Molecular and crystal structure of hydrated chitosan

The molecular and crystal structure of the hydrated form of chitosan, which was obtained by deacetylation of chitin from crab tendon, was determined by the X-ray fiber diffraction method and the linked-atom least-squares method. The chitosan chains crystallize in an orthorhombic unit cell with dimensions a = 8.95(4), b = 16.97(6), c (fiber axis) = 10.34(4) Angstrom and a space group P2(1)2(1)2(1). The chain conformation is a 2-fold helix stabilized by O3---O5 hydrogen bond with the gt orientation of O6. The unit cell contains four chains and eight water molecules. There are direct hydrogen bonds (N2---O6) between adjacent chains along the b-axis, which makes a sheet structure parallel to the be-plane. These sheets stack along the a-axis. Each sheet is related to its neighboring sheet by 2(1)-symmetry along the b-axis. Water molecules form columns between these sheets and contribute to stabilize the structure by making water-bridges between polymer chains.