ABSTRACT
Chitin is said to be the second most abundant compound on Earth, only cellulose being more abundant. As such, chitin may be considered the most abundant nitrogenous substance in nature. Chemically, chitin may be described as an unbranched polysaccharide made of N-acetylglucosamine (GlcNAc, 2-acetamido-2-deoxy-D-glucose) moieties joined by β-1, 4 links. Due to this type of bonding, the structural unit of the polysaccharide is not the monomer, but the disaccharide diacetylchitobiose (O-(2-amino2-deoxy-β-D-glucopyranosyl)-(1-4)-2-amino-2-deoxy-D-glucose). Chitin chains are made of a variable number of hexose units, but normally over 1000. These chains associate among themselves by hydrogen bonding to form microfibrils made up of 20400 molecules of the polysaccharide. Because of its crystalline arrangement, chitin is one of the most insoluble natural products, even more than cellulose itself. Scant solubilization in the order of 5% can be obtained with ethanol-containing CaCl2, dimethylacetamide containing 5-9% LiCl, or N-methyl-2-pyrrolidone containing LiCl (Muzzarelli 1999). Based on the arrangement of the polysaccharide chains, from which their x-ray diffraction patterns depend, three different crystalline forms of chitin can be recognized in nature: α, β, and γ. The most abundant one is the α-form where chains are associated in an antiparallel form, whereas in the less-abundant β-form they are parallel; and in the rarest γ-form, two parallel chains alternate with an antiparallel one (Blackwell et al. 1978). All chitin forms behave as viscoelastic polymers, but due to the different arrangement of their sugar chains they differ in their tensile strength, which is highest for the α-form (Hepburn and Chandler 1978). Why chitin chains associate in three different crystalline forms in nature, contrasting with cellulose, where only one natural form exists, remains unknown.
