation of chitosan with hydrogen halides, crystal structures of chitosan/HBr and chitosan/HI complexes were analyzed based on X-ray fiber diffraction data collected using synchrotron radiation source (BL40B2, SPring-8, Hyogo, Japan). The packing models were refined using the linked-atom least-squares method. The final R-values were 0.192 for 93 observed spots (HBr complex) and 0.193 for 44 observed spots (HI complex). Both crystals are isomorphous and belong to the space group P21. The unit cell constants are a = 9.299(9), b = 9.504(8), c(fiber axis) = 10.41(1) Å and beta = 106.93(8) deg, and a = 9.46(2), b = 9.72(2), c(fiber axis) = 10.33(2) Å and beta = 105.1(2) deg for HBr and HI complexes, respectively. The chitosan chain adopts 2/1-helical structure that is a typical conformation for beta-(1->4)-linked polysaccharides. The unit cell contains four glucosamine residues (two polymer chains) and four halide ions. The halide ions are aligned along the c-axis at intervals of about 5Å and are surrounded by four polymer chains. In an asymmetric unit, there are two halide ions. One ion accepts three hydrogen bonds from N2 nitrogen atoms. The other one participates in one hydrogen bond from N2 and two hydrogen bonds from O6 oxygen atoms. In addition, the chitosan chains are linked by N-H...O and C-H...O hydrogen bonds along the b-axis. In the hydrated form of chitosan, water molecules form a columnar structure along the chain direction. The sites of halide ions in the complex crystals are similar to those of the water molecules in the hydrated form. Therefore, it was suggested that the water column was used as a path of halide ions during the complex formation.