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Inchemistry, azwitterion (/ˈtsvɪtərˌaɪɔːn/TSVIT-ər-EYE-awn; from German Zwitter[ˈtsvɪtɐ] 'hermaphrodite'), also called aninner salt ordipolar ion,[1] is amolecule that contains an equal number of positively and negatively chargedfunctional groups.[2](1,2-dipolar compounds, such asylides, are sometimes excluded from the definition.[3])
Some zwitterions, such asamino acid zwitterions, are inchemical equilibrium with an uncharged "parent" molecule.Betaines are zwitterions that cannotisomerize to an all-neutral form, such as when the positive charge is located on aquaternary ammonium group. Similarly, a molecule containing aphosphonium group and acarboxylate group cannot isomerize.
Tautomerism of amino acids follows this stoichiometry:
The ratio of the concentrations of the two species in solution is independent ofpH.
It has been suggested, on the basis of theoretical analysis, that the zwitterion is stabilized in aqueous solution byhydrogen bonding with solvent water molecules.[4] Analysis ofneutron diffraction data forglycine 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.[6]
ThepKa values for deprotonation of the common amino acids span the approximate range2.15±0.2. This is also consistent with the zwitterion being the predominant isomer that is present in an aqueous solution. For comparison, the simple carboxylic acidpropionic acid (CH3CH2CO2H) has a pKa value of 4.88.
Sulfamic acid crystallizes in the zwitterion form.[7]
In crystals ofanthranilic acid there are two molecules in theunit cell. One molecule is in the zwitterion form, the other is not.[8]
In the solid state,H4EDTA is a zwitterion with two protons having been transferred from carboxylic acid groups to the nitrogen atoms.[9]
Inpsilocybin, the proton on the dimethyl amino group islabile and may jump to the phosphate group to form a compound which is not a zwitterion.
Insight to the equilibrium in solution may be gained from the results of theoretical calculations. For example, pyridoxal phosphate, a form ofvitamin B6, in aqueous solution is predicted to have an equilibrium favoring a tautomeric form in which a proton is transferred from the phenolic -OH group to the nitrogen atom.[10]
Because tautomers are different compounds, they sometimes have different enough structures that they can be detected independently in their mixture. This allows experimental analysis of the equilibrium.[11]
The compoundtrimethylglycine, which was isolated fromsugar beet, was named as "betaine". Later, other compounds were discovered that contain the same structural motif, aquaternary nitrogen atom with acarboxylate group attached to it via a–CH2– link. At the present time, all compounds whose structure includes this motif are known as betaines. Betaines do not isomerize because the chemical groups attached to the nitrogen atom are notlabile. These compounds may be classed as permanent zwitterions, as isomerisation to a molecule with no electrical charges does not occur, or is very slow.[12]
Other examples of permanent zwitterions include phosphatidylcholines, which also contain a quaternary nitrogen atom, but with a negatively-charged phosphate group in place of a carboxylate group;sulfobetaines, which contain a quaternary nitrogen atom and a negatively charged sulfonate group;[13] andpulmonary surfactants such asdipalmitoylphosphatidylcholine. Lauramidopropyl betaine is the major component of cocamidopropyl betaine.
Strongly polarized conjugated compounds (conjugated zwitterions) are typically very reactive, sharediradical character, activate strong bonds and small molecules, and serve as transient intermediates in catalysis.[14] Donor-acceptor entities are of vast use in photochemistry (photoinduced electron transfer),organic electronics, switching andsensing.
