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

Influence of Intramolecular Charge Transfer and Nuclear Quantum Effects on Intramolecular Hydrogen Bonds in Azopyrimidines

Intramolecular hydrogen bonds (IMHBs) in 5-azopyrimidines are investigated by NMR spectroscopy and DFT computations that involve nuclear quantum effects. A series of substituted 5-phenylazopyrimidines with one or two hydrogen bond donors able to form IMHBs with the azo group is prepared by azo coupling. The barrier of interconversion between two rotamers of the compounds with two possible IMHBs is determined by variable temperature NMR spectroscopy and it is demonstrated that the barrier is significantly affected by intramolecular charge transfer. Through-hydrogen-bond scalar coupling is investigated in ¹⁵N labeled compounds and the stability of the IMHBs is correlated with experimental NMR parameters and rationalized by path integral molecular dynamics simulations that involve nuclear quantum effects. Detailed information on the hydrogen bond geometry upon hydrogen-to-deuterium isotope exchange is obtained from a comparison of experimental and calculated NMR data.

TAUTOMERISM IN NEUTRAL AND DEPROTONATED 6-AMINO-5-FORMYLURACILS: A PM3-COSMO APPROACH

In order to identify the structure of the most stable tautomers of 6-amino-5-formyluracil and its N-methylated derivatives in different pH conditions, relative stabilities of potential tautomers 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 evaluating the possible effect of the medium over the molecular stability. The results show that, in aqueous phase, the order of basicity of protonation sites in the 6-amino-5-formyluracil derivatives is N 9 > ( N 3 > N 1) > ( O 8 > O 4 > O 2). The 6-amino-5-formyluracil derivatives act as an amino-formyl-keto tautomer with a strong imino character, although, upon successive deprotonations, a growing contribution of enolyzed forms which explain the increasing labilization of carbonyl bonds must be pointed out.

A potential antitumor agent, (6-amino-1-methyl-5-nitrosouracilato-N3)-triphenylphosphine-gold(I): structural studies and in vivo biological effects against experimental glioma

The synthesis and molecular and supramolecular structures of the compound (6-amino-1-methyl-5-nitrosouracilato-N3)-triphenylphosphine-gold(I) with interesting abilities to inhibit tumor growth in an animal model of experimental glioma are reported. Thus, its antitumor properties, effects on both enzyme and non-enzyme antioxidant defense systems and the response of several biochemical biomarkers have been analyzed. After seven days of treatment, the gold compound decreased the tumor growth to ca. one-tenth and reduced oxidative stress biomarkers (thiobarbituric acid-reactive substances (TBARS) and protein oxidation levels) compared to animals treated with the vehicle. Also, gold compound maintained non-enzyme antioxidant defense systems as in non-tumor animals and increased enzyme antioxidant defenses, such as superoxide dismutase and glutathione peroxidase activities, and decreased catalase activity. Analysis of serum levels of electrolytes, nitrogenous compounds, glucose, lipids, total protein, albumin, transaminases and alkaline phosphatase indicated that gold compound treatment showed few adverse effects, while effectively inhibiting tumor growth through mechanisms that involved endogenous antioxidant defenses.