Intermolecular Interaction in Chitosan/Hydrogen Bromide Complex Based on X-Ray Fiber Diffraction

The crystal structure of chitosan/hydrogen bromide (HBr) complex was analyzed using synchrotron X-ray fiber diffraction data. The crystals belong to the monoclinic space group P2(1). The unit cell constants are a = 9.299 (9), b = 9.504 (8), c (fiber axis) = 10.41 (1) angstrom and beta = 106.93(8)degrees. The final model was obtained by the linked-atom least-squares refinement, which gave an R-factor of 0.191 (93 observed spots). The unit cell contains two polymer chains (four glucosamine residues) and four HBr molecules. The chitosan chain adopts 2/1-helical conformation. The orientation of 06 oxygen is nearly gt; it plays a role as proton donor to bromide ions and as proton acceptor from NH(3)(+) groups (N2 nitrogen) of adjacent chitosan chains. The bromide ions are aligned along the c-axis at intervals of about 5 angstrom and are surrounded by four polymer chains. Two types of bromide ions exist in an asymmetric unit. One bromide ion participates in three hydrogen bonds from N2, and the other one participates in one hydrogen bond from N2 and two hydrogen bonds from O6.