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

The inhibitory effects of lanthanide ions on the generation of free radical s from the reaction of Fe3+ and Fe2+ with tert-butyl hydroperoxide ((BuOOH) -Bu-t) were investigated by electron spin resonance (ESR) utilizing 5,5-dim ethyl-1-pyrroline-N-oxide (DMPO) as a spin trap. Peroxyl, alkoxyl, and carb on-centered free radicals generated from Fe3+-(BuOOH)-Bu-t system were succ essfully trapped by DMPO, whereas peroxyl radicals were not trapped in Fe2-(BuOOH)-Bu-t system. Peroxyl and alkoxyl radicals are initial radical spec ies generated from Fe3+ and Fe2+ systems, respectively. The carbon-centered radicals ((CH3)-C-.) might be attributed to beta-scission reaction of thes e alkoxyl radicals. The ESR signals of DMPO adducts of these radicals were quenched in the presence of lanthanides (Ln(3+) or [Ln(cit)(2)](3-)), in co ncentration-dependent fashion. Moreover, the quenching effect of Ln(3+) is closely related to the time the Ln(3+) was added into the free-radical-trig gered systems. The results reveal that there might be various mechanisms re sponsible for inhibiting generation and transformation of the free radicals . If Ln(3+) and iron react with peroxide simultaneously, the complex format ion of Ln(3+) with (BuOOH)-Bu-t will be the main mechanism of the competiti ve inhibitory effect of Ln(3+). Whereas if Ln(3+) is added after iron, the inhibitory effect on the ESR signal of DMPO adducts might be interpreted pr eferentially by the coordination and magnetic dipole-dipole interaction bet ween Ln(3+) and DMPO adducts.