What is chitin?

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Chitin is one of the most abundant natural biopolymers, i.e. a large molecule build of many small molecules (the monomers). Chitin belongs to the class of polysaccharides (complex sugars) build of many monosaccharide units (simple sugars). The monomeric unit of chitin is N-acetylglucosamine, i.e. a sugar derivative that carries an acetic acid group. Up to about 3,000-5,000 of these monomers are connected in what is known as β-1,4-glycosidic linkages to form a linear chain. Like any other polysaccharide, chitin chains have two different ends, a reducing and a non-reducing end. Chitin chains can align in two different ways, either parallel (β-chitin) or anti-parallel (α-chitin), to form strong, crystalline chitin fibers. This makes chitin insoluble in water and an ideal structural component of e.g. fungal cell walls and insect or crab shells. Strictly speaking, only fully acetylated polymers consisting of N-acetylglucosamine residues only would be considered as chitin, the presence of a single deacetylated glucosamine unit would convert it into chitosan. However, it is not known whether chitins in nature are really fully acetylated (i.e., whether they really have a degree of acetylation of 100%). In practice, polymers that are insoluble in dilute acid are considered chitin while acid-soluble ones are defined as chitosans. The border between the two depends on the degree of acetylation of the molecule, it is somewhere in the range of a degree of acetylation of 75%.