Crystalline behavior of chitosan organic acid salts

The crystal structures of chitosan acid salts were studied by X-ray diffrac tion measurements on a fiber diagram and a new procedure to obtain an anhyd rous polymorph of chitosan was found. The salts prepared by immersing a chi tosan into a mixture of acid solution and isopropanol were classified into two types (Types I and II) depending on their conformation. Molecular confo rmation of the Type I salt retains the extended 2-fold helical structure of the original chitosan, but that of Type II salt is a twisted 2-fold helix. All the Type II salts changed to the anhydrous "Annealed" polymorph of chi tosan when soaking in 75% aqueous isopropanol, but when the Type I salts we re immersed in the solution, they returned to the hydrated "Tendon" polymor ph which is that of the original chitosan. The strange transformation obser ved in Type II salt may be related to the stability of the molecular confor mation of chitosan in the salt.