Tautomers of pyrimidine and purine bases. Each purine or pyrimidine can exist as one of several isomers that differ in the placement of a hydrogen atom and a double bond (tautomers). Shown here are two tautomers for each of the common bases found in nucleic acids. The predominant tautomer of each base at physiological pH is on the left. These predominant tautomeric forms are found in DNA and RNA and participate in canonical Watson- 6-