Molecular and Crystal Structure of a Chitosan-Zinc Chloride Complex

We determined the molecular and packing structure of a chitosan-ZnCl₂ complex by X-ray diffraction and linked-atom least-squares. Eight D-glucosamine residues-composed of four chitosan chains with two-fold helical symmetry, and four ZnCl₂ molecules-were packed in a rectangular unit cell with dimensions a = 1.1677 nm, b = 1.7991 nm, and c = 1.0307 nm (where c is the fiber axis). We performed exhaustive structure searches by examining all of the possible chain packing modes. We also comprehensively searched the positions and spatial orientations of the ZnCl₂ molecules. Chitosan chains of antiparallel polarity formed zigzag-shaped chain sheets, where N2···O6, N2···N2, and O6···O6 intermolecular hydrogen bonds connected the neighboring chains. We further refined the packing positions of the ZnCl₂ molecules by theoretical calculations of the crystal models, which suggested a possible coordination scheme of Zn(II) with an O6 atom.