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An amino acid contains both acidic (carboxylic acid fragment) and basic (amine fragment) centres. The isomer on the right is a zwitterion.

Sulfamic acid isomers, with the zwitterion (right).

In chemistry, a zwitterion (/ˈtsvɪtər.aɪ.ən/ TSVIT-ər-eye-ən; from German zwitter [ˈtsvɪtɐ] "hybrid" and formerly called a dipolar ion) is a neutral molecule with a positive and a negative electrical charge, though multiple positive and negative charges can be present. Zwitterions are distinct from dipoles, at different locations within that molecule. Zwitterions are sometimes also called inner salts.^[1]

Unlike simple amphoteric compounds that might only form either a cationic or anionic species depending on external conditions, a zwitterion simultaneously has both ionic states in the same molecule.

Examples

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

NH₂RCHCO₂H NH₃⁺RCHCO₂⁻

The zwitterionic structure of glycine in the solid state has been confirmed by neutron diffraction measurements.^[2] At least in some cases, the zwitterionic form of amino acids also persists 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 as a zwitterion.^[4] Many alkaloids, such as LSD 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, a common example are the 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 (ylide) and a 1,3-dipole.^[6]

References

^ ^a ^b IUPAC Gold Book zwitterionic compounds/zwitterions
^ 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 (6): 1827–1833. doi:10.1107/S0567740872005096.
^ 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. PMC 1364450. PMID 16479267. text
^ 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.
^ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.
^ IUPAC Gold Book dipolar compounds

English Journal

A vibrational spectroscopy study on anserine and its aqueous solutions.

Akkaya Y1, Balci K2, Goren Y2, Akyuz S3, Stricker MC4, Stover DD4, Ritzhaupt G4, Collier WB4.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.Spectrochim Acta A Mol Biomol Spectrosc.2015 Oct 5;149:812-21. doi: 10.1016/j.saa.2015.05.020. Epub 2015 May 12.
In this study based on vibrational spectroscopic measurements and Density Functional Theory (DFT), we aimed for a reliable interpretation of the IR and Raman spectra recorded for anserine in the solid phase and water (H2O) and heavy water (D2O) solutions. Initial DFT calculations at the B3LYP/6-31G(
PMID 25997178

Divalent Metal-Ion Complexes with Dipeptide Ligands Having Phe and His Side-Chain Anchors: Effects of Sequence, Metal Ion, and Anchor.

Dunbar RC1, Berden G2, Martens JK2, Oomens J2,3.
The journal of physical chemistry. A.J Phys Chem A.2015 Sep 24;119(38):9901-9. doi: 10.1021/acs.jpca.5b06315. Epub 2015 Sep 11.
Conformational preferences have been surveyed for divalent metal cation complexes with the dipeptide ligands AlaPhe, PheAla, GlyHis, and HisGly. Density functional theory results for a full set of complexes are presented, and previous experimental infrared spectra, supplemented by a number of newly
PMID 26325483

Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency.

Liu Y1, Page ZA1, Russell TP2, Emrick T3.
Angewandte Chemie (International ed. in English).Angew Chem Int Ed Engl.2015 Sep 21;54(39):11485-9. doi: 10.1002/anie.201503933. Epub 2015 Jul 31.
Three conjugated polymer zwitterions (CPZs), containing thiophene-, diketopyrrolopyrrole- (DPP), and naphthalene diimide (NDI) backbones, were synthesized with pendant zwitterions, specifically sulfobetaine groups. Diboronate-ester-functionalized bithiophene and benzothiadiazole monomers were copoly
PMID 26236017

Japanese Journal

アラインを用いた複素環化合物の合成

有機合成化学協会誌 74(4), 326-334, 2016
NAID 130005150749

Friction of zwitterionic hydrogel by dynamic polymer adsorption

Macromolecules 48(15), 5394-5401, 2015-08-11
NAID 120005819298

THREE-COMPONENT SYNTHESIS OF INDOLIZINES FROM AZAAROMATIC-ACETYLENEDICARBOXYLATE ZWITTERIONS WITH ACYLZIRCONOCENE CHLORIDE COMPLEXES (Dedicated to Professor Isao Kuwajima on the occasion of his 77th birthday)

Heterocycles : an international journal for reviews and communications in heterocyclic chemistry 90(1), 108-112, 2015-01-01
NAID 40020329983