Zwitterionic

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 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.

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.

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