Chitosan is fundamentally a naturally occurring polymer derived from chitin. Chitin is a major structural component found in the exoskeleton of crustaceans (like shrimps, lobsters, crabs), insects, and the cell walls of fungi and yeast. It is considered the second most abundant polysaccharide in nature, after cellulose.
Chitosan is obtained by the deacetylation of chitin. This process typically involves treatment with hot alkali, specifically enzymatic methods. Chitin is considered a polymer of glucosamine and N-acetyl glucosamine where the deacetylated units are in a minor proportion. Chitosan, in contrast, is where the deacetylated units are in major proportion or their distribution allows dissolution in aqueous diluted acid solutions. Chitin has a high acetyl content and is hydrophobic, whereas deacetylation into chitosan makes it more "water and organic solvents friendly".
Chemically, chitosan is a linear polysaccharide composed of β-(1-4) linked D-glucosamine and N-acetyl-D-glucosamine units. A key feature of its chemical structure is the predominant presence of units with amino groups. These amino groups are located at the C-2 position of the glucosamine units. The molecule also possesses hydroxyl groups at the C-6 (primary) and C-3 (secondary) positions. These functional groups, particularly the free amino groups, make chitosan more reactive than chitin and available for chemical reactions and salt formation with acids.
Two of the most important parameters that characterize chitosan and significantly influence its physical, chemical, and biological properties are the degree of deacetylation (DD) and the molecular weight (MW).