Tautomer

Tautomers are constitutional isomers of organic compounds that readily interconvert with each other. The chemical reaction interconverting the two is 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. The concept of tautomerizations is called tautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. Tautomerism should be distinguished from resonance, where two structures differing only in the position of electrons coexist in quantum superposition, rather than as a rapidly-interconverting mixture. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.