Nucleotide tautomerization

Tautomerization (or tautomerism) is a reaction that allows isomers of a chemical compound to interconvert (tautomers). This reaction normally results in the migration of a hydrogen atom or proton and switch of a single bond and adjacent double bound, and is a reversible reaction.  Unlike other classes of isomers, tautomeric compounds exist in mobile equilibrium with each other, so that attempts to prepare the separate substances usually result in the formation of a mixture that shows all the chemical and physical properties to be expected on the basis of the structures of the components.

An example is the Keto-enol tautomerism where the hydrogen of the ketone (an aldehyde) migrates near the oxygen molecule bound with it forming an enol (alcohol), this bounding make the double bound between the oxygen and carbon turn into a single bound, and so a double bond is created between two carbon molecules.

 

Keto-enol tautomerism.
Left the keto form; right the enol.

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. In these cases the lactam – lactim tautomerism is present, it is an amide - imidic acid tautomerism in heterocyclic rings that usually happen in the nucleobases guanine, thymine, and cytosine.
 



Thymine(a) and Guanine(b) tautomerism.