Editing Arginine (section)

== Structure ==
[[File:Betaine Arginine.png|thumb|right|250px|Delocalization of charge in guanidinium group of {{sm|l}}-Arginine]]
The [[amino acid]] [[Substituent|side-chain]] of arginine consists of a 3-carbon [[aliphatic]] straight chain, the distal end of which is capped by a [[guanidine|guanidinium]] group, which has a [[Acid dissociation constant|p''K''<sub>a</sub>]] of 13.8,<ref>{{cite journal | vauthors = Fitch CA, Platzer G, Okon M, Garcia-Moreno BE, McIntosh LP |display-authors=3| title = Arginine: Its pKa value revisited | journal = Protein Science | volume = 24 | issue = 5 | pages = 752–61 | date = May 2015 | pmid = 25808204 | pmc = 4420524 | doi = 10.1002/pro.2647 }}</ref> and is therefore always protonated and positively charged at physiological pH. Because of the [[Conjugated system|conjugation]] between the double bond and the nitrogen [[lone pair]]s, the positive charge is delocalized, enabling the formation of multiple [[hydrogen bond]]s.