Chitin

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Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in β-1,4 linkage.
A close-up of the wing of a sap beetle; the wing is composed of chitin.

Chitin (C8H13O5N)n (/ˈktɨn/ 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 the main component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans (e.g., crabs, lobsters and shrimps) and insects, the radulas of mollusks, 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. Bird plumage and butterfly wing scales are often organized into stacks of nano-layers or nano-sticks made of chitin nanocrystals that produce various iridescent colors by thin-film interference.[1]

Etymology[edit]

The English word "chitin" comes from the French word chitine, which first appeared in 1821 and derived from the Greek word χιτών (chiton), meaning covering.[2]

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[edit]

The structure of chitin was solved by Albert Hofmann in 1929.[3]

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.

A cicada sheds its chitinous exoskeleton.

In its unmodified form, chitin is translucent, pliable, resilient, and quite tough. In arthropods, however, it is often modified, becoming embedded in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton. In its pure form, chitin is leathery, but in most invertebrates it occurs largely as a component of composite materials. Combined with calcium carbonate, as in the shells of Crustacea and molluscs, chitin produces a much stronger composite. This composite material is much harder and more stiff than pure chitin, and is tougher and less brittle than calcium carbonate.[4] Another difference between pure and composite forms can be seen by comparing the flexible body wall between the segments of a caterpillar (mainly chitin) to the stiff, light elytron of a beetle (containing a large proportion of sclerotin).[5]

Fossil record[edit]

Chitin was probably present in the exoskeletons of Cambrian arthropods such as trilobites. The oldest preserved chitin dates to the Oligocene, about 25 million years ago, comprising a scorpion encased in amber.[6]

Uses[edit]

Agriculture[edit]

Most recent studies point out that chitin is a good inducer of defense mechanisms in plants.[7] It has also been assessed as a fertilizer that can improve overall crop yields.[8] The EPA regulates chitin for agricultural use within the USA.[9] Chitosan is prepared from chitin by deacetylation.

Industrial[edit]

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[10] and pharmaceuticals. It also acts as a binder in dyes, fabrics, and adhesives.[citation needed] Industrial separation membranes and ion-exchange media can be made from chitin. Processes to size and strengthen paper employ chitin and chitosan.[11][12]

Medicine[edit]

Chitin's properties as a flexible and strong material 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.[13][14][15][full citation needed][16]

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.[17]

Biomedical research[edit]

Chitin may be employed for affinity purification of recombinant protein. A chitin binding domain is genetically fused to a protein of interest and then contacted to beads coated with chitin. The immobilized protein is purified and released from the beads by cleaving off the chitin binding domain.[clarification needed][citation needed]

See also[edit]

References[edit]

  1. ^ Morganti P (2012). "Nanoparticles and Nanostroctures Man-made or Naturally recovered: the biomimetic activity of Chitin Nanofibrils". J of Nanomaterials & Molecular Nanotechnology 1: 2–4. 
  2. ^ 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) …)
  3. ^ Hofmann hydrolyzed chitin using a crude preparation of the enzyme chitinase, which he obtained from the snail Helix pomatia. See:
    • A. Hofmann (1929) "Über den enzymatischen Abbau des Chitins und Chitosans" (On the enzymatic degradation of chitin and chitosan), Ph.D. thesis, University of Zurich (Zurich, Switzerland).
    • P. Karrer and A. Hofmann (1929) "Polysaccharide XXXIX. Über den enzymatischen Abbau von Chitin and Chitosan I," Helvetica Chimica Acta, 12 (1) : 616-637.
    • Nathaniel S. Finney and Jay S. Siegel (2008) "In Memorian: Albert Hofmann (1906-2008)," Chimia, 62 (5) : 444-447 ; see page 444. Available on-line at: University of Zurich
  4. ^ Campbell, N. A. (1996) Biology (4th edition) Benjamin Cummings, New Work. p.69 ISBN 0-8053-1957-3
  5. ^ Gilbert, Lawrence I. (2009). Insect development : morphogenesis, molting and metamorphosis. Amsterdam Boston: Elsevier/Academic Press. ISBN 978-0-12-375136-2. 
  6. ^ Briggs, DEG (29 January 1999). "Molecular taphonomy of animal and plant cuticles: selective preservation and diagenesis". Philosophical Transactions of the Royal Society B: Biological Sciences 354 (1379): 7–17. doi:10.1098/rstb.1999.0356. PMC 1692454. 
  7. ^ "Linden, J., Stoner, R., Knutson, K. Gardner-Hughes, C. "Organic Disease Control Elicitors". Agro Food Industry Hi-Te (p12-15 Oct 2000)". 
  8. ^ "Chitosan derived from chitin, Chitosan Natural Biocontrol for Agricultural & Horticultural use". 
  9. ^ "EPA: Chitin; Poly-N-acetyl-D-glucosamine (128991) Fact Sheet". 
  10. ^ Shahidi,F., Arachchi, J.K.V. and Jeon, Y.-J. (1999) Food applications of chitin and chitosans. Trends in Food Science & Technology, 10 37-51
  11. ^ Hosokawa J, Nishiyama M, Yoshihara K and Kubo T (1990). "Biodegradable film derived from chitosan & homogenized cellulose". Ind.Eng.Chem.Res. 44: 646–650. 
  12. ^ Gaellstedt M, Brottman A, and Hedenqvist MS (2005). "Packaging related properties lf protein and chitosan coated paper". Packag. Technol. Sci 18: 160–170. 
  13. ^ Bhuvanesh Gupta, Abha Arorab, Shalini Saxena and Mohammad Sarwar Alam (July 2008). "Preparation of chitosan–polyethylene glycol coated cotton membranes for wound dressings: preparation and characterization". Polymers for Advanced Technologies 20: 58–65. doi:10.1002/pat.1280. 
  14. ^ SK Kim (2013). Marine Biomaterials. CRC-Press. pp. 149–159. 
  15. ^ Morganti P, Tishenko G, et al. Nanoparticles and nanocomposite chitin nanofibrils/chitosan films in health-care-prepartion and characterization of biomimetic films for wounds in humans. pp. 681–715. 
  16. ^ Morganti P, Fabrizi G et al. (2012). "Anti-aging activity of chitin nanofibrils complexes". The Journal of Nutrition, Health & Aging 16 (3): 242–245. doi:10.1007/s12603-011-0358-0. 
  17. ^ Tiffany A. Reese, Hong-Erh Liang, Andrew M. Tager, Andrew D. Luster, Nico Van Rooijen, David Voehringer & Richard M. Locksley (3 May 2007). "Chitin induces accumulation in tissue of innate immune cells associated with allergy". Nature 447 (7140): 92–96. doi:10.1038/nature05746. PMC 2527589. PMID 17450126. 

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