Editing Histamine (section)

== Properties ==
Histamine base, obtained as a mineral oil [[Mulling (spectroscopy)|mull]], melts at 83–84&nbsp;°C.<ref>{{cite web|url=http://webbook.nist.gov/cgi/cbook.cgi?ID=C51456&Mask=80|title=Histamine|website=webbook.nist.gov|url-status=dead|archive-url=https://web.archive.org/web/20180427205956/https://webbook.nist.gov/cgi/cbook.cgi?ID=C51456&Mask=80|archive-date=2018-04-27|access-date=2015-01-04}}</ref> Hydrochloride<ref>{{cite web|url=http://www.sigmaaldrich.com/catalog/product/sigma/h7250?lang=en&region=US|title=Histamine dihydrochloride H7250|website=Sigma-Aldrich|url-status=live|archive-url=https://web.archive.org/web/20150809131738/http://www.sigmaaldrich.com/catalog/product/sigma/h7250?lang=en&region=US|archive-date=2015-08-09}}</ref> and phosphorus<ref>{{cite book | chapter-url=http://lib.njutcm.edu.cn/yaodian/ep/EP501E/16_monographs/17_monographs_d-k/histamine_phosphate/0144e.pdf | chapter = Histamine phosphate | title = European Pharmacopoeia | edition = 5th | isbn = 9287152810 |access-date=2015-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20150104181128/http://lib.njutcm.edu.cn/yaodian/ep/EP501E/16_monographs/17_monographs_d-k/histamine_phosphate/0144e.pdf |archive-date=2015-01-04 }}</ref> salts form white [[hygroscopic]] [[crystal]]s and are easily dissolved in [[properties of water|water]] or [[ethanol]], but not in [[diethyl ether|ether]]. In [[aqueous solution]], the imidazole ring of histamine exists in two [[tautomer]]ic forms, identified by which of the two nitrogen atoms is protonated. The nitrogen farther away from the side chain is the 'tele' nitrogen and is denoted by a lowercase tau sign and the nitrogen closer to the side chain is the 'pros' nitrogen and is denoted by the pi sign. The tele tautomer, ''N<sup>τ</sup>-H''-histamine, is preferred in solution as compared to the pros tautomer, ''N<sup>π</sup>-H''-histamine.

[[File:Histamine tautomers.svg|400px|thumbnail|center|The tele tautomer (''N<sup>τ</sup>-H''-histamine), on the left is more stable than the pros tautomer (''N<sup>π</sup>-H''-histamine) on the right.]]

Histamine has two [[base (chemistry)|basic]] centres, namely the [[aliphatic]] amino group and whichever [[nitrogen]] atom of the imidazole ring does not already have a [[hydrogen|proton]]. Under physiological conditions, the aliphatic amino group (having a [[dissociation constant|p''K'']]<sub>a</sub> around 9.4) will be [[protonated]], whereas the second nitrogen of the imidazole ring (p''K''<sub>a</sub> ≈ 5.8) will not be protonated.<ref name="Paiva 1970">{{cite journal | vauthors = Paiva TB, Tominaga M, Paiva AC | title = Ionization of histamine, N-acetylhistamine, and their iodinated derivatives | journal = Journal of Medicinal Chemistry | volume = 13 | issue = 4 | pages = 689–92 | date = July 1970 | pmid = 5452432 | doi = 10.1021/jm00298a025 }}</ref>
Thus, histamine is normally protonated to a singly charged [[cation]]. Since human [[blood]] is slightly basic (with a normal pH range of 7.35 to 7.45) therefore the predominant form of histamine present in human blood is monoprotic at the aliphatic nitrogen. Histamine is a [[monoamine neurotransmitter]].