Conformational behavior of chitosan in the acetate salt: An X-ray study

A well-defined X-ray fiber pattern of chitosan acetate was obtained by immersing a tendon chitosan, prepared from a crab tendon chitin by a solid-state N-deacetylation, in an aqueous acetic acid-isopropanol solution at 110 degrees C, This pattern was very similar to that of chitosan salts with some inorganic acids, such as HF, HCl, and H2SO4, in which chitosan chains form an 8/5 helix, indicating that chitosan acetate also take ap this conformation, This information may give an influential clue to the chitosan conformation in the aqueous acetic acid solution, the most popular solvent for chitosan. However, after one month of storage of the chitosan acetate, the fiber pattern, the density and its IR spectrum changed to those of the anhydrous polymorph of chitosan, suggesting that the acetic acid was removed accompanied with water molecules from the crystal during storage and that the polymorph can be obtained not only by annealing chitosan, brat also through the chitosan acetate.