Dimer Mechanism for the Tautomeric Conversion of 4-Amino-2-oxopyrimidine

Values of the enthalpy (ΔH), atomic charges (q_i), and bond orders (P_ij) for six tautomeric forms of 4-amino-2-oxopyrimidine have been calculated by the semiempirical AM1 quantum-chemical method. It was found that the most stable of these is 4-amino-2-oxo-1H-pyrimidine (cytosine). By analogy with complementary pairs of nucleotide bases, the tautomeric conversion of cytosine and other oxo forms can come about via a dimeric intermolecular mechanism. A novel interpretation is proposed for the conversion of cytosine to 4-amino-2-oxo-3H-pyrimidine as a result of a 1H-3H two stage proton transfer.__________Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 564–568, April, 2005.