Induction of single strand breaks, and base lesions in plasmid DNA films induced by carbon, nitrogen, and oxygen KLL Auger process.

PURPOSE

To reveal the reaction process in DNA by Auger electrons using the observed single strand breaks (SSB) and base lesions induced by monochromatic ultrasoft X-rays in dry plasmid DNA film. Desorbed ions from DNA and 2-deoxy-D-ribose thin films were also measured to ascertain the decomposition site in DNA.

MATERIALS AND METHODS

Dry plasmid DNA (pUC18) films were irradiated with synchrotron monochromatic ultrasoft X-rays (USX). Two photon energies, 270, and 560 eV, were chosen for the irradiation experiments. Irradiated plasmid DNA was analyzed by agarose gel electrophoresis. The yield of base lesions was determined by the post-irradiation-treatment of the DNA with enzymatic probes (formamidpyrimidine DNA glycosylase [Fpg] and endonuclease III [Nth]). Desorbed ions induced by 540 eV USX irradiation from calf thymus DNA and 2-deoxy-D-ribose thin films were detected by quadrupole-mass spectrometer.

RESULTS

Yield of strand breaks and base lesions were obtained by 270 and 560 eV photon energies, respectively. Each yield showed characteristic of the photon energy spectrum. The characteristics of the desorbed ion mass spectra from 2-deoxy-D-ribose and DNA films were strikingly similar with each other.

CONCLUSIONS

In this paper we report, for the first time, the yields of base lesions and SSB induced by monochromatic USX. The yield of SSB induced by core-ionization of carbon, nitrogen, and oxygen was two times more frequent than that of valence-electrons. From the comparison of desorbed ion mass spectra of 2-deoxy-D-ribose with DNA films we predict these breaks are likely to be induced by the decomposition of the sugar sites in DNA backbone.