Two isomeric forms of an amino acid. The isomer on the right is a zwitterion.
sulfamic acid isomers

In chemistry, a zwitterion (play /ˈtsvɪtər..ən/ tsvit-ər-eye-ən; from German zwitter [ˈtsvɪtɐ] "hybrid") is a neutral molecule with a positive and a negative electrical charge (n.b. not dipoles) at different locations within that molecule. Zwitterions are sometimes also called inner salts.[1]


Amino acids are the best-known examples of zwitterions. These compounds contain an ammonium and a carboxylate groups, and can be viewed as arising via a kind of intramolecular acid-base reaction: The amine group deprotonates the carboxylic acid.

NH2RCHCO2H is in equilibrium with NH3+RCHCO2

The zwitterionic structure of glycine in the solid state has been confirmed by neutron diffraction measurements.[2] At least in some cases, zwitterionic form of amino acids also persist in the gas phase.[3]

In addition to the amino acids, many other compounds that contain both acidic and basic centres tautomerize to the zwitterionic form. Examples, such as bicine and tricine, contain a basic secondary or tertiary amine fragment together with a carboxylic acid fragment. Neutron diffraction measurements show that solid sulfamic acid exists a zwitterion.[4] Many alkaloids, such as lysergic acid and psilocybin, exist as zwitterions because they contain carboxylates and ammonium centres.

Many zwitterions contain quaternary ammonium cations. Since it lacks N-H bonds, the ammonium center cannot participate in tautomerization. Zwitterions containing quaternary-ammonium centers are common in biology, e.g., betaines, which serve as electrolytes 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.[5]

The vesicle-forming phospholipid phosphatidylcholine, palmitoyl-oleyl-sn-phosphatidylcholine.

Related compounds

Dipolar compounds are usually not classified as zwitterions. For example amine oxides, which are often written as R3N+O-, are not zwitterions in terms of the definition,[1] which specifies that there are unit electrical charges on atoms. The distinction lies in the fact that the plus and minus signs on the amine oxide signify formal charges, not electrical charges. Formal charges are used in valence bond theory for some canonical forms used to construct a resonance hybrid. In fact another theoretical representation of an amine oxide uses a dative covalent bond ≡N→O with no formal charges. Other compounds that are sometimes referred to as zwitterions, mistakenly according to the definition above, include nitrones and "dipolar compounds", such as a 1,2-dipole and a 1,3-dipole.[6]


  1. ^ a b IUPAC Gold Book zwitterionic compounds/zwitterions
  2. ^ Jönsson, P.-G.; Kvick, Å. (1972). "Precision neutron diffraction structure determination of protein and nucleic acid components. III. The crystal and molecular structure of the amino acid α-glycine". Acta Cryst. B 28: 1827–1833. doi:10.1107/S0567740872005096. 
  3. ^ Price, William D.; Jockusch, Rebecca A.; Williams, Evan R. (1997). "Is Arginine a Zwitterion in the Gas Phase?". J. Am. Chem. Soc. 119 (49): 11988–11989. doi:10.1021/ja9711627.  text
  4. ^ R. L. Sass (April 1960). "A neutron diffraction study on the crystal structure of sulfamic acid". Acta Cryst. 13 (4): 320–324. doi:10.1107/S0365110X60000789. 
  5. ^ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
  6. ^ IUPAC Gold Book dipolar compounds

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  • zwitteríon — s. m. [Química] Íon com uma carga positiva e negativa no mesmo grupo de átomos. • [Brasil] Plural: zwitteríones ou zwitteríons.   ‣ Etimologia: alemão Zwitterion, de zwitter, híbrido + ion, íon   ♦ Grafia em Portugal: zwitterião …   Dicionário da Língua Portuguesa

  • zwitterion — Ion que tiene a la vez regiones con carga negativa y positiva. Aminoácidos, como la glicina, pueden actuar como zwitteriones cuando se encuentran en una solución neutra. Diccionario Mosby Medicina, Enfermería y Ciencias de la Salud, Ediciones… …   Diccionario médico

  • zwitterion — ● zwitterion nom masculin Ion dipolaire …   Encyclopédie Universelle

  • zwitterion — [zwit′ər ī΄än] n. [Ger < zwitter, hybrid (< OHG zwitarn < zwi , double, akin to OE twi : for IE base see TWO) + ion,ION] an ion carrying both a positive and a negative charge in different parts of the molecule, as in certain amino acids… …   English World dictionary

  • Zwitterion — Eine Aminosäure, links in unionisiertem Zustand, rechts als Zwitterion Ein Zwitterion (siehe auch Ion) ist ein Molekül mit zwei oder mehreren funktionellen Gruppen, von denen eine positiv und eine andere negativ geladen ist. Das Molekül ist… …   Deutsch Wikipedia

  • Zwitterion — Un acide aminé dans sa forme non ionisée(à gauche) et zwitterionique (à droite) Un zwitterion (de l allemand Zwitter « hybride », « hermaphrodite » prononcé [t͜svɪtɛrjɔ̃] ) est une espèce chimique moléculaire possédant des… …   Wikipédia en Français

  • Zwitterión — Un zwitterión (del alemán zwitter híbrido , hermafrodita ) es un compuesto químico que es eléctricamente neutro pero que tiene cargas formales positivas y negativas sobre átomos diferentes.[1] Los zwitteriones son especies polares y usualmente… …   Wikipedia Español

  • zwitterion — zwitterionic /tsvit euhr uy on ik/, adj. /tsvit euhr uy euhn/, n. Physical Chem. an ion with both a positive and a negative charge. [ < G Zwitterion (1897), equiv. to Zwitter hybrid, hermaphrodite + Ion ION] * * * …   Universalium

  • zwitterion — zwit·ter·ion tsvit ər .ī .änalso zwit n a dipolar ion zwit·ter·ion·ic .tsvit ər ī än ik adj * * * n. an ion that bears a positive and a negative charge. Amino acids can yield zwitterions. * * * zwit·ter·ion (tsvitґer i″on) a dipolar ion,… …   Medical dictionary

  • zwitterion — [ zwɪtərʌɪən, tsvɪ ] noun Chemistry an ion having separate positively and negatively charged groups. Derivatives zwitterionic adjective Origin early 20th cent.: Ger. Zwitterion, from Zwitter a hybrid + Ion ion …   English new terms dictionary

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