Competitive Measurement of Rate Constants for Hydroxyl Radical Reactions Using Radiolytic Hydroxylation of Benzoate.

Abstract

Hydroxyl radicals were generated from N2O-saturated phosphate-buffered saline solution (pH 7.5) by irradiation with a 3.7 TBq 137Cs source. The radicals attacked benzoate to form highly fluorescent products. The induction of fluorescence was tested in 0.002 to 2mM of benzoate solution; the fluorescence intensity was approximately constant at benzoate concentrations more than 0.1 mM during irradiation, and increased linearly with irradiation dose up to 53 Gy tested. The major primary products detected by high-performance liquid chromatography were three monohydroxybenzoates, 2-, 3- and 4-hydroxybenzoate (HOBZ). The G-values obtained from γ-irradiation for 60 min of a 0.2 mM benzoate solution were G(2-HOBZ)=0.97, G(3-HOBZ)=0.48 and G(4-HOBZ)=0.45. As the fluorescence of irradiated benzoate solution arose mostly from 2- and 3-HOBZ and the intensity of 2-HOBZ is 30 times that of 3-HOBZ, 98% of the fluorescence intensity induced was ascribed to 2-HOBZ. A hydroxyl radical scavenger competed with benzoate for hydroxyl radicals produced, and diminished the fluorescent products. A rate constant for the reaction of a scavenger with hydroxyl radical could be determined from the reduction in fluorescence intensity, using 0.2 mM benzoate and various concentrations of the scavenger. The fluorescence intensity was detectable down to 0.15 μM of 2-HOBZ produced. For various compounds, rate constants obtained in this way were similar to those measured by pulse-radiolysis.