Zwitterion

File:Amino Acid Zwitterion Structural Formulae V.1.svg

An 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

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.

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. At least in some cases, the zwitterionic form of amino acids also persists in the gas phase.

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. 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.

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, 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.