[A theoretical investigation of tautomerism of adenine, purine, guanine, and cytosine].

The parameters of tautomeric equilibrium and stability of lactam-lactim, amino-imino, and N9H <--> N7H tautomeric forms of adenine, purine, guanine, and cytosine for isolated states have been calculated. It was found that the Ade-N9H tautomeric form is most stable; in this case the intramolecular proton transfer and tautomeric transformation Ade-N9H --> Ade-N7H can occur. The parameters of tautomeric equilibrium and stability for seven tautomeric forms of guanine were determined. For cytosine, the simultaneous existence of the amino-hydroxy, imino-oxo and valence amino-hydroxy of tautomeric forms was predicted.