Zwitterion

In chemistry, a zwitterion (/ˈtsvɪtər.aɪ.ən/ TSVIT-ər-eye-ən; from German zwitter [ˈtsvɪtɐ], meaning "hermaphrodite"), formerly called a dipolar ion, is a neutral molecule with both positive and negative electrical charges. (In some cases multiple positive and negative charges may be present.) Zwitterions are distinct from molecules that have dipoles at different locations within the molecule. Zwitterions are sometimes called inner salts.

Unlike simple amphoteric compounds that may only form either a cationic or anionic species, a zwitterion simultaneously has both ionic states.

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.

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.

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