Studies of soft X-ray-induced Auger effect on the induction of DNA damage

Abstract

Purpose: To understand the characteristics of DNA damage induced by Auger effect in DNA by ultrasoft X-irradiation. In situ electron paramagnetic resonance (EPR) spectroscopy as well as biochemical analysis has been applied to examine the DNA damage induction in both viewpoints of intermediate species and final products.

Materials and methods: Unpaired electron species induced in a calf thymus DNA film irradiated with monochromatic ultrasoft X-rays (270–580 eV) was observed using an X-band EPR spectrometer installed in a synchrotron beamline. To determine the yield of single strand break (SSB), pUC18 plasmid DNA was irradiated and then analyzed by agarose gel electrophoresis. To analyze molecular change in a single strand DNA, a new technique using DNA-denaturation-treatment has been applied to quantify multiple SSB arising in both DNA strands.

Results: Short-lived EPR spectra were observed during irradiation. The intensity of transient EPR spectrum shows the similar energy dependence with that of the SSB yield around oxygen K-edge in particular. The fraction of the single-strand plasmid DNA (SS-DNA) after irradiation could be determined using a low-temperature–denaturation condition. The obtained slope of the dose-response for SS-DNA shows half of that of closed circular DNA as expected under the diluted solution condition.

Conclusion: The availability of an EPR apparatus installed in a synchrotron beamline is demonstrated by detecting very short-lived unpaired electron species. Transient EPR spectra of DNA show the similar energy dependence to that of the SSB yield. The proposed DNA-denaturation assay works as expected using the low-temperature–denaturation condition.

Keywords:

• Auger effect
• DNA damage
• K-shell photoabsorption
• synchrotron soft X-rays
• Electron paramagnetic resonance
• Glycosylase