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[[File:Amino Acid Zwitterion Structural Formulae V.1.svg|thumb|280px|An [[Chemistry:Amino acid|amino acid]] contains both acidic (carboxylic acid fragment) and basic (amine fragment) centres. The isomer on the right is a zwitterion.]]
[[File:Zwitterion Structural Formulae V.1.svg|thumb|280px|Sulfamic acid isomers, with the zwitterion (right).]]
In chemistry, a '''zwitterion''' ({{IPAc-en|ˈ|t|s|v|ɪ|t|ə|ˌ|r|aɪ|ə|n}} {{respell|TSVIT|ə|rye|ən}}; {{ety|de|Zwitter ''{{IPA-de|ˈtsvɪtɐ|}}''<nowiki />|[[Biology:Hermaphrodite|hermaphrodite]]}}), formerly called a '''dipolar ion''', is a [[Physics:Molecule|molecule]] with two or more [[Chemistry:Functional group|functional groups]], of which at least one has a positive and one has a negative electrical charge and the net charge of the entire molecule is zero. Because they contain at least one positive and one negative charge, zwitterions are also sometimes called '''inner salts'''.<ref name=zwitter>IUPAC Gold Book [http://goldbook.iupac.org/Z06752.html ''zwitterionic compounds/zwitterions'']</ref> The charges on the different functional groups balance each other out, and the molecule as a whole is electrically neutral. The [[Chemistry:PH|pH]] where this happens is known as the [[Biology:Isoelectric point|isoelectric point]].<ref>{{Cite book|url=https://books.google.com/books?id=Gx0fZ_DF5BoC&pg=PA54|title=Biochemistry|last=Das|first=Debajyoti|date=1978|publisher=Academic Publishers|year=|isbn=9788187504825|location=|pages=54|language=en}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=NYa45_BxgukC&pg=PA74|title=Biochemistry|last=Campbell|first=Mary|last2=Farrell|first2=Shawn|date=2007-11-20|publisher=Cengage Learning|year=|isbn=0495390410|location=|pages=74|language=en}}</ref>

Unlike simple amphoteric compounds that may only form ''either'' a cationic or anionic species, a zwitterion simultaneously has both ionic states.<ref>[http://www.chemicool.com/definition/amphoteric "Definition of amphoteric"] (2014), Chemicool. Retrieved 2016-06-14.</ref>

==Examples==
[[Chemistry:Amino acid|Amino acid]]s are examples of zwitterions. These compounds contain an [[Chemistry:Ammonium|ammonium]] and a carboxylate group, and can be viewed as arising via a kind of intramolecular [[Chemistry:Acid–base reaction|acid–base reaction]]: The [[Chemistry:Amine|amine]] group deprotonates the [[Chemistry:Carboxylic acid|carboxylic acid]].
:{{chem|H|2|NRCHCOOH}} ⇌ {{chem||+|H|3|NRCHCOO|−}}
The zwitterionic structure of [[Chemistry:Glycine|glycine]] in the solid state has been confirmed by [[Physics:Neutron diffraction|neutron diffraction]] measurements.<ref>{{cite journal|last=Jönsson|first=P.-G.|author2=Kvick, 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}}</ref> At least in some cases, the zwitterionic form of amino acids also persists in the gas phase.<ref>{{cite journal|last=Price|first=William D.|author2=Jockusch, Rebecca A. |author3=Williams, 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>

In addition to the amino acids, many other compounds that contain both acidic and basic centres [[Chemistry:Tautomer|tautomer]]ize to the zwitterionic form. Examples, such as [[Chemistry:Bicine|bicine]] and [[Chemistry:Tricine|tricine]], contain a basic secondary or tertiary amine fragment together with a carboxylic acid fragment. [[Physics:Neutron diffraction|Neutron diffraction]] measurements show that solid [[Chemistry:Sulfamic acid|sulfamic acid]] exists as a zwitterion.<ref name="Sass">{{ cite journal | author = R. L. Sass |date=April 1960 | title = A neutron diffraction study on the crystal structure of sulfamic acid | journal = Acta Crystallogr. | volume = 13 | issue = 4 | pages = 320–324 | doi = 10.1107/S0365110X60000789 }}</ref> Many [[Chemistry:Alkaloid|alkaloid]]s, such as [[Chemistry:Psilocybin|psilocybin]], exist as zwitterions because they contain carboxylates and ammonium centres.

Many zwitterions contain [[Chemistry:Quaternary ammonium cation|quaternary ammonium cation]]s. Since it lacks N–H bonds, the ammonium center cannot participate in [[Chemistry:Tautomer|tautomer]]ization. Zwitterions containing quaternary ammonium centers are common in biology; a common example are the [[Chemistry:Betaine|betaine]]s, which serve as [[Physics:Electrolyte|electrolyte]]s in fish. The membrane-forming phospholipids are also commonly zwitterions. The polar head groups in these compounds are zwitterions, resulting from the presence of the anionic phosphate and cationic quaternary ammonium centres.<ref>Nelson, D. L.; Cox, M. M. ''Lehninger, Principles of Biochemistry'' 3rd Ed. Worth Publishing: New York, 2000. {{ISBN|1-57259-153-6}}.</ref> [[Image:1-palmitoyl-2-oleoylphosphatidylcholine.svg|300px|thumb|The vesicle-forming phospholipid phosphatidylcholine, palmitoyl-oleyl-''sn''-phosphatidylcholine.]]

A [[Chemistry:Ylide|ylide]] or ylid is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. It is a type of zwitterion.

== Calculation of the isoelectric point ==
The pH value at the isoelectric point can be calculated from the [[Chemistry:Acid dissociation constant|acid dissociation constant]]s of the acidic and basic groups of the zwitterion (except at high dilution):
:<math chem>\ce{pH}=\frac{\ce pK_\ce{a_1}+\ce pK_\ce{a_2}} {2}</math>
'''Derivation:'''

The acid dissociation constants are defined as:
: <math chem>\begin{align}
       K_\ce{a_1}=c\left(\ce{H3O+}\right)\cdot\frac{c(\ce{Zwitterion})}{c(\ce{Cation})}\\[10px]
       K_\ce{a_2}=c\left(\ce{H3O+}\right)\cdot\frac{c(\ce{Anion})}{c(\ce{Zwitterion})}
             \end{align}</math>
Multiplication of these two equations eliminates the concentration of the zwitterion:
:<math chem>K_\ce{a_1}\cdot K_\ce{a_2}=c\left(\ce{H3O+}\right)^2 \cdot\frac{c(\ce{Anion})}{c(\ce{Cation})}</math>
Because the concentrations of the anion and cation are equal at the isoelectric point, the equation simplifies to:
:<math chem>K_\ce{a_1}\cdot K_\ce{a_2} = c\left(\ce{H3O+}\right)^2</math>
Taking the square root and logarithm on both sides yields:
:<math chem>\ce{pH}=\frac{\ce pK_\ce{a_1}+\ce pK_\ce{a_2}} {2}</math>

=== Multiprotic systems ===
In the case of acidic amino acids (e.g. [[Chemistry:Aspartic acid|aspartic acid]]) or basic amino acids (e.g. [[Chemistry:Lysine|lysine]]), the p''K''<sub>a</sub> values of the two similar groups are taken into consideration: in the case of amino acids with basic side chains, the side chain and the amino group can both take a positive charge at low pH values, and in the case of amino acids with acidic side chains, the two carboxylic acid groups can both take a negative charge at high pH values. The molecule is therefore neutral halfway between these two p''K'' values, when one of the two similar groups is neutral, so the p''K'' values of the two similar groups are substituted into the equation above<ref name="Kuhn">Reinhard Kuhn: ''Capillary Electrophoresis: Principles and Practice.'' Springer Science & Business Media, 2013, {{ISBN|978-3-642-78058-5}}, S.&nbsp;79.</ref> and the average is taken,<ref name="Kim">Cherng-ju Kim: ''Advanced Pharmaceutics.'' CRC Press, 2004, {{ISBN|978-0-203-49291-8}}, S.&nbsp;86–99.</ref> for example for lysine: {{sfrac|8.95 + 10.53|2}} = 9.74<ref name="Godbey">W. T. Godbey: ''An Introduction to Biotechnology.'' Elsevier, 2014, {{ISBN|978-1-908-81848-5}}, S.&nbsp;15.</ref> and for aspartic acid: {{sfrac|2.09 + 3.86|2}} = 2.98.<ref name="Chang">Raymond Chang: ''Physical Chemistry for the Biosciences.'' University Science Books, 2005, {{ISBN|978-1-891-38933-7}}, S.&nbsp;291.</ref>

==Related compounds==
The [[Chemistry:Resonance|resonance structures]] that are used to represent the charge delocalization of [[Chemistry:Dipolar compound|dipolar compound]]s frequently include zwitterions, even when there is not complete separation of charges like with a stable zwitterion.<ref name="dipolar compounds">IUPAC Gold Book [http://goldbook.iupac.org/D01753.html ''dipolar compounds'']</ref> Zwitterionic compounds, on the other hand, have stable, separated unit electrical charges on atoms.<ref name=zwitter />

Also related are [[Chemistry:Mesoionic|mesoionic]] compounds, which are dipolar heterocyclic compounds in which both the negative and the positive charge are delocalized.

==References==
{{reflist}}

[[Category:Zwitterions| ]]
[[Category:Ions]]

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