Substituent effect on the rate of the hydroxyl and phenyl radical spin trapping with nitrones

Hydroxyl and phenyl radical spin trapping rates by α-phenyl-N-t-butylnitrone (PBN, N-benzylidene-t-butylamine N-oxide) and its analogs were determined using a competitive trapping method. Hydroxyl radical was generated from hydrogen peroxide in water using UV photolysis, in the presence of the selected spin trap plus 5,5-dimethyl-pyrroline N-oxide (DMPO). Phenyl radical was produced with UV photolysis of tetraphenyllead, and spin trapping was performed in benzene. Spin trapping rate constants were calculated using EPR signal intensity ratios for the DMPO spin adducts vs PBN-type spin adducts. The rate constants were strongly dependent on the kind of substituent in the spin trap; the magnitude of the substituent effect was also dependent on the kind of free radicals trapped, i.e., hydroxyl or phenyl radicals. For example, in phenyl radical trapping, the spin trapping rate in hydroxy-substituted PBNs followed Hammett's equation, while there is no such correlation in hydroxyl radical trapping. Hydroxy-substituted PBNs such as 2-, 3-, and 4-hydroxy-PBNs showed much lower apparent spin trapping rates than that of PBN. The reaction between hydroxyl radical and phenoxyl group is a likely cause.