Theoretical investigations on the structure and relative stabilities of the tautomeric forms of 6-amino-5-nitrosouracil and violuric acid derivatives (PM3-COSMO calculation)

In order to identify the structure of the most stable tautomers of 6-amino-5-nitrosouracil and violuric acid derivatives in different pH conditions, relative stabilities of potential tautomers (112) in aqueous phase have been calculated taking into account the entropy effects over the tautomeric equilibria. In each medium, the tautomer with lower energy must be the most representative form at the corresponding pH and the knowledge of the effect of the medium in the tautomerization energies allows to evaluate the possible effect of the medium over the molecular stability. Clearly, the results show that, in aqueous phase, the order of basicity of protonation sites in the 6-amino-5-nitrosouracil derivatives is N6 > (N3 > N1) > (O4, O5, O2), whereas for the violuric ones, the basicity order is (N3, N1) > O5 > (O2, O4, O6). The 6-amino-5-nitrosouracil derivatives use to behave as an amino-nitroso-keto tautomer with a strong imino character, whereas, upon coordination, the violuric acids change from a keto–oxime tautomer (free acids) to a partially enolyzed keto–nitroso coordinated one, despite usually the authors refer to these forms as 5-oximato ligands.