Editing Chitin (section)

== Chemistry, physical properties and biological function ==

[[File:Chitin glucose and cellulose.svg|200px|thumb|right|Chemical configurations of the different monosaccharides (glucose and N-acetylglucosamine) and polysaccharides (chitin and cellulose) presented in [[Haworth projection]]]]

The structure of chitin was determined by [[Albert Hofmann]] in 1929. Hofmann hydrolyzed chitin using a crude preparation of the enzyme chitinase, which he obtained from the snail ''Helix pomatia''.<ref>{{cite thesis| first= A. |last= Hofmann | year=1929| title= Über den enzymatischen Abbau des Chitins und Chitosans| trans-title= On the enzymatic degradation of chitin and chitosan| publisher= University of Zurich |place= Zurich, Switzerland}}</ref><ref>{{cite journal| first1= P.| last1= Karrer |first2= A. |last2= Hofmann | year= 1929| title= Polysaccharide XXXIX. Über den enzymatischen Abbau von Chitin and Chitosan I| journal= Helvetica Chimica Acta| language= de| volume= 12| number= 1| pages= 616–637 | doi= 10.1002/hlca.19290120167}}</ref><ref>{{cite journal| first1= Nathaniel S.| last1= Finney| first2= Jay S.| last2= Siegel| year= 2008| title= In Memoriam: Albert Hofmann (1906-2008)| journal= CHIMIA| volume= 62| number= 5| pages= 444–447| url= http://www.zora.uzh.ch/9154/2/Siege_Finney_Hoffmann_2008V.pdf| publisher= University of Zurich| doi= 10.2533/chimia.2008.444| access-date= 2013-04-14| archive-date= 2013-06-16| archive-url= https://web.archive.org/web/20130616034406/http://www.zora.uzh.ch/9154/2/Siege_Finney_Hoffmann_2008V.pdf| url-status= dead}}</ref>

Chitin is a modified [[polysaccharide]] that contains nitrogen; it is [[biosynthesis|synthesized]] from units of [[N-acetylglucosamine|''N''-acetyl-<small>D</small>-glucosamine]] (to be precise, 2-(acetylamino)-2-deoxy-<small>D</small>-glucose). These units form covalent β-(1→4)-linkages (like 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 [[arthropod]]s, however, it is often modified, occurring largely as a component of [[composite material]]s, such as in [[sclerotin]], a tanned [[protein]]aceous matrix, which forms much of the [[exoskeleton]] of [[insect]]s. Combined with [[calcium carbonate]], as in the shells of [[crustacean]]s and [[mollusc]]s, 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]].<ref name="Campbell">Campbell, N. A. (1996) ''Biology'' (4th edition) Benjamin Cummings, New Work. p.69 {{ISBN|0-8053-1957-3}}</ref> 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]]).<ref>{{cite book | author = Gilbert, Lawrence I. | title = Insect development : morphogenesis, molting and metamorphosis | publisher = Elsevier/Academic Press | location = Amsterdam Boston | year = 2009 | isbn = 978-0-12-375136-2 }}</ref>

In butterfly wing scales, chitin is organized into stacks of [[gyroid]]s constructed of chitin [[photonic crystal]]s that produce various [[iridescent]] colors serving [[phenotype|phenotypic]] signaling and communication for mating and foraging.<ref name="wings">{{cite journal|journal=Proc Natl Acad Sci U S A|year=2010|volume=107|issue=26|pages=11676–81|doi=10.1073/pnas.0909616107|title=Structure, function, and self-assembly of single network gyroid (I4132) photonic crystals in butterfly wing scales|vauthors=Saranathan V, Osuji CO, Mochrie SG, Noh H, Narayanan S, Sandy A, Dufresne ER, Prum RO|pmid=20547870|pmc=2900708|bibcode=2010PNAS..10711676S|doi-access=free}}</ref> The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in [[biomimicry]].<ref name="wings" /> [[Scarab beetle]]s in the genus ''[[Cyphochilus]]'' also utilize chitin to form extremely thin [[Scale (anatomy)|scales]] (five to fifteen [[micrometre]]s thick) that diffusely reflect white light. These scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of [[nanometre]]s, which serve to scatter light. The [[Scattering#Single and multiple scattering|multiple scattering]] of light is thought to play a role in the unusual whiteness of the scales.<ref>{{cite web|url=https://www.bbc.co.uk/news/science-environment-28811232|date=16 August 2014|title=Beetles' whiteness understood|author=Dasi Espuig M|publisher=BBC News: Science and Environment|access-date=15 November 2014}}</ref><ref name="Burresi">{{cite journal |first1 = Matteo |last1 = Burresi |first2 = Lorenzo |last2 = Cortese| first3 = Lorenzo |last3 = Pattelli | first4 = Mathias | last4 = Kolle | first5 = Peter | last5 = Vukusic | first6 = Diederik S. | last6 = Wiersma | first7 =  Ullrich | last7 = Steiner |first8 = Silvia | last8 = Vignolini |title=Bright-white beetle scales optimise multiple scattering of light |journal=Scientific Reports |volume = 4 |pages = 6075 |year = 2014 |doi = 10.1038/srep06075 | pmid=25123449 | pmc=4133710|bibcode = 2014NatSR...4E6075B }}</ref> In addition, some social wasps, such as ''[[Protopolybia chartergoides]]'', orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper.<ref>{{Cite journal |last1=Kudô |first1=K. |last2=Yamane |first2=Sô. |last3=Mateus |first3=S. |last4=Tsuchida |first4=K. |last5=Itô |first5=Y. |last6=Miyano |first6=S. |last7=Yamamoto |first7=H. |last8=Zucchi |first8=R. |date=2001-10-01 |title=Nest materials and some chemical characteristics of nests of a New World swarm-founding polistine wasp, Polybia paulista (Hymenoptera Vespidae) |url=https://doi.org/10.1080/08927014.2001.9522766 |journal=Ethology Ecology & Evolution |volume=13 |issue=4 |pages=351–360 |doi=10.1080/08927014.2001.9522766 |bibcode=2001EtEcE..13..351K |s2cid=86452110 |issn=0394-9370}}</ref>

[[Chitosan]] is produced commercially by [[deacetylation]] of chitin by treatment with [[sodium hydroxide]]. Chitosan has a wide range of biomedical applications including wound healing, drug delivery and tissue engineering.<ref name=":0" /><ref name=":1" /> Due to its specific intermolecular hydrogen bonding network, dissolving chitin in water is very difficult.<ref name=Bedian2017rev>{{cite journal|last1=Bedian|first1=L|last2=Villalba-Rodríguez|first2=AM|last3=Hernández-Vargas|first3=G|last4=Parra-Saldivar|first4=R|last5=Iqbal|first5=HM|title=Bio-based materials with novel characteristics for tissue engineering applications - A review.|journal=International Journal of Biological Macromolecules|date=May 2017|volume=98|pages=837–846|doi=10.1016/j.ijbiomac.2017.02.048|pmid=28223133}}</ref> 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<ref>{{Cite journal |last1=Cho |first1=Yong-Woo |last2=Jang |first2=Jinho |last3=Park |first3=Chong Rae |last4=Ko |first4=Sohk-Won |date=2000-12-01 |title=Preparation and Solubility in Acid and Water of Partially Deacetylated Chitins |url=https://pubs.acs.org/doi/10.1021/bm000036j |journal=Biomacromolecules |language=en |volume=1 |issue=4 |pages=609–614 |doi=10.1021/bm000036j |pmid=11710189 |issn=1525-7797}}</ref><ref>{{Citation |last1=Rouhani Shirvan |first1=Anahita |title=5 - Recent advances in application of chitosan and its derivatives in functional finishing of textiles |date=2019-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780081024911000058 |work=The Impact and Prospects of Green Chemistry for Textile Technology |pages=107–133 |editor-last=Shahid-ul-Islam |access-date=2023-12-18 |series=The Textile Institute Book Series |publisher=Woodhead Publishing |isbn=978-0-08-102491-1 |last2=Shakeri |first2=Mina |last3=Bashari |first3=Azadeh |editor2-last=Butola |editor2-first=B. S.}}</ref>

===Humans and other mammals===
Humans and other mammals have [[chitinase]] and [[CHI3L1|chitinase-like proteins]] that can degrade chitin; they also possess several [[immune receptor]]s that can recognize chitin and its degradation products, initiating an [[immune response]].<ref name=Komi2017rev>{{cite journal|last1=Elieh Ali Komi|first1=D|last2=Sharma|first2=L|last3=Dela Cruz|first3=CS|title=Chitin and Its Effects on Inflammatory and Immune Responses.|journal=Clinical Reviews in Allergy & Immunology|volume=54|issue=2|pages=213–223|date=1 March 2017|doi=10.1007/s12016-017-8600-0|pmid=28251581|pmc=5680136}}</ref>

Chitin is sensed mostly in the lungs or [[gastrointestinal tract]] where it can activate the [[innate immune system]] through [[eosinophil]]s or  [[macrophage]]s, as well as an [[adaptive immune response]] through [[T helper]] cells.<ref name=Komi2017rev/> [[Keratinocyte]]s in skin can also react to chitin or chitin fragments.<ref name=Komi2017rev/>

===Plants===
Plants also have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein.<ref name=Komi2017rev/>  The first chitin receptor was cloned in 2006.<ref name=Sanchez2015rev>{{cite journal|last1=Sánchez-Vallet|first1=A|last2=Mesters|first2=JR|last3=Thomma|first3=BP|title=The battle for chitin recognition in plant-microbe interactions.|journal=FEMS Microbiology Reviews|date=March 2015|volume=39|issue=2|pages=171–83|doi=10.1093/femsre/fuu003|pmid=25725011|issn=0168-6445|doi-access=free|hdl=20.500.11850/97275|hdl-access=free}}</ref>  When the receptors are activated by chitin, genes related to plant defense are expressed, and [[jasmonate]] hormones are activated, which in turn activate systemic defenses.<ref name=Sharp2013rev/>  [[Commensalism|Commensal]] fungi have ways to interact with the host immune response that, {{as of|2016|lc=y}}, were not well understood.<ref name=Sanchez2015rev/>

Some pathogens produce chitin-binding proteins that mask the chitin they shed from these receptors.<ref name=Sharp2013rev>{{cite journal|last1=Sharp|first1=Russell G.|title=A Review of the Applications of Chitin and Its Derivatives in Agriculture to Modify Plant-Microbial Interactions and Improve Crop Yields|journal=Agronomy|date=21 November 2013|volume=3|issue=4|pages=757–793|doi=10.3390/agronomy3040757|language=en|doi-access=free}}</ref><ref>{{cite journal|last1=Rovenich|first1=H|last2=Zuccaro|first2=A|last3=Thomma|first3=BP|title=Convergent evolution of filamentous microbes towards evasion of glycan-triggered immunity.|journal=The New Phytologist|date=December 2016|volume=212|issue=4|pages=896–901|doi=10.1111/nph.14064|pmid=27329426|doi-access=free}}</ref> ''[[Zymoseptoria tritici]]'' is an example of a fungal pathogen that has such blocking proteins; it is a major pest in [[wheat]] crops.<ref name=Kettles2016rev/>