Tautomers

Tautomers are a particular class of isomers, which belong to the constitutional isomers. They are molecules characterized by differ among themselves usually in the position of a hydrogen atom. Thus, reactions involving the transfer of a H + to another position in the same molecule (i.e., intramolecular transfer of a proton), are tautomerization reactions.

The tautomers have a high rate of interconversion, and due to this fact often both tautomeric forms are "ignored" and they are considered as the same molecule.
The most common situation is called keto-enol tautomerism. But what does this mean? When we say "keto", we are referring to a molecule which has a carbonyl group, whilst when we say "enol", it is a molecule having one hydroxyl group and the carbon which has this group is simultaneously involved in a double bond. Hence the name enol (en = double bond; ol = hydroxyl group). Normally the form "keto" is the most favored, but in certain (bio)chemical contexts, it may originate the enol form.

In Biochemistry, tautomers have an important role in the metabolism of nitrogen bases, in particular in the pairing between nitrogen bases which occurs in DNA and mainly in RNA.