In chemistry, a zwitterion (/ˈtsvɪtəˌraɪən/ TSVIT-ə-ry-ən; from German Zwitter [ˈtsvɪtɐ] 'hermaphrodite'), also called an inner salt or dipolar ion,[1] is a molecule that contains an equal number of positively and negatively charged functional groups.[2] With amino acids, for example, in solution a chemical equilibrium will be established between the "parent" molecule and the zwitterion.
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.