Chitin and chitosan

Chitin is a linear polysaccharide of β-1,4-linked N-acetyl-D-glucosamine — cellulose with an acetamido group replacing the hydroxyl at C2. It is the structural polymer of arthropod cuticle, fungal cell wall and mollusc pen, and by mass it is second only to cellulose among biopolymers. Almost none of it is used, and the reason is solubility.

Why chitin is intractable

The acetamido group adds a hydrogen-bond donor and acceptor to every sugar unit, so chitin chains pack into a denser, more extensively hydrogen-bonded crystal than cellulose. There is no ordinary solvent for it: it dissolves only in aggressive systems such as concentrated lithium chloride in dimethylacetamide, hexafluoroisopropanol, or certain ionic liquids, none of which scale cheaply.

Polymorph matters. In α-chitin, the dominant form in crustacean shell and insect cuticle, chains are antiparallel and hydrogen bonding is maximal — the most stable and least accessible form. In β-chitin, found in squid pen and some diatoms, chains are parallel, interchain bonding between sheets is weaker, and the crystal swells in water. β-chitin therefore deacetylates and hydrolyses under far milder conditions, which is why the source organism, not just the purity, changes the process.

Deacetylation and the pKa that runs everything

Chitosan is made by removing acetyl groups, conventionally with hot concentrated sodium hydroxide, in the crustacean route after demineralising the shell with acid and deproteinising it with alkali. As acetamido groups become free primary amines, the crystal disorders and the polymer becomes soluble in dilute acid. The transition is conventionally placed above roughly 50 % deacetylation, but it is a continuum, not a threshold.

The amine has a pKa near 6.3–6.5. Below it the polymer is protonated and dissolves as a polycation; above it the charge is lost and chitosan precipitates. That single fact explains most of its behaviour. Its antimicrobial activity comes from electrostatic association of the polycation with anionic phosphoryl and carboxyl groups on the bacterial envelope, and it accordingly falls away as pH rises toward neutrality — which is why claims of antimicrobial performance must state the pH at which they were measured. Its haemostatic and mucoadhesive behaviour, its ability to flocculate suspended solids, and its film formation are all the same charge interaction under different names.

The constraint on regulated use

Chitosan is not a defined molecule. Degree of deacetylation, molecular weight distribution and residual protein and mineral all depend on the shell batch and the process, and every functional property depends on those parameters. For food, wound-care and pharmaceutical applications this is the binding limit: there is no chitosan monograph that fixes the material the way a small molecule is fixed, so each producer’s grade is validated separately. Crustacean-derived material also carries residual shellfish protein, and tropomyosin is the dominant shellfish allergen, which is one reason fungal chitosan — produced from controlled fermentation with a consistent chitin–glucan wall — attracts interest despite lower yields. Enzymatic deacetylation by chitin deacetylases avoids the strongly alkaline step and gives more defined products, but is still slow on crystalline α-chitin.