Editing Keratin (section)

===Disulfide bridges===
In addition to intra- and intermolecular [[hydrogen bond]]s, the distinguishing feature of keratins is the presence of large amounts of the [[sulfur]]-containing amino acid [[cysteine]], required for the [[disulfide bond|disulfide bridges]] that confer additional strength and rigidity by permanent, thermally stable [[cross-link|crosslinking]]<ref>{{cite web | title = What is Keratin? | url = http://www.wisegeek.org/what-is-keratin.htm | publisher = WiseGEEK | access-date = 11 May 2014 | archive-date = 13 May 2014 | archive-url = https://web.archive.org/web/20140513010609/http://www.wisegeek.org/what-is-keratin.htm | url-status = live }}</ref>—in much the same way that non-protein sulfur bridges stabilize [[vulcanization|vulcanized]] [[rubber]]. Human hair is approximately 14% cysteine. The [[pungency|pungent]] smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the [[soluble|insolubility]] of keratins, except in a small number of solvents such as [[dissociation (chemistry)|dissociating]] or [[redox|reducing]] agents.
[[File:Toe nail.jpg|thumb|A human [[Nail (anatomy)|toenail]] that fell off after a small trauma.]]
The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in [[mammalian]] [[fingernail]]s, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes.<ref>{{cite journal | vauthors = H Bragulla H, G Homberger D | title = Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia | journal = Journal of Anatomy | volume = 214 | issue = 4 | pages = 516–559 | date = Apr 2009 | pmid = 19422428 | pmc = 2736122 | doi = 10.1111/j.1469-7580.2009.01066.x }}</ref> Hair and other α-keratins consist of [[alpha helix|α-helically]] coiled single protein strands (with regular intra-chain [[hydrogen bond|H-bonding]]), which are then further twisted into superhelical [[rope]]s that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.

Thiolated polymers ([[thiomers]]) can form disulfide bridges with cysteine substructures of keratins getting covalently attached to these proteins.<ref>{{cite journal | vauthors = Leichner C, Jelkmann M, Bernkop-Schnürch A | title = Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature | journal = Advanced Drug Delivery Reviews | volume = 151-152 | pages = 191–221 | date = 2019 | pmid = 31028759 | doi = 10.1016/j.addr.2019.04.007 }}</ref> Thiomers therefore exhibit high binding properties to keratins found in hair,<ref>{{cite patent | title = Cosmetic compositions containing thiomers for hair color retention | number = 20110229430A1 | inventor = Hawkins G, Afriat IR, Xavier JH, Popescu LC | country = US | pubdate = 22 September 2011 }}</ref> on skin<ref>{{cite journal | vauthors = Grießinger J, Bonengel S, Partenhauser A, Ijaz M, Bernkop-Schnürch A | title = Thiolated polymers: Evaluation of their potential as dermoadhesive excipients | journal = Drug Development and Industrial Pharmacy | volume = 43 | issue = 2 | pages = 204–212 | date = 2017 | pmid = 27585266 | doi = 10.1080/03639045.2016.1231809 }}</ref><ref>{{cite journal | vauthors = Partenhauser A, Zupančič O, Rohrer J, Bonengel S, Bernkop-Schnürch A | title = Thiolated silicone oils as adhesive skin protectants for improved barrier function | journal = International Journal of Cosmetic Science | volume = 38 | issue = 3 | pages = 257–265 | date = 2015 | pmid = 26444859 | doi = 10.1111/ics.12284 }}</ref> and on the surface of many cell types.<ref>{{cite journal | vauthors = Le-Vinh B, Steinbring C, Nguyen Le N, Matuszczak B, Bernkop-Schnürch A | title = S-Protected thiolated chitosan versus thiolated chitosan as cell adhesive biomaterials for tissue engineering. | journal = ACS Applied Materials & Interfaces | volume = 15 | issue = 34 | pages = 40304–40316 | date = 2023 | pmid = 37594415 | pmc = 10472333 | doi = 10.1021/acsami.3c09337 }}</ref>