Chitin (C8H13O5N)n (// KY-tin) is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is a characteristic component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g., crabs, lobsters and shrimps) and insects, the radulae of molluscs, and the beaks and internal shells of cephalopods, including squid and octopuses. The structure of chitin is comparable to the polysaccharide cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has also proven useful for several medical and industrial purposes. In butterfly wing scales, chitin is often organized into stacks of nano-layers or nano-sticks made of chitin nanocrystals that produce various iridescent colors by thin-film interference: similar, analogous structures made of keratin are found in iridescent bird plumage.
A similar word, "chiton", refers to a marine animal with a protective shell (also known as a "sea cradle").
Chemistry, physical properties and biological function
Chitin is a modified polysaccharide that contains nitrogen; it is synthesized from units of N-acetylglucosamine (to be precise, 2-(acetylamino)-2-deoxy-D-glucose). These units form covalent β-1,4 linkages (similar to the linkages between glucose units forming cellulose). Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer replaced with an acetyl amine group. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.
In its pure, unmodified form, chitin is translucent, pliable, resilient, and quite tough. In most arthropods, however, it is often modified, occurring largely as a component of composite materials, such as in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the shells of crustaceans and molluscs, chitin produces a much stronger composite. This composite material is much harder and stiffer than pure chitin, and is tougher and less brittle than pure calcium carbonate. Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar (mainly chitin) to the stiff, light elytron of a beetle (containing a large proportion of sclerotin).
Scarab beetles in the genus Cyphochilus demonstrate an exceptional form of chitin in the white scales covering their bodies, as they are whiter than paper or any artificial material produced so far. What makes these scales so white is the special arrangement of the chitin elements inside them, together with their varied shapes and sizes; the chitin filaments are just a few millionths of a metre thick, and tightly packed, scattering light efficiently, deflecting all colours with equal strength, but still able keep a degree of disorder in their shape. In addition, some social wasps species, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to build envelopes for their nests.
Most recent studies point out that chitin is a good inducer of defense mechanisms in plants. It has also been assessed as a fertilizer that can improve overall crop yields. The EPA regulates chitin for agricultural use within the USA. Chitosan is prepared from chitin by deacetylation.
Chitin is used in industry in many processes. Examples of the potential uses of chemically modified chitin in food processing include the formation edible films and as an additive to thicken and stabilize foods and pharmaceuticals. It also acts as a binder in dyes, fabrics, and adhesives. Industrial separation membranes and ion-exchange media can be made from chitin. Processes to size and strengthen paper employ chitin and chitosan. Researchers have developed a method for using chitosan as a reproducible form of biodegradable plastic and as a promising substrate for engineering human tissues by use of three-dimensional bioprinting.
Chitin's flexibility and strength make it favorable as surgical thread. Its biodegradibility means it wears away with time as the wound heals. Moreover, chitin has been reported to have some unusual properties that accelerate healing of wounds in humans. Occupations associated with high environmental chitin levels, such as shellfish processors, are prone to high incidences of asthma. Recent studies have suggested that chitin may play a role in a possible pathway in human allergic disease. To be specific, mice treated with chitin develop an allergic response, characterized by a build-up of interleukin-4-expressing innate immune cells. In these treated mice, additional treatment with a chitinase enzyme abolishes the response.
Chitin may be employed for affinity purification of recombinant protein. A chitin binding moiety is genetically fused to a protein of interest and then connected to beads coated with chitin. The immobilized protein is purified and released from the beads by cleaving off the chitin binding moiety.[clarification needed]
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- Auguste Odier (presented: 1821 ; published: 1823) "Mémoire sur la composition chimique des parties cornées des insectes" (Memoir on the chemical composition of the horny parts of insects), Mémoires de la Société d'Histoire Naturelle de Paris, 1 : 29-42. From page 35: "… la Chitine (c'est ainsi que je nomme cette substance de chiton, χιτον, enveloppe) …" (… chitine (it is thus that I name this substance from chiton, χιτον, covering) …)
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- Martín-Gil FJ, Leal JA, Gómez-Miranda B, Martín-Gil J, Prieto A, Ramos-Sánchez MC (1992). "Low temperature thermal behaviour of chitins and chitin-glucans". Thermochim. Acta 211: 241–254. doi:10.1016/0040-6031(92)87023-4.
- Chitin at the US National Library of Medicine Medical Subject Headings (MeSH)