2. Identification of chitin and chitosan
Chitin is a linear polymer of 1,4-linked N-acetyl-D-glucosamine, which looks like 1,4-D-anhydroglucopyranose chain of cellulose except for the acetamide group at C-2 position of anhydroglucopyranoside residue. Similarly to cellulose, native chitin occurs in fibrous crystalline states, i.e. microfibrils [61]. This natural biopolymer (chitin) can be presented in different structural forms, according to its biological function and its natural source; these forms are differentiated according to the arrangement of the carbohydrate chains. The α form has chains arranged alternately antiparallel; the β form has all chains in parallel and the γ-chitin has two chains in one direction with an additional inverted chain [22]. In other words, α-chitin has a rhombic structure with two symmetrical, alternately antiparallel chitin chains. The carbonyl and hydroxyl groups in α-type chitin participate in an increased degree of intermolecular hydrogen bonding, leading to a more compact and stable structure. Therefore, α-type chitin is less likely to swell in water. β-type chitin is monoclinic with two parallel chitin chains, and fewer hydrogen bonds exist between the molecules; hence, it is more loosely structured [82]. Although both allomorphs of chitin (α and β-type) are insoluble in aqueous and common organic solvents, β-chitin displays higher reactivity, swelling and solubility as compared to α-chitin, but as this latter is more widely spread in the biomass, mainly as a major component of the crustaceans shells, it is preferentially used in industries and in research laboratories [4].