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Two tautomers of an amino acid: (1) neutral and (2) zwitterionic forms.

Tautomers are constitutional isomers of organic compounds that readily interconvert.^[1]^[2]^[3] This reaction commonly results in the relocation of a proton. Although it is a complicated concept, tautomerism is relevant to the behavior of amino acids and nucleic acids, two of the fundamental building blocks of life.

The concept of tautomerizations is called tautomerism. The chemical reaction interconverting the two is called tautomerization.

Examples

Aromaticity provides some stability to the triple lactim form of these triple lactam-lactim tautomers.

Tautomerization is pervasive in organic chemistry. It is typically associated with polar molecules and ions containing functional groups that are at least weakly acidic. Most commonly tautomers exist in pairs, which means that the proton is located at one of two positions. Common tautomeric pairs include:^[4]

amino acid-ammonium carboxylate, which applies to the building blocks of the proteins.
ketone - enol, e.g., for acetone (see: keto–enol tautomerism);
enamine - imine;
lactam - lactim, an amide-imidic acid tautomerism in the nucleobases guanine, thymine, and cytosine;
enamine - enamine, e.g., during pyridoxalphosphate-catalyzed enzymatic reactions; and

Keto-enol tautomerization typically strongly favors the keto tautomer, but an important exception is the case of 1,3-diketones such as acetylacetone.

Some examples of tautomers

Some more specialized examples are known including nitroso - oxime, ketene - ynol, amide - imidic acid (e.g., the latter is encountered during nitrile hydrolysis reactions);

Prototropy

Prototropy is the most common form of tautomerism and refers to the relocation of a proton.^[5] Prototropic tautomerism may be considered as a subset of acid-base behavior. Prototropic tautomers are sets of isomeric protonation states with the same empirical formula and total charge. Tautomerizations are catalyzed by:^{[citation needed]}

bases, involving a series of steps: deprotonation, formation of a delocalized anion (e.g., an enolate), and protonation at a different position of the anion; and
acids, involving a series of steps: protonation, formation of a delocalized cation, and deprotonation at a different position adjacent to the cation).

Two specific further subcategories of tautomerizations:

Annular tautomerism is a type of prototropic tautomerism wherein a proton can occupy two or more positions of a heterocyclic system, for example, 1H- and 3H-imidazole; 1H-, 2H- and 4H- 1,2,4-triazole; 1H- and 2H- isoindole.^[6]^{[non-primary source needed]}^{[better source needed]}
Ring–chain tautomers occur when the movement of the proton is accompanied by a change from an open structure to a ring, such as the open chain and pyran forms of glucose and furan form of fructose.^[4]

Valence tautomerism

Valence tautomerism is a type of tautomerism in which single and/or double bonds are rapidly formed and ruptured, without migration of atoms or groups.^[7] It is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons. An example of this type of tautomerism can be found in bullvalene. Another example is open and closed forms of certain heterocycles, such as azide - tetrazole or mesoionic münchnone-acylamino ketene. Valence tautomerism requires a change in molecular geometry and should not be confused with canonical resonance structures or mesomers.

References

^ Antonov L (2013). Tautomerism: Methods and Theories (1st ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-33294-6.
^ Smith MB, March J (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. pp. 1218–1223. ISBN 0-471-58589-0.
^ Katritzky AR, Elguero J, et al. (1976). The Tautomerism of heterocycles. New York: Academic Press. ISBN 0-12-020651-X.
^ ^a ^b Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Tautomerism".
^ Roman M. Balabin (2009). "Tautomeric equilibrium and hydrogen shifts in tetrazole and triazoles: Focal-point analysis and ab initio limit". J. Chem. Phys. 131 (15): 154307. Bibcode:2009JChPh.131o4307B. doi:10.1063/1.3249968. ^{[non-primary source needed]}^{[better source needed]}
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Valence tautomerization".

English Journal

Two fluorescent Schiff base sensors for Zn(2+): the Zn(2+)/Cu(2+) ion interference.

Jiménez-Sánchez A1, Ortíz B, Ortiz Navarrete V, Farfán N, Santillan R.
The Analyst.Analyst.2015 Aug 10;140(17):6031-9. doi: 10.1039/c5an00789e.
Two simple and low cost 2,4-di-tert-butyl-6-[(1-hydroxycyclohexylmethylimino)methyl]phenol () and 2-[{(1-hydroxycyclohexyl)methylimino}methyl]phenol () Schiff base sensors exhibiting selectivity for Zn(2+) in water : methanol (95 : 5, v/v, 10 mM HEPES) are described. and display an "off-on"
PMID 26192046

N-acetylglycine Cation Tautomerization Enabled by the Peptide Bond.

Kocisek J, Piekarski DG, Delaunay R, Huber BA, Adoui L, Martin F, Alcami M, Rousseau P, Domaracka A, Kopyra J, Díaz-Tendero S.
The journal of physical chemistry. A.J Phys Chem A.2015 Aug 5. [Epub ahead of print]
We present a combined experimental and theoretical study of the ionization of N- acetylglycine molecules by 48 keV O6+ ions. We focus on the single ionization channel of this interaction. In addition to the prompt fragmentation of the N-acetylglycine cation, we observe also the formation of metastab
PMID 26243533

DFT studies of the bonding mechanism of 8-hydroxyquinoline and derivatives on the (111) aluminum surface.

Chiter F1, Lacaze-Dufaure C, Tang H, Pébère N.
Physical chemistry chemical physics : PCCP.Phys Chem Chem Phys.2015 Aug 5. [Epub ahead of print]
The 8-hydroxyquinoline (8-HQ) molecule is an efficient corrosion inhibitor for aluminum and is also used in organic electronic devices. In this paper, the adsorption modes of 8-HQ and its derivatives (tautomer, dehydrogenated and hydrogenated species) on the Al(111) surface are characterized using d
PMID 26243038

Japanese Journal

Photophysics of cytosine tautomers : new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

Nakayama Akira,Harabuchi Yu,Yamazaki Shohei,Taketsugu Tetsuya
Physical chemistry chemical physics 15(29), 12322-12339, 2013-08-07
… Nonadiabatic molecular dynamics simulations provide a time-dependent picture of the deactivation processes, including the excited-state lifetime of each tautomer. … In particular, the decay time of the imino tautomer is predicted to be only [similar]100 fs. … A, 2011, 115, 8406], supporting the coexistence of more than one tautomer in the photophysics of isolated cytosine and that each tautomer exhibits a different excited-state lifetime. …
NAID 120005462410

Excitation wavelength dependence of photo-induced intramolecular proton transfer reaction of 4′-N,N-diethylamino-3-hydroxyflavone in various liquids

Suda Kayo,Terazima Masahide,Kimura Yoshifumi
Chemical Physics Letters 531, 70-74, 2012-04
… It was found that the relative fluorescence intensity of the tautomer excited state to that of the normal excited state significantly decreases in ionic liquids with changing the excitation wavelength from 380 to 470 nm. …
NAID 120003988360

配位構造内の回転運動による電子移動制御系の構築

久米晶子
Bulletin of Japan Society of Coordination Chemistry 59(0), 48-55, 2012
In this review, a family of new bistable copper complexes has been rationally assembled from the basic components, which undergoes the coordination isomerization by rotation of coordinated asymmetric …
NAID 130002145022