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.
Keywords: X-ray fiber diffraction; chitosan−ZnCl2 complex; crystal structure.