A comparative study on the TiO2 mediated photo-oxidation of uracil, thymine and 6-methyluracil

The semiconductor TiO2 assisted photo-oxidation of uracil, thymine and 6-methyluracil has been carried out in aqueous suspension with an excitation wavelength of 365 nm. The rate and quantum yield (empty set) of this heterogeneous photoprocess were measured based on the disappearance of the pyrimidine bases spectrophotometrically (265 nm) and hence the quantum yield (empty set) has been evaluated as a function of concentration of the substrate (pyrimidine bases), weight of TiO2 and pH. The reaction rates are found to conform to the Langmuir-Hinshelwood isotherm and the k and K values thus determined are reported. The influence of pH on the empty set values reveals a varying proportion of contribution of h(vb)(+) and (OH)-O-. on the reactivity of the substrate biomolecules, and the effect of [Ag+](0) on empty set points to a decreased h(+)-e(-) recombination with a concomitant increase in h(+) capture by the substrate biomolecules, leading to an enhancement of the reaction rate. A mechanism consistent with the observed data is proposed. (C) 1997 Elsevier Science S.A.