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Keto-enol tautomerism refers to a chemical equilibrium between a keto and enol forms. The term keto refers to ketone or aldehyde and enol refers to alkene and alcohol group.This involves interconversion of forms, because keto and enol forms are said to be a tautomer of each other, this interconversion of two forms also involves the movement of an alpha hydrogen and reconstruction of bonding electrons takes place, thus this isomerism is known as tautomersism. The keto-enol tautomerization is dependent upon the stability factor of both the keto and enol tautomer.

Presence of alpha hydrogen atom is the necessary condition for having keto-enol tautomerization.

Guanine (a hydroxypurine) and cytosine are more stable in the keto form than in the enol form because C=O is more stable than C=C.

The enol form of guanine and cytosine are shown below: