A novel mechanism of DNA damage induction by K-shell resonancephotoexcitations at nitrogen and oxygen as observed using an electron
paramagnetic resonance spectrometer in SPring-8

T. Oka 1), A. Yokoya 1), K. Fujii 1), Y. Fukuda 2) and M. Ukai 3)1) R.G. for Radiation and Biomolecular Science, JAEA
2) Kansai Photon Science Institute (SPring-8), JAEA
3) Tokyo Univ., Agriculture and Technology

Many studies using soft X-rays (<10 keV) as fine probes ofthe energy deposition to biological systems have highlighted
their unique biological effects due to the highly localized spatial
properties on the scale of DNA [1]. In order to obtain moredetailed insights into the physicochemical mechanism of DNA
damage induction in terms of unpaired electron species, asintermediates of DNA damage, resulting from ionization, we
have developed an X-band EPR spectrometer set up at asynchrotron soft X-ray beamline (BL23SU) in SPring-8, Japan[2]. This apparatus makes it possible for us to perform in situ
observation of short lived unpaired species just appearing during
irradiation.
Using this unique apparatus we have examined EPR signal ofDNA thin films irradiated in the EPR cavity in a vacuum
chamber [3]. Figure 1 shows the variation in EPR intensity (spinconcentration of the unpaired electron species) for DNA thinfilms as a function of X-ray energy (red line). The right-handaxis in the figure shows X-ray absorption (blue line). The EPR
intensities were significantly enhanced at energies slightly
above the K-shell thresholds of nitrogen and oxygen. By normal
K-shell ionization, two electrons emitted from nitrogen oroxygen atom in DNA, and the doubly charged parent cation will
be produced (Auger effect). On the other hand, when the energy
is slightly higher than the ionization threshold (* excitationenergy), the post collision interaction (PCI), which is adeceleration of the ejected electron could be induced. The
significant PCI effect leads the recapture of a photoelectron into
an unoccupied orbital or a high-lying Rydberg orbital of DNA,
which then produces unpaired electron species. In order toevaluate the PCI effect on the enhancement of the EPR
intensities, the recapture cross section after the Auger effect in aDNA film was calculated using a semiclassical approximation.
The calculated result shows the cross section increased sharply
and then gradually decreased as X-ray energy above the ionizing
threshold, which is similar to the significant enhancement of theEPR intensities was observed in this study. This indicates that
only the enhanced spin concentration above the K-shellthresholds can be attributed to the recapture of a photoelectron.
The PCI effect is usually studied in the gas phase byphotoemission spectroscopy, whereas our results may be thefirst observation of the PCI effect measured in a DNA, whosemolecular weight is as large as a polymer, by EPR (Fig. 2). The
result suggests that the DNA film itself forms unpaired electron
species through the excitation of enhanced electron recapturing.
Different from the well-known low photoelectron attachment toan electronegative site in DNA followed by dissociation [5] or
the recombination of slow electrons with ion holes, theionization process induced by the characteristic X-ray energy isa novel mechanism of DNA damage.

Fig. 1 Variation in EPR intensity for DNA thin film and X-rayabsorbance as a function of X-ray energy. The EPR
intensity significant enhances around both of nitrogenand oxygen K-edge as indicated by circle in the figure.

Fig. 2 X-ray absorption processes to the K-shell of specific atomin DNA. Photoelectron ejected from the DNA byirradiating X-ray with energy slightly higher than theionization threshold decelerated and recaptured by PCI,
and then detected as an unpaired electron species.
References[1] A. Yokoya et al., Charged particle and photon interactions
with matter-recent advances, applications and interfaces, Eds.,
Y. Hatano, Y. Katsumura, and A. Mozumder, CRC/Taylor &Francis Group, USA, pp543-574. (2011).
[2] A. Yokoya and K. Akamatsu, Nucl. Instr. Meth. A, 467-468,1333-1337 (2001).
[3] T. Oka et al., Phys. Rev. Lett. 109, 213001 (2012).