Nano-Geometry of Hydroxyl Radical Generated by Low-LET Irradiation in Water

For suppressing the side effect of radiation therapy, the regulation of hydroxyl radical (OH) is essential, particularly in low-LET radiation such as X-ray or gamma-ray. Primary generation of reactive species, called "spur," attributed to radiation-induced ionization and/or excitation of water can be characterized as cluster of radicals. In this study, the density of OH generation in spur was evaluated by electron paramagnetic resonance (EPR) spin-trapping technique. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) was used as EPR spin trapping agent. Several concentrations of DMPO (0.5-1680 mM) were prepared and then irradiated with 32 Gy X-rays (3.2, 4.8, or 6.4 Gy/min) or gamma-ray (7.9 Gy/min). The amount of OH adduct of DMPO (DMPO-OH) in the irradiated sample was measured by X-band EPR. The relationship between molecular density of DMPO in the reaction mixture and DMPO-OH generated was analyzed, where molecular distance was defined as reciprocal of molecule-to-molecule distance of DMPO. The amount of DMPO-OH was increased in proportion to the molecular density of DMPO and reached plateau. The DMPO concentration at inflection point was 3.3 mM. Again increasing DMPO density, another linear trend that passed through the origin was observed when the DMPO concentrations were above 490 mM. Two different OH density, i.e., sparse (around 3 mM; molecule-to-molecule distance is 8 nm) and dense (much higher than 1700 mM; molecule-to-molecule distance is less than 1 nm), were predicted as in sub-spur structure.第1回QST国際シンポジウム「量子生命科学 -Quantum Life Science-