Structure of the chitin molecule, showing two of theN-acetylglucosamine units that repeat to form long chains in β-(1→4)-linkage.Haworth projection of the chitin molecule.A close-up of the wing of aleafhopper; the wing is composed of chitin.Acicada emerges from its nymphal exoskeleton; the shed exoskeleton is mostly modified chitin (sclerotin) but the wings and much of the adult body are still unsclerotized chitin at this stage
Chitin (C8H13O5N)n (/ˈkaɪtɪn/KY-tin) is a long-chainpolymer ofN-acetylglucosamine, anamide derivative ofglucose. Chitin is the second most abundantpolysaccharide in nature (behind onlycellulose); an estimated 1 billion tons of chitin are produced each year in thebiosphere.[1] It is a primary component ofcell walls infungi (especially filamentous and mushroom-forming fungi), theexoskeletons ofarthropods such as crustaceans and insects, theradulae,cephalopod beaks andgladii ofmolluscs and in some nematodes and diatoms.[2][3]It is also synthesised by at least some fish andlissamphibians.[4] Commercially, chitin is extracted from the shells of crabs, shrimps, shellfish and lobsters, which are major by-products of the seafood industry.[2][3] The structure of chitin is comparable to cellulose, forming crystalline nanofibrils or whiskers. It is functionally comparable to the proteinkeratin. Chitin has proved useful for several medicinal, industrial and biotechnological purposes.[3][5]
Chemical configurations of the different monosaccharides (glucose and N-acetylglucosamine) and polysaccharides (chitin and cellulose) presented inHaworth projection
The structure of chitin was determined byAlbert Hofmann in 1929. Hofmann hydrolyzed chitin using a crude preparation of the enzyme chitinase, which he obtained from the snailHelix pomatia.[7][8][9]
Chitin is a modifiedpolysaccharide that contains nitrogen; it issynthesized from units ofN-acetyl-D-glucosamine (to be precise, 2-(acetylamino)-2-deoxy-D-glucose). These units form covalent β-(1→4)-linkages (like the linkages betweenglucose units formingcellulose). Therefore, chitin may be described ascellulose with onehydroxyl group on eachmonomer replaced with anacetylamine group. This allows for increasedhydrogen bonding between adjacentpolymers, giving the chitin-polymer matrix increased strength.
In its pure, unmodified form, chitin is translucent, pliable, resilient, and quite tough. In mostarthropods, however, it is often modified, occurring largely as a component ofcomposite materials, such as insclerotin, a tannedproteinaceous matrix, which forms much of theexoskeleton ofinsects. Combined withcalcium carbonate, as in the shells ofcrustaceans andmolluscs, chitin produces a much stronger composite. This composite material is much harder and stiffer than pure chitin, and is tougher and less brittle than purecalcium carbonate.[10] Another difference between pure and composite forms can be seen by comparing the flexible body wall of acaterpillar (mainly chitin) to the stiff, lightelytron of abeetle (containing a large proportion ofsclerotin).[11]
In butterfly wing scales, chitin is organized into stacks ofgyroids constructed of chitinphotonic crystals that produce variousiridescent colors servingphenotypic signaling and communication for mating and foraging.[12] The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations inbiomimicry.[12]Scarab beetles in the genusCyphochilus also utilize chitin to form extremely thinscales (five to fifteenmicrometres thick) that diffusely reflect white light. These scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds ofnanometres, which serve to scatter light. Themultiple scattering of light is thought to play a role in the unusual whiteness of the scales.[13][14] In addition, some social wasps, such asProtopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper.[15]
Chitosan is produced commercially bydeacetylation of chitin by treatment withsodium hydroxide. Chitosan has a wide range of biomedical applications including wound healing, drug delivery and tissue engineering.[2][3] Due to its specific intermolecular hydrogen bonding network, dissolving chitin in water is very difficult.[16] Chitosan (with a degree of deacetylation of more than ~28%), on the other hand, can be dissolved in dilute acidic aqueous solutions below a pH of 6.0 such as acetic, formic and lactic acids. Chitosan with a degree of deacetylation greater than ~49% is soluble in water[17][18]
Plants also have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein.[19] The first chitin receptor was cloned in 2006.[20] When the receptors are activated by chitin, genes related to plant defense are expressed, andjasmonate hormones are activated, which in turn activate systemic defenses.[21]Commensal fungi have ways to interact with the host immune response that, as of 2016[update], were not well understood.[20]
Some pathogens produce chitin-binding proteins that mask the chitin they shed from these receptors.[21][22]Zymoseptoria tritici is an example of a fungal pathogen that has such blocking proteins; it is a major pest inwheat crops.[23]
For more on the preservation potential of chitin and other biopolymers, seetaphonomy.
Chitin was probably present in the exoskeletons ofCambrian arthropods such astrilobites. The oldest preserved (intact) chitin samples thus far reported are dated to theOligocene, about25 million years ago, from specimens encased inamber where the chitin has not completely degraded.[24]
Chitin is used in many industrial processes. Examples of the potential uses of chemically modified chitin infood processing include the formation of edible films and as an additive to thicken and stabilize foods and food emulsions.[28][29] Processes tosize and strengthenpaper employ chitin and chitosan.[30][31]
How chitin interacts with theimmune system of plants and animals has been an active area of research, including the identity of keyreceptors with which chitin interacts, whether the size of chitin particles is relevant to the kind of immune response triggered, and mechanisms by which immune systems respond.[32][23] Chitin is deacetylated chemically or enzymatically to producechitosan, a highlybiocompatible polymer which has found a wide range of applications in the biomedical industry.[2][33][34] Chitin and chitosan have been explored as avaccine adjuvant due to its ability to stimulate an immune response.[2][19]
Chitin and chitosan are under development asscaffolds in studies of how tissue grows and howwounds heal, and in efforts to invent betterbandages,surgical thread, and materials forallotransplantation.[2][16][35]Sutures made of chitin have been experimentally developed, but their lack of elasticity and problems making thread have prevented commercial success so far.[36]
Chitosan has been demonstrated and proposed to make a reproducible form ofbiodegradable plastic.[37] Chitinnanofibers are extracted from crustacean waste and mushrooms for possible development of products intissue engineering, drug delivery and medicine.[2][38]
Chitin has been proposed for use in building structures, tools, and other solid objects from acomposite material, combining chitin withMartian regolith.[39] To build this, thebiopolymers in the chitin are suggested as thebinder for the regolithaggregate to form aconcrete-likecomposite material. The authors believe that waste materials from food production (e.g. scales from fish, exoskeletons from crustaceans and insects, etc.) could be put to use as feedstock for manufacturing processes.
^Odier, Auguste (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 (in French).1. presented: 1821:29–42.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)]"
^Hofmann, A. (1929).Über den enzymatischen Abbau des Chitins und Chitosans [On the enzymatic degradation of chitin and chitosan] (Thesis). Zurich, Switzerland: University of Zurich.
^Karrer, P.; Hofmann, A. (1929). "Polysaccharide XXXIX. Über den enzymatischen Abbau von Chitin and Chitosan I".Helvetica Chimica Acta (in German).12 (1):616–637.doi:10.1002/hlca.19290120167.
^Campbell, N. A. (1996)Biology (4th edition) Benjamin Cummings, New Work. p.69ISBN0-8053-1957-3
^Gilbert, Lawrence I. (2009).Insect development : morphogenesis, molting and metamorphosis. Amsterdam Boston: Elsevier/Academic Press.ISBN978-0-12-375136-2.
^Sarathchandra, S. U.; Watson, R. N.; Cox, N. R.; di Menna, M. E.; Brown, J. A.; Burch, G.; Neville, F. J. (1996-05-01). "Effects of chitin amendment of soil on microorganisms, nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenneL.)".Biology and Fertility of Soils.22 (3):221–226.Bibcode:1996BioFS..22..221S.doi:10.1007/BF00382516.ISSN1432-0789.S2CID32594901.
^Tzoumaki, Maria V.; Moschakis, Thomas; Kiosseoglou, Vassilios; Biliaderis, Costas G. (August 2011). "Oil-in-water emulsions stabilized by chitin nanocrystal particles".Food Hydrocolloids.25 (6):1521–1529.doi:10.1016/j.foodhyd.2011.02.008.ISSN0268-005X.
^Shahidi, F.; Arachchi, J.K.V.; Jeon, Y.-J. (1999). "Food applications of chitin and chitosans".Trends in Food Science & Technology.10 (2):37–51.doi:10.1016/s0924-2244(99)00017-5.
