chitin

Structure of the chitin molecule, showing two of the N-acetylglucosamine units that repeat to form long chains in beta-1,4 linkage.

Chitin (C₈H₁₃O₅N)_n (pronounced /ˈkaɪtɨn/) 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 of cephalopods, including squid and octopuses. Chitin has also proven useful for several medical and industrial purposes. Chitin may be compared to the polysaccharide cellulose and to the protein keratin. Although keratin is a protein, and not a carbohydrate like chitin, keratin and chitin have similar structural functions.

Chemistry, physical properties and biological function

Chitin is a polysaccharide; it is synthesized from units of N-acetylglucosamine (more completely, N-acetyl-D-glucos-2-amine). These units form covalent β-1,4 linkages (similar to the linkages between glucose units forming cellulose). Chitin may therefore be described as cellulose with one hydroxyl group on each monomer substituted 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 a hardened proteinaceous matrix, which forms much of the exoskeleton. In its pure form it is leathery, but when encrusted in calcium carbonate it becomes much harder.^[1] The difference between the unmodified and modified forms can be seen by comparing the body wall of a caterpillar (unmodified) to a beetle (modified).

Fossil record

Chitin first appeared in the exoskeletons of Cambrian arthropods, e.g. trilobites. The oldest preserved chitin dates to the Oligocene, about 25 million years ago.^[2]

Etymology

The English word "chitin" comes from the French word "chitine", which first appeared in 1836. These words were derived from the Greek word "chitōn", meaning mollusk. That is either influenced by, or related to the Greek word khitōn, meaning "tunic" or "frock", the Central Semitic word "*kittan", the Akkadian words "kitû" or "kita’um", meaning flax or linen, and the Sumerian word "gada" or "gida".^[3]

A similar word, "chiton", refers to a marine animal with a protective shell (also known as a "sea cradle").

Uses

Agriculture

Most recent studies point out that chitin is a good inducer for defense mechanisms in plants.^[4] It was recently tested as a fertilizer that can help plants develop healthy immune responses, and have a much better yield and life expectancy.^[5] The EPA regulates chitin for agricultural use.^[6] Chitosan is derived from chitin, which is used as a biocontrol elicitor in agriculture and horticulture.

Industrial

Chitin is used industrially in many processes. It is used in water purification, 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 resins can be made from chitin. Processes to size and strengthen paper employ chitin.

Medicine

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 some unusual properties that accelerate healing of wounds in humans.^[7]

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.^{[citation needed]}

Specifically, 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.^[8]

References

1. ^ Campbell, N. A. (1996) Biology (4th edition) Benjamin Cummings, New Work. p.69 ISBN 0-8053-1957-3
2. ^ Briggs, D.E.G. (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, http://journals.royalsociety.org/index/7TTY8KM0Y9PADF1X.pdf 
3. ^ American Heritage dictionary of the English Language: Fourth Edition. 2000. entry for chiton
4. ^ "Linden, J., Stoner, R., Knutson, K. Gardner-Hughes, C. “Organic Disease Control Elicitors”. Agro Food Industry Hi-Te (p12-15 Oct 2000)". http://www.yeacrops.com/Crop%20Protection%20Article.pdf. 
5. ^ "Chitosan derived from chitin, Chitosan Natural Biocontrol for Agricutlural & Horticultural use". http://en.wikipedia.org/wiki/Chitosan#Agricultural_.26_Horticultural_use. 
6. ^ "EPA: Chitin; Poly-N-acetyl-D-glucosamine (128991) Fact Sheet". http://www.epa.gov/pesticides/biopesticides/ingredients/factsheets/factsheet_128991.htm. 
7. ^ Bhuvanesh Gupta,Abha Arorab,Shalini Saxenaa 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. http://www3.interscience.wiley.com/cgi-bin/fulltext/121487185/PDFSTART. </
8. ^ 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: 92-96. doi:10.1038/nature05746. PMID 17450126. http://www.nature.com/nature/journal/v447/n7140/edsumm/e070503-13.html. 

• Martín-Gil FJ, Leal JA, Gómez-Miranda B, Martín-Gil J, Prieto A, Ramos-Sánchez MC. "Low temperature thermal behaviour of chitins and chitin-glucans". Thermochim. Acta, 1992, vol. 211, pp. 241-254

See also

• Chitosan (Natural Biocontrol for Agricultural & Horticultural use)
• Chitobiose
• Biopesticide

External links

• Chitin Product Information from China GreatVista Chemicals
• Horseshoe Crab Chitin Research
• MeSH Chitin