Density of Hydroxyl Radicals Generated in an Aqueous Solution by Irradiating Carbon-Ion Beam

摘要

The density of hydroxyl radicals ((OH)-O-center dot) produced in aqueous samples by exposure to X-ray or carbon-ion beams was investigated. The generation of (OH)-O-center dot was detected by the electron paramagnetic resonance (EPR) spin-trapping technique using 5,5-dimethy1-1-pyrroline-N-oxide (DMPO) as the spin-trapping agent. When the concentration of DMPO is in excess of the generated OH, the production of DMPO-OH (spin-trapped (OH)-O-center dot) should be saturated. Reaction mixtures containing several concentrations (0.5-1685 mm) of DMPO were then irradiated by a 32 Gy 290MeV carbon-ion beam (C290-beam) or X-ray. C290-beam irradiation was performed at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, National Institute of Radiological Sciences, Chiba, Japan), applying different linear energy transfers (LET) (20-169 keV/mu m). The amount of DMPO-OH in the irradiated samples was detected by EPR spectroscopy. The generation of DMPO-OH increased with the concentration of initial DMPO, displayed a shoulder around 3.3 mm DMPO, and reached a plateau. This plateau suggests that the generated (OH)-O-center dot were completely trapped. Another linear increase in DMPO-OH measured in solutions with higher DMPO concentrations suggested very dense (OH)-O-center dot generation (>1.7m). Generation of (OH)-O-center dot is expected to be localized on the track of the radiation beam, because the maximum concentration of measured DMPO-OH was 40, mu M. These results suggested that both sparse (approximate to 3.3 mM) and dense (>1.7M) (OH)-O-center dot generation occurred in the irradiated samples. The percentage of dense (OH)-O-center dot generation increased with increasing LET. Different types of dense (OH)-O-center dot generation may be expected for X-ray and C290-beams.