X-RAY FIBER DIFFRACTION STUDIES OF CHITOSAN AND CHITOSAN GELS

Fibres obtained from freshly prepared films of ungelled chitosan yielded X-ray diffraction patterns indicative of high crystallinity and characteristic of 8(5) helices arranged in a monoclinic unit cell (a = 1.38, b = 1.63, c = 4.07 nm, alpha = 96.46-degrees). This is the first report of an 8-fold helical structure for chitosan. Exposure of these fibres to high humidity led to a progressive change from an 8-fold to a 2-fold helical structure arranged in the more orthodox orthorhombic unit cell. Gels were prepared by dissolving chitosan in acetic acid and then inducing gelation by (a) raising the pH of the solution, (b) N-acetylation, or (c) N- and O-acetylation. The gels formed in (a) were non-crystalline, and those in (b) and (c) gave diffraction patterns similar to those for chitin gels.