Zwitterion

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.

Betaines are zwitterions that cannot isomerize to an all-neutral form, such as when the positive charge is located on a quaternary ammonium group.

It has been suggested, on the basis of theoretical analysis, that the zwitterion is stabilized in aqueous solution by hydrogen bonding with solvent water molecules.

[4] Analysis of neutron diffraction data for glycine showed that it was in the zwitterionic form in the solid state and confirmed the presence of hydrogen bonds.

[5] Theoretical calculations have been used to show that zwitterions may also be present in the gas phase for some cases different from the simple carboxylic acid-to-amine transfer.

Later, other compounds were discovered that contain the same structural motif, a quaternary nitrogen atom with a carboxylate group attached to it via a –CH2– link.