Editing Zwitterion (section)

==Amino acids==
[[File:Amino Acid Zwitterion Structural Formulae V.1.svg|thumb|280px|An [[amino acid]] contains both acidic (carboxylic acid fragment) and basic (amine fragment) centres. The isomer on the right is a zwitterion.]]

[[tautomer|Tautomerism]] of amino acids follows this stoichiometry:
:{{chem2|RCH(NH2)CO2H <-> RCH(N+H3)CO2-}}
The ratio of the concentrations of the two species in solution is independent of [[pH]].

It has been suggested, on the basis of theoretical analysis,  that the zwitterion is stabilized in aqueous solution by [[hydrogen bonding]] with solvent water molecules.<ref>{{cite journal |title= On the Number of Water Molecules Necessary to Stabilize the Glycine Zwitterion |first1= Jan H. |last1= Jensen |first2= Mark S. |last2= Gordon |journal= Journal of the American Chemical Society |year= 1995 |volume= 117 |issue= 31 |pages= 8159–8170 |doi= 10.1021/ja00136a013 |url= https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1283&context=chem_pubs |access-date= 2020-08-28 |archive-date= 2020-12-02 |archive-url= https://web.archive.org/web/20201202083526/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1283&context=chem_pubs |url-status= live }}</ref> Analysis of [[neutron diffraction]] data for [[glycine]]  showed that it was in the zwitterionic form in the solid state and confirmed the presence of hydrogen bonds.<ref>{{cite journal|last1=Jönsson|first1=P.-G.|last2=Kvick|first2=Å.|year=1972|journal=Acta Crystallographica Section B|title=Precision neutron diffraction structure determination of protein and nucleic acid components. III. The crystal and molecular structure of the amino acid α-glycine|volume=28|pages=1827–1833|doi=10.1107/S0567740872005096|issue=6|url=http://journals.iucr.org/b/issues/1972/06/00/a09113/a09113.pdf|access-date=2019-09-03|archive-date=2020-03-14|archive-url=https://web.archive.org/web/20200314030331/http://journals.iucr.org/b/issues/1972/06/00/a09113/a09113.pdf|url-status=live}}</ref> Theoretical calculations have been used to show that zwitterions may also be present in the gas phase for some cases different from the simple carboxylic acid-to-amine transfer.<ref>{{cite journal|last1=Price|first1=William D.|last2=Jockusch|first2=Rebecca A. |last3=Williams|first3=Evan R. |year=1997|title=Is Arginine a Zwitterion in the Gas Phase?|journal=Journal of the American Chemical Society |volume=119 |issue=49 |pages=11988–11989 |doi=10.1021/ja9711627 |pmid=16479267 |pmc=1364450 }}</ref> 

The [[Proteinogenic amino acid|p''K''<sub>a</sub> values]] for deprotonation of the common amino acids span the approximate range {{val|2.15|0.2}}. This is also consistent with the zwitterion being the predominant isomer that is present in an aqueous solution. For comparison, the simple carboxylic acid [[propionic acid]] ({{chem2|CH3CH2CO2H}}) has a p''K''<sub>a</sub> value of 4.88.