Chitin and its association with other molecules

Abstract

Abstract Chitin or poly‐N‐acetyl‐D‐glucosamine occurs in three crystallographic forms α, β and γ. The α form is the most stable and consists of antiparallel chains. The β form (parallel chains – one chain per unit cell) and the γ form (probably two chains up and one down per unit cell) are both converted to the antiparallel arrangement (α form) on treatment in strong HCl. Chitin in the cuticles, apodemes and chaetae of animals occurs together with a considerable quantity of protein. X‐ray diffraction diagrams of the intact structures show repetition along the fiber axis corresponding to six NAG residues, or (more rarely observed) to eight NAG residues. The structural studies of Phillips and his colleagues on the reaction of lysozyme with NAG oligomers are taken as a relevant model for those chitin/protein complexes showing a repetition of six NAG residues/protein unit. In the case of locust tibia cuticles, evidence is presented for the existence of corpuscular protein units along chitin microfibrils and these repeat at 31 Å i.e. every six NAG residues. It is suggested that these protein units are basically a chitin synthetase but serving also as a “plasticiser” to modify the mechanical and water‐holding properties of the chitin.