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Tautomers are structural isomers of biological compounds that differ solely in hydrogen position and are easily interconverted. Due to the movement of a proton, they are in dynamic equilibrium. Tautomerizationis the isomerization process that results in the relocation of the hydrogen atom.

Cytosine can be found in two forms: amino and imino. Cytosine has hydrogen donors in the amino groups and hydrogen acceptors in the ring nitrogen atoms (N-1) It depicts cytosine in its tautomeric state, in which hydrogen bonds from the ring to the amino groups are interconverted.

The most stable tautomer of cytosine is the enol form with the hydroxyl atom trans to N3 (tautomer C₂ t), followed by the N1-H keto-amino (C₁) and cis enol (C₂c) tautomers.