Zwitterion
In chemistry, a zwitterion (Template:IPAc-en Template:Respell; Template:Etymology), also called an inner salt or dipolar ion,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is a molecule that contains an equal number of positively and negatively charged functional groups.<ref Name=Skoog>Template:Cite book
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(1,2-dipolar compounds, such as ylides, are sometimes excluded from the definition.<ref name=GoldBook>Template:GoldBookRef</ref>)
Some zwitterions, such as amino acid zwitterions, are in chemical equilibrium with an uncharged "parent" molecule. 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.
Amino acidsEdit
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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
The pKa values for deprotonation of the common amino acids span the approximate range Template:Val. 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 (Template:Chem2) has a pKa value of 4.88.
Other compoundsEdit
- Zwitterion Structural Formulae V.1.svg
Sulfamic acid isomers, with the zwitterion (right)
- Anthranilsäure.svg
- EDTA-xtal-2D-skeletal.png
Structure of H4EDTA
- Psilocybin, Kekulé, skeletal formula of canonical psilocybin.svg
Sulfamic acid crystallizes in the zwitterion form.<ref name="Sass">Template:Cite journal</ref>
In crystals of anthranilic acid there are two molecules in the unit cell. One molecule is in the zwitterion form, the other is not.<ref>Template:Cite journal</ref>
In the solid state, H4EDTA is a zwitterion with two protons having been transferred from carboxylic acid groups to the nitrogen atoms.<ref>Template:Cite journal</ref>
In psilocybin, the proton on the dimethyl amino group is labile and may jump to the phosphate group to form a compound which is not a zwitterion.
Theoretical studiesEdit
Insight to the equilibrium in solution may be gained from the results of theoretical calculations. For example, pyridoxal phosphate, a form of vitamin 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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
Betaines and similar compoundsEdit
The compound trimethylglycine, which was isolated from sugar beet, was named as "betaine". 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. 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 not labile. These compounds may be classed as permanent zwitterions, as isomerisation to a molecule with no electrical charges does not occur, or is very slow.<ref>Template:Cite book</ref>
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;<ref>Template:Cite journal</ref> and pulmonary surfactants such as dipalmitoylphosphatidylcholine. Lauramidopropyl betaine is the major component of cocamidopropyl betaine.
- Betaine Formula V.1.svg
Trimethylglycine (trivial name betaine)
- 1-Oleoyl-2-almitoyl-phosphatidylcholine Structural Formulae V.1.png
Example of a phosphatidylcholine
- Cocamidopropyl betaine.svg
Conjugated zwitterionsEdit
Strongly polarized conjugated compounds (conjugated zwitterions) are typically very reactive, share diradical character, activate strong bonds and small molecules, and serve as transient intermediates in catalysis.<ref>Template:Cite journal</ref> Donor-acceptor entities are of vast use in photochemistry (photoinduced electron transfer), organic electronics, switching and sensing.