Zwitterion
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In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
that contains an equal number of positively- and negatively-charged
functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...
s. : With
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s, 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.


Amino acids

The equilibrium is established in two stages. In the first stage, a proton is transferred from the carboxyl group to a water molecule: :H2N(R)CO2H + H2O <=> H2N(R)CO2- + H3O+ In the second stage, a proton is transferred from the
hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is di ...
ion to the amine group: :H2N(R)CO2- + H3O+ <=> H3N+ (R)CO2- + H2O Overall, the reaction is an isomerization reaction :H2N(R)CO2H <=> H3N+ (R)CO2- The ratio of the concentrations of the two species in solution is independent of pH as it is equal to the value of the equilibrium constant for the isomerization reaction: :K= \frac where signifies the concentration of the chemical species X at equilibrium. It is generally assumed that , that is, that the zwitterion is the predominant amino acid isomer in aqueous solution. It has been suggested, on the basis of theoretical analysis, that the zwitterion is stabilized in aqueous solution by
hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...
with solvent water molecules. Analysis of neutron diffraction data for
glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
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 p''K''a values for deprotonation of the common amino acids span the approximate range . 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 p''K''a value of 4.88.


Other compounds

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 H4EDTA File:Psilocybn.svg,
Psilocybin Psilocybin ( , ) is a naturally occurring psychedelic prodrug compound produced by more than 200 species of fungi. The most potent are members of the genus ''Psilocybe'', such as '' P. azurescens'', '' P. semilanceata'', and '' P.&n ...
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. 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 studies

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. 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.


Betaines and similar compounds

The compound trimethylglycine, which was isolated from
sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose and which is grown commercially for sugar production. In plant breeding, it is known as the Altissima cultivar group of the common beet ('' Beta vulgaris''). Together ...
, 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. 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; and pulmonary surfactants such as dipalmitoylphosphatidylcholine. Lauramidopropyl betaine is the major component of cocamidopropyl betaine. File:Betaine Formula V.1.svg, Trimethylglycine (trivial name betaine) File:1-Oleoyl-2-almitoyl-phosphatidylcholine Structural Formulae V.1.png, Example of a
phosphatidylcholine Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup. They are a major component of biological membranes and can be easily obtained from a variety of readily available sources, such as egg yolk or so ...
File:Cocamidopropyl betaine2.png, cocamidopropyl betaine


Conjugated zwitterions

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. Donor-acceptor entities are of vast use in photochemistry (
photoinduced electron transfer Photoinduced electron transfer (PET) is an excited state electron transfer process by which an excited electron is transferred from donor to acceptor."Organic and Inorganic Photochemistry" V. Ramamurthy and Kirk S. Schanze 1998 Marcel Dekker Due ...
), organic electronics, switching and sensing.


See also

*
Amphoterism In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphip ...
* Azomethine ylide


References

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