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{{Short description|Molecule containing an equal number of positive and negative functional groups}} | {{Short description|Molecule containing an equal number of positive and negative functional groups}} | ||
In [[chemistry]], a '''zwitterion''' ({{IPAc-en|ˈ|t|s|v|ɪ|t| | In [[chemistry]], a '''zwitterion''' ({{IPAc-en|ˈ|t|s|v|ɪ|t|ɚ|ˌ|aɪ|ɔː|n|}} {{respell|TSVIT|ər|EYE|awn}}; {{etymology|de|{{wikt-lang|de|Zwitter}} ''{{IPA|de|ˈtsvɪtɐ|}}''<nowiki />|[[hermaphrodite]]}}), also called an '''inner salt''' or '''dipolar ion''',<ref>{{cite web |date=2015-11-03 |title=Zwitterion |url=https://chem.libretexts.org/Ancillary_Materials/Reference/Organic_Chemistry_Glossary/Zwitterion |access-date=2022-02-11 |website=Chemistry LibreTexts |language=en |archive-date=2023-06-21 |archive-url=https://web.archive.org/web/20230621214448/https://chem.libretexts.org/Ancillary_Materials/Reference/Organic_Chemistry_Glossary/Zwitterion |url-status=live}}</ref> is a [[molecule]] that contains an equal number of positively and negatively charged [[functional group]]s.<ref Name=Skoog>{{cite book |last1=Skoog|first1=Douglas A. |last2=West|first2=Donald M. |last3=Holler|first3=F. James |last4=Crouch|first4=Stanley R. |title=Fundamentals of Analytical Chemistry |date=2004 |publisher=Thomson/Brooks/Cole |isbn=0-03-035523-0 |pages=231, 385, 419, 460 |edition=8th}} | ||
:{{cite book |last1=Skoog |first1=Douglas A. |last2=West |first2=Donald M. |last3=Holler |first3=F. James |display-authors=0 |title=Fundamentals of Analytical Chemistry |edition=9th |date=2013 |pages=415–416 |isbn=978-1-285-60719-1}}</ref> | :{{cite book |last1=Skoog |first1=Douglas A. |last2=West |first2=Donald M. |last3=Holler |first3=F. James |display-authors=0 |title=Fundamentals of Analytical Chemistry |edition=9th |date=2013 |pages=415–416 |publisher=Cengage Learning |isbn=978-1-285-60719-1}}</ref> | ||
(1,2-[[dipolar compound]]s, such as [[ylide]]s, are sometimes excluded from the definition.<ref name=GoldBook>{{GoldBookRef|title=Zwitterionic compounds/zwitterions|file=Z06752}}</ref>) | (1,2-[[dipolar compound]]s, such as [[ylide]]s, are sometimes excluded from the definition.<ref name=GoldBook>{{GoldBookRef|title=Zwitterionic compounds/zwitterions|file=Z06752}}</ref>) | ||
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The ratio of the concentrations of the two species in solution is independent of [[pH]]. | 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>{{cite journal |title= On the Number of Water Molecules Necessary to Stabilize the Glycine Zwitterion |first1= Jan H. |last1= Jensen |first2= Mark S. |last2= Gordon |journal= Journal of the American Chemical Society |year= 1995 |volume= 117 |issue= 31 |pages= 8159–8170 |doi= 10.1021/ja00136a013 |url= https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1283&context=chem_pubs |access-date= 2020-08-28 |archive-date= 2020-12-02 |archive-url= https://web.archive.org/web/20201202083526/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1283&context=chem_pubs |url-status= live }}</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>{{cite journal|last1=Jönsson|first1=P.-G.|last2=Kvick|first2=Å.|year=1972|journal=Acta Crystallographica Section B|title=Precision neutron diffraction structure determination of protein and nucleic acid components. III. The crystal and molecular structure of the amino acid α-glycine|volume=28|pages=1827–1833|doi=10.1107/S0567740872005096|issue=6|url=http://journals.iucr.org/b/issues/1972/06/00/a09113/a09113.pdf|access-date=2019-09-03|archive-date=2020-03-14|archive-url=https://web.archive.org/web/20200314030331/http://journals.iucr.org/b/issues/1972/06/00/a09113/a09113.pdf|url-status=live}}</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>{{cite journal|last1=Price|first1=William D.|last2=Jockusch|first2=Rebecca A. |last3=Williams|first3=Evan R. |year=1997|title=Is Arginine a Zwitterion in the Gas Phase?|journal=Journal of the American Chemical Society |volume=119 |issue=49 |pages=11988–11989 |doi=10.1021/ja9711627 |pmid=16479267 |pmc=1364450 }}</ref> | 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>{{cite journal |title= On the Number of Water Molecules Necessary to Stabilize the Glycine Zwitterion |first1= Jan H. |last1= Jensen |first2= Mark S. |last2= Gordon |journal= Journal of the American Chemical Society |year= 1995 |volume= 117 |issue= 31 |pages= 8159–8170 |doi= 10.1021/ja00136a013 |bibcode= 1995JAChS.117.8159J |url= https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1283&context=chem_pubs |access-date= 2020-08-28 |archive-date= 2020-12-02 |archive-url= https://web.archive.org/web/20201202083526/https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1283&context=chem_pubs |url-status= live }}</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>{{cite journal|last1=Jönsson|first1=P.-G.|last2=Kvick|first2=Å.|year=1972|journal=Acta Crystallographica Section B|title=Precision neutron diffraction structure determination of protein and nucleic acid components. III. The crystal and molecular structure of the amino acid α-glycine|volume=28|pages=1827–1833|doi=10.1107/S0567740872005096|issue=6|bibcode=1972AcCrB..28.1827J |url=http://journals.iucr.org/b/issues/1972/06/00/a09113/a09113.pdf|access-date=2019-09-03|archive-date=2020-03-14|archive-url=https://web.archive.org/web/20200314030331/http://journals.iucr.org/b/issues/1972/06/00/a09113/a09113.pdf|url-status=live}}</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>{{cite journal|last1=Price|first1=William D.|last2=Jockusch|first2=Rebecca A. |last3=Williams|first3=Evan R. |year=1997|title=Is Arginine a Zwitterion in the Gas Phase?|journal=Journal of the American Chemical Society |volume=119 |issue=49 |pages=11988–11989 |doi=10.1021/ja9711627 |pmid=16479267 |pmc=1364450 |bibcode=1997JAChS.11911988P }}</ref> | ||
The [[Proteinogenic amino acid|p''K''<sub>a</sub> values]] for deprotonation of the common amino acids span the approximate range {{val|2.15|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 acid [[propionic acid]] ({{chem2|CH3CH2CO2H}}) has a p''K''<sub>a</sub> value of 4.88. | The [[Proteinogenic amino acid|p''K''<sub>a</sub> values]] for deprotonation of the common amino acids span the approximate range {{val|2.15|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 acid [[propionic acid]] ({{chem2|CH3CH2CO2H}}) has a p''K''<sub>a</sub> value of 4.88. | ||
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File:Psilocybin, Kekulé, skeletal formula of canonical psilocybin.svg| [[Psilocybin]] | File:Psilocybin, Kekulé, skeletal formula of canonical psilocybin.svg| [[Psilocybin]] | ||
</gallery> | </gallery> | ||
[[Sulfamic acid]] crystallizes in the zwitterion form.<ref name="Sass">{{ cite journal | first= R. L.|last= Sass |date=1960 | title = A neutron diffraction study on the crystal structure of sulfamic acid | journal = Acta Crystallographica | volume = 13 | issue = 4 | pages = 320–324 | doi = 10.1107/S0365110X60000789 | doi-access = free }}</ref> | [[Sulfamic acid]] crystallizes in the zwitterion form.<ref name="Sass">{{ cite journal | first= R. L.|last= Sass |date=1960 | title = A neutron diffraction study on the crystal structure of sulfamic acid | journal = Acta Crystallographica | volume = 13 | issue = 4 | pages = 320–324 | doi = 10.1107/S0365110X60000789 |bibcode= 1960AcCry..13..320S | doi-access = free }}</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>{{cite journal |last1=Brown |first1=C. J. |last2=Ehrenberg |first2=M. |title=Anthranilic acid, C<sub>7</sub>H<sub>7</sub>NO<sub>2</sub>, by neutron diffraction |journal=Acta Crystallographica C |date=1985 |volume=41 |issue=3 |pages=441–443 |doi=10.1107/S0108270185004206}}</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>{{cite journal |last1=Brown |first1=C. J. |last2=Ehrenberg |first2=M. |title=Anthranilic acid, C<sub>7</sub>H<sub>7</sub>NO<sub>2</sub>, by neutron diffraction |journal=Acta Crystallographica C |date=1985 |volume=41 |issue=3 |pages=441–443 |doi=10.1107/S0108270185004206 |bibcode=1985AcCrC..41..441B }}</ref> | ||
In the solid state, [[EDTA|H<sub>4</sub>EDTA]] is a zwitterion with two protons having been transferred from carboxylic acid groups to the nitrogen atoms.<ref>{{cite journal |first= Par Michel |last= Cotrait |title= La structure cristalline de l'acide éthylènediamine tétraacétique, EDTA |trans-title=The crystalline structure of ethylenediamine tetraacetic acid, EDTA |year= 1972 |journal= Acta Crystallographica B |volume= 28 |issue= 3 |pages= 781–785 |doi= 10.1107/S056774087200319X}}</ref> | In the solid state, [[EDTA|H<sub>4</sub>EDTA]] is a zwitterion with two protons having been transferred from carboxylic acid groups to the nitrogen atoms.<ref>{{cite journal |first= Par Michel |last= Cotrait |title= La structure cristalline de l'acide éthylènediamine tétraacétique, EDTA |trans-title=The crystalline structure of ethylenediamine tetraacetic acid, EDTA |year= 1972 |journal= Acta Crystallographica B |volume= 28 |issue= 3 |pages= 781–785 |doi= 10.1107/S056774087200319X |bibcode= 1972AcCrB..28..781C }}</ref> | ||
In [[psilocybin]], the proton on the dimethyl amino group is [[lability#chemistry|labile]] and may jump to the phosphate group to form a compound which is not a zwitterion. | In [[psilocybin]], the proton on the dimethyl amino group is [[lability#chemistry|labile]] and may jump to the phosphate group to form a compound which is not a zwitterion. | ||
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==Theoretical studies== | ==Theoretical studies== | ||
[[Image:Pyridoxal-phosphate.svg|thumb|140px|pyridoxal phosphate]] | [[Image:Pyridoxal-phosphate.svg|thumb|140px|pyridoxal phosphate]] | ||
Insight to the equilibrium in solution may be gained from the results of theoretical calculations. For example, pyridoxal phosphate, a form of [[vitamin B6|vitamin B<sub>6</sub>]], 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>{{cite journal |last1=Kiruba |first1=G. S. M. |last2=Ming |first2=Wah Wong |title=Tautomeric Equilibria of Pyridoxal-5′-phosphate and 3-Hydroxypyridine Derivatives: A Theoretical Study of Solvation Effects|journal=Journal of Organic Chemistry |date=2003 |volume=68 |issue=7 |pages=2874–2881 |doi=10.1021/jo0266792|pmid=12662064 }}</ref> | Insight to the equilibrium in solution may be gained from the results of theoretical calculations. For example, pyridoxal phosphate, a form of [[vitamin B6|vitamin B<sub>6</sub>]], 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>{{cite journal |last1=Kiruba |first1=G. S. M. |last2=Ming |first2=Wah Wong |title=Tautomeric Equilibria of Pyridoxal-5′-phosphate and 3-Hydroxypyridine Derivatives: A Theoretical Study of Solvation Effects|journal=Journal of Organic Chemistry |date=2003 |volume=68 |issue=7 |pages=2874–2881 |doi=10.1021/jo0266792|pmid=12662064 |url=http://scholarbank.nus.edu.sg/handle/10635/95196 }}</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>{{cite journal |title= Theoretical and Experimental Studies of the Zwitterion ⇌ Neutral Form Equilibrium of Ampholytes in Pure Solvents and Mixtures |first1= Peter I. |last1= Nagy |first2= Krisztina |last2= Takács-Novák |journal= J. Am. Chem. Soc. |year= 1997 |volume= 119 |issue= 21 |pages= 4999–5006 |doi= 10.1021/ja963512f }}</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>{{cite journal |title= Theoretical and Experimental Studies of the Zwitterion ⇌ Neutral Form Equilibrium of Ampholytes in Pure Solvents and Mixtures |first1= Peter I. |last1= Nagy |first2= Krisztina |last2= Takács-Novák |journal= J. Am. Chem. Soc. |year= 1997 |volume= 119 |issue= 21 |pages= 4999–5006 |doi= 10.1021/ja963512f |bibcode= 1997JAChS.119.4999N }}</ref> | ||
==Betaines and similar compounds== | ==Betaines and similar compounds== | ||
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==Conjugated zwitterions== | ==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.<ref>{{cite journal |title=Charge frustration in ligand design and functional group transfer. |first1= Dominik |last1= Munz |first2= Meyer |last2= Karsten |journal= Nat. Rev. Chem. |year= 2021 |volume= 5 |issue= 6 |pages= 422–439 |doi= 10.1038/s41570-021-00276-3|s2cid= 235220781 }}</ref> Donor-acceptor entities are of vast use in photochemistry ([[photoinduced electron transfer]]), [[organic electronics]], switching and [[Reichardt's dye|sensing]]. | 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>{{cite journal |title=Charge frustration in ligand design and functional group transfer. |first1= Dominik |last1= Munz |first2= Meyer |last2= Karsten |journal= Nat. Rev. Chem. |year= 2021 |volume= 5 |issue= 6 |pages= 422–439 |doi= 10.1038/s41570-021-00276-3|pmid= 37118028 |s2cid= 235220781 }}</ref> Donor-acceptor entities are of vast use in photochemistry ([[photoinduced electron transfer]]), [[organic electronics]], switching and [[Reichardt's dye|sensing]]. | ||
==See also== | ==See also== | ||
Latest revision as of 12:34, 1 October 2025
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In chemistry, a zwitterion (Template:IPAc-en Template:Respell; Template:Etymology), also called an inner salt or dipolar ion,[1] is a molecule that contains an equal number of positively and negatively charged functional groups.[2] (1,2-dipolar compounds, such as ylides, are sometimes excluded from the definition.[3])
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 acids
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.[4] 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.[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]
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 compounds
-
Sulfamic acid isomers, with the zwitterion (right)
-
Structure of H4EDTA
Sulfamic acid crystallizes in the zwitterion form.[7]
In crystals of anthranilic acid there are two molecules in the unit 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]
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.[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]
Betaines and similar compounds
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.[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] and pulmonary surfactants such as dipalmitoylphosphatidylcholine. Lauramidopropyl betaine is the major component of cocamidopropyl betaine.
-
Trimethylglycine (trivial name betaine)
-
Example of a phosphatidylcholine
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.[14] Donor-acceptor entities are of vast use in photochemistry (photoinduced electron transfer), organic electronics, switching and sensing.
See also
References
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