Chitin

Abstract

Chitin has long been defined as a substance that contains nitrogen, is insoluble in strong alkali solutions at 140° to 160°, and which on hydrolysis yields glucosamine and acetic acid. It has been established that some preparations of both animal and fungal origin are almost certainly straight chain, β 1–4 linked, polymers of acetylglucosamine (N-acetyl-2-amino-2-deoxy-d-glucose). On hydrolysis such a substance should yield 29,5% of its weight as acetic acid and 88% as glucosamine (106% as glucosamine hydrochloride). Its nitrogen content should be 6,9%. There is, as yet, no evidence that substances giving all the reactions of chitin described in Section B do not contain such a polymer, and X-ray evidence lends support to the conclusion that all so examined do contain it. A preparation that contains the theoretical amount of nitrogen and acetyl groups is a rarity owing to the difficulties of purification without degradation. It will be assumed in this article that materials satisfying the tests in Section B do in fact contain a pure acetylglucosamine polymer and that this is chitin. The reasons for this cautious approach will become apparent. It must be emphasised at the outset that to show that a material is “insoluble” and that on hydrolysis glucosamine is formed is no justification for assuming that it contains chitin. There are many polymers in both animals and lower plants that are insoluble under some conditions and that contain acetylglucosamine units. They are, however, mixed polymers and chitin is the only unmixed polymer of acetylglucosamine for which evidence has yet been secured.