Chemical reactivity characteristics of β-chitin from squid pen and its comparison with α-chitin

Being different from the literature results, the chemical reactivity of beta-chitin was found to be poorer than that of alpha-chitin for O-propionylation reaction (basically a heterogeneous reaction). But the chemical reactivity of beta-chitin was almost the same as that of alpha-chitin for N-acetylation reaction(a homogeneous reaction). The poor reactivity of beta-chitin is explained by the higher crystallinity shown by the results of X-ray diffraction, the closer fibrillar structure shown by microscopy, and the higher stability of crystals shown by higher decomposition temperature in the thermogravimetric curves. After beta-chitin was fibrillated by wet grinding, the O-propionylation reactivity increases evidently, but was still lower than that of achitin. Four levels of fibril structure can be distinguished in fibrillated beta-chitin by polarized optical microscopy and scanning electron microscopy, i.e. fibrillar bundle, fibril, microfibril and elementary fibril with diameter in the order of 10(1), 10(0), 10(-1) and 10(-2) mum respectively. A model for four levels of fibril structure of beta-chitin is first suggested. Because more free hydroxyl groups are exposed on all levels of fibrils, the chemical reactivity of fibrillated P-chitin enhances.