The role of copper(II) ions in the oxidation of 5-hydroxy-6-methyluracil in the ground and electronically excited states with molecular oxygen in aqueous solutions

A spectral fluorescence study of photoinduced reactions in aqueous solutions has been carried out in order to examine the mechanisms of the oxidation of 5-hydroxy-6-methyluracil (I) in the ground and electronically excited states by molecular oxygen in the presence of copper(II) chloride. It has been found that 5,5,6-trihydroxy-6-methylpyrimidine-2,4-dione (II) is formed upon the photolysis of I. The spectral parameters (λmax) and the quantum yields (φ) of fluorescence (FL) of compounds I (φ = 8 × 10–4; λmax = 362 nm) and II (φ = 17 × 10–4; λmax = 306, 330 nm) have been determined. A reaction scheme was proposed, according to which the photooxidation of I occurs through the steps of the generation of the radical cation I•+ and the superoxide anion О2•- with the subsequent formation of 5,5,6-trihydroxy-6-methylpyrimidine-2,4-dione. The catalytic and inhibitory effects of Cu(II) ions on the oxidation of 5-hydroxy-6-methyluracil in the ground and electronically excited states, respectively, by the oxygen radical anion О2•- have been revealed.