Zwitterion
In chemistry, a zwitterion (ˈtsvɪtəˌraɪən ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups. 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. Tautomerism of amino acids follows this stoichiometry: The ratio of the concentrations of the two species in solution is independent of pH. 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. 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. 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. The pKa values for deprotonation of the common amino acids span the approximate range 2.15. This is also consistent with the zwitterion being the predominant isomer that is present in an aqueous solution. For comparison, the simple carboxylic acid propionic acid () has a pKa value of 4.88. File:Zwitterion Structural Formulae V.1.svg|[[Sulfamic acid]] isomers, with the zwitterion (right) File:Anthranilsäure.svg|[[Anthranilic acid]] File:EDTA-xtal-2D-skeletal.png|Structure of [[EDTA|H4EDTA]] File:Psilocybn.svg| [[Psilocybin]] Sulfamic acid crystallizes in the zwitterion form. In crystals of anthranilic acid there are two molecules in the unit cell. One molecule is in the zwitterion form, the other is not. In the solid state, H4EDTA is a zwitterion with two protons having been transferred from carboxylic acid groups to the nitrogen atoms.