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Zwitterion

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In chemistry, a zwitterion ( TSVIT-ər-EYE-awn; from German Zwitter [ˈtsvɪtɐ] 'hermaphrodite'), also called an inner salt or dipolar ion is a molecule that contains an equal number of positively and negatively charged functional groups.[1] (1,2-dipolar compounds, such as ylides, are sometimes excluded from the definition.[2])

Some zwitterions, such as amino acid zwitterions, are in chemical equilibrium with an uncharged "parent" molecule. Betaines are zwitterions that cannot isomerize to an all-neutral form, such as when the positive charge is located on a quaternary ammonium group. Similarly, a molecule containing a phosphonium group and a carboxylate group cannot isomerize.

Because tautomers are different compounds, they have distinct structures. By detecting each, the equilibrium between the zwitterion and its uncharged tautomer can be assessed.[3]

Sulfamic acid crystallizes in the zwitterion form.[4]

In crystals of anthranilic acid there are two molecules in the unit cell. One molecule is in the zwitterion form, the other is not.[5]

In the solid state, H4EDTA is a zwitterion with two protons having been transferred from carboxylic acid groups to the nitrogen atoms.[6]

pyridoxal phosphate

Vitamin B6, also known as pyridoxal phosphate, exists as the zwitterion in aqueous solution.[7]

The amino acids are a particularly important family of zwitterions. The zwitterions arise by tautomerism, which follows this stoichiometry:

RCH(NH2)CO2H ⇌ RCH(N+H3)CO−2

The ratio of the concentrations of the two species in solution is independent of pH. The zwitterionic form in the solid state is stabilized by hydrogen bonds.[8] Zwitterions may also be present in the gas phase for some cases different from the simple carboxylic acid-to-amine transfer.[9]

Betaines and similar compounds

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The compound trimethylglycine, named as "betaine", contain the same structural motif, a quaternary nitrogen atom with a carboxylate group attached to it via a –CH2– link. All compounds whose structure includes this motif are known as betaines. Betaines do not isomerize because the chemical groups attached to the nitrogen atom are not labile. These compounds may be classed as permanent zwitterions, as isomerisation to a molecule with no electrical charges does not occur, or is very slow.[10]

Other examples of permanent zwitterions include phosphatidylcholines, which also contain a quaternary nitrogen atom, but with a negatively-charged phosphate group in place of a carboxylate group; sulfobetaines, which contain a quaternary nitrogen atom and a negatively charged sulfonate group;[11] and pulmonary surfactants such as dipalmitoylphosphatidylcholine. Lauramidopropyl betaine is the major component of cocamidopropyl betaine.

Containing both amines or imines and phosphoric acid groups, the nucleotides such as AMP, ADP, and ATP exist significantly as zwitterions, although the positive and negative charges are not necessarily balanced.[12]

Conjugated zwitterions

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Strongly polarized conjugated compounds (conjugated zwitterions) are typically very reactive, share diradical character, activate strong bonds and small molecules, and serve as transient intermediates in catalysis.[13] Donor-acceptor entities are of vast use in photochemistry (photoinduced electron transfer), organic electronics, switching and sensing.

  1. ^ Skoog, Douglas A.; West, Donald M.; Holler, F. James; Crouch, Stanley R. (2004). Fundamentals of Analytical Chemistry (8th ed.). Thomson/Brooks/Cole. pp. 231, 385, 419, 460. ISBN 0-03-035523-0.
    Fundamentals of Analytical Chemistry (9th ed.). Cengage Learning. 2013. pp. 415–416. ISBN 978-1-285-60719-1.
  2. ^ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Zwitterionic compounds/zwitterions". doi:10.1351/goldbook.Z06752
  3. ^ Nagy, Peter I.; Takács-Novák, Krisztina (1997). "Theoretical and Experimental Studies of the Zwitterion ⇌ Neutral Form Equilibrium of Ampholytes in Pure Solvents and Mixtures". J. Am. Chem. Soc. 119 (21): 4999–5006. Bibcode:1997JAChS.119.4999N. doi:10.1021/ja963512f.
  4. ^ Sass, R. L. (1960). "A neutron diffraction study on the crystal structure of sulfamic acid". Acta Crystallographica. 13 (4): 320–324. Bibcode:1960AcCry..13..320S. doi:10.1107/S0365110X60000789.
  5. ^ Brown, C. J.; Ehrenberg, M. (1985). "Anthranilic acid, C7H7NO2, by neutron diffraction". Acta Crystallographica C. 41 (3): 441–443. Bibcode:1985AcCrC..41..441B. doi:10.1107/S0108270185004206.
  6. ^ Cotrait, Par Michel (1972). "La structure cristalline de l'acide éthylènediamine tétraacétique, EDTA" [The crystalline structure of ethylenediamine tetraacetic acid, EDTA]. Acta Crystallographica B. 28 (3): 781–785. Bibcode:1972AcCrB..28..781C. doi:10.1107/S056774087200319X.
  7. ^ Kiruba, G. S. M.; Ming, Wah Wong (2003). "Tautomeric Equilibria of Pyridoxal-5′-phosphate and 3-Hydroxypyridine Derivatives: A Theoretical Study of Solvation Effects". Journal of Organic Chemistry. 68 (7): 2874–2881. doi:10.1021/jo0266792. PMID 12662064.
  8. ^ 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" (PDF). Acta Crystallographica Section B. 28 (6): 1827–1833. Bibcode:1972AcCrB..28.1827J. doi:10.1107/S0567740872005096. Archived (PDF) from the original on 2020-03-14. Retrieved 2019-09-03.
  9. ^ Price, William D.; Jockusch, Rebecca A.; Williams, Evan R. (1997). "Is Arginine a Zwitterion in the Gas Phase?". Journal of the American Chemical Society. 119 (49): 11988–11989. Bibcode:1997JAChS.11911988P. doi:10.1021/ja9711627. PMC 1364450. PMID 16479267.
  10. ^ Nelson, D. L.; Cox, M. M. (2000). Lehninger, Principles of Biochemistry (3rd ed.). New York: Worth Publishing. ISBN 1-57259-153-6.
  11. ^ Gonenne, Amnon; Ernst, Robert (1978-06-15). "Solubilization of membrane proteins by sulfobetaines, novel zwitterionic surfactants". Analytical Biochemistry. 87 (1): 28–38. doi:10.1016/0003-2697(78)90565-1. ISSN 0003-2697. PMID 677454.
  12. ^ Kaszubowski, Oskar; Ślepokura, Katarzyna (2025). "Post-crystallization Rearrangement of Crystal Architecture, Intermolecular Interactions, and Nucleotide Conformation in Adenosine Monophosphates Crystals Induced by Single Crystal-to-Single Crystal Dehydration". Crystal Growth & Design. 25 (12): 4586–4600. doi:10.1021/acs.cgd.5c00490.
  13. ^ Munz, Dominik; Karsten, Meyer (2021). "Charge frustration in ligand design and functional group transfer". Nat. Rev. Chem. 5 (6): 422–439. doi:10.1038/s41570-021-00276-3. PMID 37118028. S2CID 235220781.