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Zwitterion

Zwitterion

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In chemistry, a zwitterion (icon ; from German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

 zwitter ˈtsvɪtɐ "hybrid") is a neutral molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 with a positive and a negative electrical charge (n.b. not dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

s) at different locations within that molecule. Zwitterions are sometimes also called inner salts.

Examples


Amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s are the best-known examples of zwitterions. These compounds contain an ammonium and a carboxylate group, and can be viewed as arising via a kind of intramolecular acid-base reaction: The amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 group deprotonates the carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

.
NH2RCHCO2H NH3+RCHCO2

The zwitterionic structure of glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

 in the solid state has been confirmed by neutron diffraction
Neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material: A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of...

 measurements. At least in some cases, zwitterionic form of amino acids also persist in the gas phase.

In addition to the amino acids, many other compounds that contain both acidic and basic centres tautomer
Tautomer
Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...

ize to the zwitterionic form. Examples, such as bicine
Bicine
Bicine is an organic compound used as a buffering agent. It is one of Good's buffers and has a pKa of 8.35 at 20 °C....

 and tricine
Tricine
Tricine is an organic compound that is used in buffer solutions. The name tricine comes from tris and glycine from which it was derived. It is a zwitterionic amino acid with a useful buffering range of pH 7.4-8.8. It is a white crystalline powder that is moderately soluble in water. It has a pH...

, contain a basic secondary or tertiary amine fragment together with a carboxylic acid fragment. Neutron diffraction
Neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material: A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of...

 measurements show that solid sulfamic acid exists a zwitterion. Many alkaloid
Alkaloid
Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Also some synthetic compounds of similar structure are attributed to alkaloids...

s, such as lysergic acid
Lysergic acid
Lysergic acid, also known as D-lysergic acid and -lysergic acid, is a precursor for a wide range of ergoline alkaloids that are produced by the ergot fungus and some plants. Amides of lysergic acid, lysergamides, are widely used as pharmaceuticals and as psychedelic drugs...

 and psilocybin
Psilocybin
Psilocybin is a naturally occurring psychedelic prodrug, with mind-altering effects similar to those of LSD and mescaline, after it is converted to psilocin. The effects can include altered thinking processes, perceptual distortions, an altered sense of time, and spiritual experiences, as well as...

, exist as zwitterions because they contain carboxylates and ammonium centres.

Many zwitterions contain quaternary ammonium cation
Quaternary ammonium cation
Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR4+, R being an alkyl group or an aryl group. Unlike the ammonium ion and the primary, secondary, or tertiary ammonium cations, the quaternary ammonium cations are permanently charged,...

s. Since it lacks N-H bonds, the ammonium center cannot participate in tautomer
Tautomer
Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...

ization. Zwitterions containing quaternary-ammonium centers are common in biology, e.g., betaines, which serve as electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

s in fish. The membrane-forming phospholipids are also commonly zwitterions. The polar head groups in these compounds are zwitterions, resulting from the presence of the anionic phosphate and cationic quaternary ammonium centres.

Related compounds


Dipolar compounds are usually not classified as zwitterions. For example amine oxide
Amine oxide
An amine oxide, also known as amine-N-oxide and N-oxide, is a chemical compound that contains the functional group R3N+-O−, an N-O bond with three additional hydrogen and/or hydrocarbon side chains attached to N. Sometimes it is written as R3N→O or, wrongly, as R3N=O.In the strict sense the...

s, which are often written as R3N+O-, are not zwitterions in terms of the definition, which specifies that there are unit electrical charges on atoms. The distinction lies in the fact that the plus and minus signs on the amine oxide signify formal charge
Formal charge
In chemistry, a formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity....

s, not electrical charges. Formal charges are used in valence bond theory
Valence bond theory
In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...

 for some canonical forms used to construct a resonance hybrid. In fact another theoretical representation of an amine oxide uses a dative covalent bond ≡N→O with no formal charges. Other compounds that are sometimes referred to as zwitterions, mistakenly according to the definition above, include nitrone
Nitrone
A nitrone is the N-oxide of an imine and a functional group in organic chemistry. The general structure is R1R2C=NR3+O- where R3 is different from H.A nitrone is 1,3-dipole in 1,3-dipolar cycloadditions...

s and "dipolar compounds", such as a 1,2-dipole and a 1,3-dipole
1,3-dipole
In organic chemistry, a 1,3-dipolar compound or 1,3-dipole is a dipolar compound with delocalized electrons and a separation of charge over three atoms...

.