title: Deactivation of triplet-excited riboflavin by purine derivatives: important role of uric acid in light-induced oxidation of milk sensitized by riboflavin.
abstract: 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) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +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 = +1.106 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 (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 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.
authors.affiliation: Departamento de Química e Física Molecular, Instituto de Química de São Carlos, Universidade de São Paulo, Avenue Trabalhador São Carlense 400, CP 780, CEP 13560-970, São Carlos SP, Brazil.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Cardoso
authors.forename: Paula
authors.initials: P
authors.lastname: Homem-de-Mello
authors.forename: Karsten
authors.initials: K
authors.lastname: Olsen
authors.forename: Albérico B F
authors.initials: AB
authors.lastname: da Silva
authors.forename: Douglas W
authors.initials: DW
authors.lastname: Franco
authors.forename: Leif H
authors.initials: LH
authors.lastname: Skibsted
mesh_terms: D000818:Animals; D004579:Electron Transport; D006863:Hydrogen-Ion Concentration; D019271:Hypoxanthine / Q000737:chemistry; D008027:Light; D008892:Milk / Q000737:chemistry*; D010084:Oxidation-Reduction; D010782:Photolysis; D011687:Purines / Q000494:pharmacology*; D012256:Riboflavin / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000494:pharmacology; D012996:Solutions; D013816:Thermodynamics; D014527:Uric Acid / Q000737:chemistry*; D019820:Xanthine / Q000737:chemistry
chemical_list: D011687:Purines; D012996:Solutions; D019820:Xanthine; D014527:Uric Acid; D019271:Hypoxanthine; D012256:Riboflavin