Characterization of free radicals generated from the reaction of Fe3+/Fe2+ withtert-butyl hydroperoxide and the effect of lanthanide ions

Abstract

The inhibitory effects of lanthanide ions on the generation of free radicals from the reaction of Fe3+ and Fe2+ withtert-butyl hydroperoxide (tBuOOH) were investigated by electron spin resonance (ESR) utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap. Peroxyl, alkoxyl, and carbon-centered free radicals generated from Fe3+-tBuOOH system were successfully trapped by DMPO, whereas peroxyl radicals were not trapped in Fe2+-tBuOOH system. Peroxyl and alkoxyl radicals are initial radical species generated from Fe3+ and Fe2+ systems, respectively. The carbon-centered radicals (⋅CH3) might be attributed to β-scission reaction of these alkoxyl radicals. The ESR signals of DMPO adducts of these radicals were quenched in the presence of lanthanides (Ln3+ or [Ln(cit)2]3−), in concentration-dependent fashion. Moreover, the quenching effect of Ln3+ is closely related to the time the Ln3+ was added into the free-radical-triggered systems. The results reveal that there might be various mechanisms responsible for inhibiting generation and transformation of the free radicals. If Ln3+ and iron react with peroxide simultaneously, the complex formation of Ln3+ withtBuOOH will be the main mechanism of the competitive inhibitory effect of Ln3+. Whereas if Ln3+ is added after iron, the inhibitory effect on the ESR signal of DMPO adducts might be interpreted preferentially by the coordination and magnetic dipole-dipole interaction between Ln3+ and DMPO adducts.