Deactivation of triplet-excited riboflavin by purine derivatives: Important role of uric acid in light-induced oxidation of milk sensitized by riboflavin

The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M-1 s(-1) (Delta H-double dagger = 14.7 kJ mol(-1), Delta S-double dagger = -15.6 J mol(-1) K-1), 1.2 x 10(9) M-1 s(-1) (Delta H-double dagger = 34.3 kJ mol(-1), Delta S-double dagger = +45.3 J mol(-1) K-1), and 1.7 x 10(8) M-1 s(-1) (Delta H-double dagger = 122 kJ mol(-1), Delta S-double dagger = +319 J mol(-1) K-1), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +11.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (Delta G degrees = -114 kJ mol(-1)), xanthine (Delta G degrees = -73.5 kJ mol(-1)), hypoxanthine (Delta G degrees = -20.6 kJ mol-1), and purine (Delta G degrees > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.