The two tautomers of an amino acid: (1) neutral and (2)zwitterionic forms
Inchemistry,tautomers (/ˈtɔːtəmər/)[1] arestructural isomers (constitutional isomers) ofchemical compounds that readily interconvert.[2][3][4][5] Thechemical reaction interconverting the two is calledtautomerization. This conversion commonly results from the relocation of ahydrogen atom within the compound. The phenomenon of tautomerization is calledtautomerism, also calleddesmotropism. Tautomerism is for example relevant to the behavior ofamino acids andnucleic acids, two of the fundamental building blocks of life.
Care should be taken not to confuse tautomers with depictions of "contributing structures" in chemicalresonance. Tautomers are distinctchemical species that can be distinguished by their differing atomic connectivities, molecular geometries, and physicochemical and spectroscopic properties,[6] whereas resonance forms are merely alternativeLewis structure (valence bond theory) depictions of a single chemical species, whose true structure is aquantum superposition, essentially the "average" of the idealized, hypothetical geometries implied by these resonance forms.
The termtautomer is derived from Ancient Greekταὐτό (tautó)'the same' and μέρος (méros)'part'.
Some examples of tautomersKeto-enol tautomerization typically strongly favors the keto tautomer, but an important exception is the case of 1,3-diketones such asacetylacetone.[6]
Tautomerization is pervasive inorganic chemistry.[2][7] It is typically associated with polar molecules and ions containing functional groups that are at least weakly acidic. Most common tautomers exist in pairs, which means that the hydrogen is located at one of two positions, and even more specifically the most common form involves a hydrogen changing places with a double bond:H−X−Y=Z ⇌ X=Y−Z−H. Common tautomeric pairs include:[3][4]
guanidine – guanidine – guanidine: With a central carbon surrounded by threenitrogens, a guanidine group allows this transform in three possible orientations
ketene –ynol, which involves a triple bond:H−C=C=O ⇌ C≡C−O−H
amino acid – ammonium carboxylate, which applies to the building blocks of the proteins. This shifts the proton more than two atoms away, producing azwitterion rather than shifting a double bond:H2N−CH2−COOH ⇌ H3N+−CH2−CO−2
phosphite –phosphonate:P(OR)2(OH) ⇌ HP(OR)2(=O) between trivalent and pentavalent phosphorus.
Prototropy is the most common form of tautomerism and refers to the relocation of a hydrogen atom.[7] Prototropic tautomerism may be considered a subset ofacid-base behavior. Prototropic tautomers are sets of isomericprotonation states with the sameempirical formula and totalcharge. Tautomerizations arecatalyzed by:[4]
bases, involving a series of steps: deprotonation, formation of adelocalizedanion (e.g., anenolate), andprotonation 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).
Glucose can exist in both a straight-chain and ring form.
Two specific further subcategories of tautomerizations:
Annular tautomerism is a type of prototropic tautomerism wherein a proton can occupy two or more positions of theheterocyclic systems found in many drugs, for example, 1H- and 3H-imidazole; 1H-, 2H- and 4H-1,2,4-triazole; 1H- and 2H-isoindole.[8]
Ring–chain tautomers occur when the movement of the proton is accompanied by a change from an open structure to a ring, such as theopen chain and cyclichemiacetal (typicallypyranose orfuranose forms) of many sugars.[4]: 102 (SeeCarbohydrate § Ring-straight chain isomerism.) The tautomeric shift can be described as H−O ⋅ C=O ⇌ O−C−O−H, where the "⋅" indicates the initial absence of a bond.
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.[9] It is distinct from prototropic tautomerism, and involves processes with rapid reorganisation of bonding electrons.
Oxepin – benzene oxide equilibrium
A pair of valence tautomers with formula C6H6O are benzene oxide andoxepin.[9][10]
In inorganic extended solids, valence tautomerism can manifest itself in the change of oxidation states its spatial distribution upon the change of macroscopic thermodynamic conditions. Such effects have been calledcharge ordering or valence mixing to describe the behavior in inorganic oxides.[12]
The existence of multiple possible tautomers for individualchemical substances can lead to confusion. For example, samples of 2-pyridone and2-hydroxypyridine do not exist as separate isolatable materials: the two tautomeric forms are interconvertible and the proportion of each depends on factors such as temperature, solvent, and additionalsubstituents attached to the main ring.[8][13]
Historically, each form of the substance was entered into databases such as those maintained by theChemical Abstracts Service and given separateCAS Registry Numbers.[14] 2-Pyridone was assigned [142-08-5][15] and 2-hydroxypyridine [109-10-4].[16] The latter is now a "replaced" registry number so that look-up by either identifier reaches the same entry. The facility to automatically recognise such potential tautomerism and ensure that all tautomers are indexed together has been greatly facilitated by the creation of theInternational Chemical Identifier (InChI) and associated software.[17][18][19] Thus thestandard InChI for either tautomer isInChI=1S/C5H5NO/c7-5-3-1-2-4-6-5/h1-4H,(H,6,7).[20]
^abSmith, Kyle T.; Young, Sherri C.; DeBlasio, James W.; Hamann, Christian S. (27 January 2016). "Measuring Structural and Electronic Effects on Keto–Enol Equilibrium in 1,3-Dicarbonyl Compounds".Journal of Chemical Education.93 (4):790–794.Bibcode:2016JChEd..93..790S.doi:10.1021/acs.jchemed.5b00170.
^E. Vogel and H. Günther (1967). "Benzene Oxide-Oxepin Valence Tautomerism".Angewandte Chemie International Edition in English.6 (5):385–401.doi:10.1002/anie.196703851.
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