X-ray study of chitosan L- and D-ascorbates

Well-defined X-ray fiber patterns of L- and D-ascorbates of chitosan were obtained by immersing a tendon chitosan, prepared from a crab tendon chitin by a solid-state N-deacetylation, in respective ascorbic acid water-isopropyl alcohol solutions, and heating them at 70 degrees C. The crystalline unit cell of the L-ascorbate of chitosan was a monoclinic (pseudoorthorhombic) with the following dimensions: a = 1.122; b = 1.177; c (fiber axis) = 1.040 nm; and beta = 90 degrees. The presence of a P2(1) space group with b axis unique was suggested. The visible reflections on the fiber diagram of the chitosan D-ascorbate could be indexed in terms of a monoclinic unit cell with a = 0.962, b = 1.349, c (fiber axis) = 1.030 nm, and gamma = 96.7 degrees; no space group could be assigned, however. Although no 2(1) symmetry is present along the fiber axes in these crystals, the similarity of both fiber axes to that of the unreacted chitosan (1.043 nm) suggested that the extended 2-fold helical conformation of chitosan is retained in the backbone chitosan chain of each ascorbate. The observed density values of both ascorbates suggested that two chitosan chains, but no water molecule are present in each unit cell of L- and D-ascorbates. It was suggested during the preparation of respective L- and D-ascorbate of chitosan that D-ascorbic acid has a higher reactivity with chitosan to make the salt than the L isomer has.