102 - Comparison of effects of plateau carbon-ion beams and x-rays on the redox states of aqueous and organic solutions of 2,2-diphenyl-1-picrylhydrazyl radical

Abstract

Carbon-ion (C-ion) beams are known to exhibit a sharp maximum of the energy deposition, called as a Bragg peak, at a certain penetration depth. Thus, the cancer cells are precisely killed by adjusting the Bragg peak to a tumor. However, the dose in the plateau region of the C-ion beams before the Bragg peak is not necessarily zero. In this study, the redox states of aqueous and organic solutions after irradiation by the plateau C-ion beams were evaluated by 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) as a redox probe. Irradiation of the C-ion beams (290 MeV per nucleon, 13 keV µm–1) was carried out using the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences, Japan. A Shimadzu Pantak HF-320 was used for X-irradiation (X-ray tube voltage: 200 kV, X-ray tube current: 20 mA, pre-filter: 0.5 mm Cu + 0.5 mm Al, dose rate: 1.78 Gy min–1). C-ion irradiation of aqueous solutions of DPPH• solubilized by β-cyclodextrin in water resulted in the decrease in the absorption band at 527 nm due to DPPH•. The absorbance at 527 nm linearly decreased with increasing the radiation dose. When C-ion beams were replaced by X-rays, the slope of the linear plot is about the same as that in the case of C-ion beams. Similar results were obtained when H2O was replaced by methanol or acetonitrile without β-cyclodextrin, although the slope values were twice larger as compared to the cases in H2O. On the other hand, DPPH• was more susceptible to the C-ion beams or X-rays in isopropyl myristate (IPM), a saturated fatty acid ester, than in H2O, methanol, and acetonitrile. Furthermore, the C-ion beams consumed DPPH• more effectively than X-rays in IPM, suggesting that lipid molecules may be more susceptible to the C-ion beams than to X-rays. The mechanism will also be discussed based on the product analysis by the electron spin resonance (ESR) spin-trapping.