Advanced Science Research

7-4

Novel Mechanism of Radiation Damage to DNA－ An Advance in Elucidating the Radiation Process in DNA Damage －40.100.08

30.06

20.04
0.02

1nitrogen K-edge

oxygen K-edge

00400

450

500

550600650700

Energy / eV

X-ray absorbance (arb. units)

EPR intensity (arb. units)

X-rays

5’3’DNANN

3’

Two electrons are emitted bya normal ionization process.

5’

A photoelectron ejected from theK-shell can be captured by PCI andthen detected by EPR as an unpairedelectron species originating from aspecific excitation state.

Fig.7-9 Anomalous enhancement of EPR signal at nitrogenand oxygen K-edges
EPR signal intensity (― line, left axis) is anomalously enhanced(○) at energies slightly above the nitrogen and oxygen K-edges
when compared with the dependence of the X-ray absorption
probability of DNA (― line, right axis) on the X-ray energy(abscissa).

Fig.7-10 K-shell photoabsorption processes at specificatoms in DNA (nitrogen K-shell photoabsorption isshown as an example)
We revealed that when DNA is exposed to X-rays with anenergy slightly above the K-shell ionization threshold, anejected electron is re-captured by the DNA as an unpairedelectron.

Risk assessment for low-dose irradiation has been one of themost urgent issues. In particular, the research field of radiation
damage to DNA coding genetic information and its biologicalrepair has not yet been fully understood. In this study werevealed a novel mechanism of the radiation process in DNA
damage.
When a living cell is exposed to ionizing radiation,
particularly high-energy charged particles, energy over a wide
range is deposited from the particle to the DNA. Dependingon the deposited energy level, various DNA damage processes
might proceed simultaneously. To fully understand the damage
processes, each one should be individually analyzed. Previous
studies, however, lacked suitable techniques to realize thisgoal.
We addressed this difficulty using highly monochromatizedsoft X-rays. In an irradiated DNA molecule, an unpaired
electron remaining in the outermost shell after ionization is
one of the short-lived intermediates that cause damage. We
exposed DNA to soft X-rays obtained from a synchrotron
facility (SPring-8) while sweeping the energy. The precise
dependence of the yields of the unpaired electron species on
the soft X-ray energy was investigated, particularly around theK-shell ionization thresholds of the DNA constituent atoms

nitrogen and oxygen. Previously it was difficult to directly
observe the highly reactive and short-lived species. To realize“in situ” observation of the short-lived species, we connected
an electron paramagnetic resonance (EPR) apparatus to thesoft X-ray beamline.
The result shows that the EPR intensity (― line in Fig.7-9)
corresponding to the yield of the unpaired electron specieschanges roughly in proportion to the X-ray absorption
probability by K-shell electrons (― line in Fig.7-9). Further,
exposure to X-rays with an energy slightly above theionization thresholds causes, very interestingly, an anomalousenhancement of the EPR signal intensity (shown by circles inFig.7-9). A theoretical analysis revealed that the enhancement
is caused by recapturing of an ejected slow electron at theionized atom through a Post Collision Interaction (PCI)
process, which is a novel mechanism of radiation damage toDNA (Fig.7-10).
These results are expected to advance understanding ofDNA damage, particularly that originating from K-shellionization.
This work was supported by a Japan Society for thePromotion of Science (JSPS) KAKENHI Grant-in-Aid for
Scientific Research (B) (No.21310041).

Reference
Oka, T., Yokoya, A. et al., Unpaired Electron Species in Thin Films of Calf-Thymus DNA Molecules Induced by Nitrogen and Oxygen K -Shell
Photoabsorption, Physical Review Letters, vol.109, issue 21, 2012, p.213001-1-213001-5.
JAEA R&D Review 201381