Chitin is a highly abundant, naturally occurring substance that serves a foundational role in the biology of countless organisms. It is classified chemically as a polysaccharide, a complex carbohydrate structured as a long chain of smaller sugar molecules linked together. This biopolymer is considered the second most common polysaccharide on Earth, surpassed only by cellulose. Chitin’s primary function in nature is a structural one, providing rigidity, strength, and form to the organisms that synthesize it.
The Chemical Building Block
The fundamental unit that makes up the chitin molecule is a simple sugar derivative called N-acetylglucosamine (NAG). Chitin is formed by linking thousands of these identical NAG monomers into a single, extended chain. Each \(text{N-acetylglucosamine}\) unit is chemically bonded to the next via a \(beta\)-(1\(rightarrow\)4) glycosidic linkage, a specific connection similar to that found in plant cellulose.
The defining chemical difference from cellulose is the presence of a nitrogen-containing acetamido group (\(text{N-acetyl}\)) attached to the \(text{C-2}\) carbon of the glucose ring. This nitrogen group allows for extensive hydrogen bonding between adjacent polymer chains, which gives chitin its exceptional strength and stability. These parallel chains pack tightly together, forming highly ordered crystalline structures, with the most common form being \(alpha\)-chitin. This dense, hydrogen-bonded structure makes the raw material notably insoluble and resistant to degradation.
Where Chitin Exists in Nature
Chitin is the primary structural component in the external skeletons of one of the largest groups of organisms: arthropods. This includes the hard, protective exoskeletons of insects, spiders, and crustaceans like crabs, shrimp, and lobsters. In these animals, chitin is embedded within a protein matrix and often mineralized with calcium carbonate, which dramatically increases its overall hardness and durability.
The substance also forms a significant part of the cell walls in nearly all fungi, where it provides structural integrity and resistance against osmotic pressure. Unlike plants, whose cell walls are made of cellulose, fungal cell walls rely on chitin for mechanical strength.
The Transformation into Chitosan
Raw chitin is not easily processed or used by humans because its tightly bound, crystalline structure makes it insoluble in most common solvents. To make it industrially useful, chitin is chemically modified through a process called deacetylation to create its derivative, chitosan. This transformation typically involves treating chitin with a hot, concentrated alkaline solution, such as sodium hydroxide.
The deacetylation process removes the \(text{N-acetyl}\) groups from the \(text{N-acetylglucosamine}\) monomers, converting them into \(text{glucosamine}\) units that possess free amine groups (\(text{NH}_2\)). The resulting chitosan polymer is defined by its degree of deacetylation, which is the percentage of converted units. Chitosan is valuable because the free amine groups make it soluble in acidic solutions and, more importantly, give it a positive charge, making it the only naturally occurring cationic polymer.
This positive charge allows chitosan to interact readily with negatively charged surfaces and molecules, which informs its wide range of applications. In the medical field, its biocompatibility and ability to promote clotting make it useful for advanced wound dressings and dissolvable sutures. Its strong binding properties are also applied industrially in water purification systems, where the cationic nature allows it to effectively adsorb heavy metal ions and other contaminants.