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) Å and a
space group P212121.
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 bc-plane.
These
sheets stack along the a-axis. Each sheet is related to
its neighboring sheet by 21-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.
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