Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydrogen atom within the compound. The phenomenon of tautomerization is called tautomerism, also called desmotropism. Tautomerism is for example relevant to the behavior of amino acids and nucleic acids, two of the fundamental building blocks of life. Care should be taken not to confuse tautomers with depictions of "contributing structures" in chemical resonance. Tautomers are distinct chemical species that can be distinguished by their differing atomic connectivities, molecular geometries, and physicochemical and spectroscopic properties, whereas resonance forms are merely alternative Lewis structure (valence bond theory) depictions of a single chemical species, whose true structure is a quantum superposition, essentially the "average" of the idealized, hypothetical geometries implied by these resonance forms. Examples 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 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: ketone – enol: H−O−C=C ⇌ O=C−C−H, see keto–enol tautomerism enamine – imine: H−N−C=C ⇌ N=C−C−H cyanamide – carbodiimide guanidine – guanidine – guanidine: With a central carbon surrounded by three nitrogens, a guanidine group allows this transform in three possible orientations amide – imidic acid: H−N−C=O ⇌ N=C−O−H (e.g., the latter is encountered during nitrile hydrolysis reactions) lactam – lactim, a cyclic form of amide-imidic acid tautomerism in 2-pyridone and derived structures such as the nucleobases guanine, thymine, and cytosine imine – imine, e.g., during pyridoxal phosphate catalyzed enzymatic reactions R1R2C(=NCHR3R4) ⇌ (R1R2CHN=)CR3R4 nitro – aci-nitro (nitronic acid): RR'HC–N+(=O)(O–) ⇌ RR'C=N+(O–)(OH) nitroso – oxime: H−C−N=O ⇌ C=N−O−H 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 a zwitterion rather than shifting a double bond: H2N−CH2−COOH ⇌ H3N−CH2−CO phosphite – phosphonate: P(OR)2(OH) ⇌ HP(OR)2(=O) between trivalent and pentavalent phosphorus. Prototropy Prototropy is the most common form of tautomerism and refers to the relocation of a hydrogen atom. Prototropic tautomerism may be considered a subset of acid-base