Roles of Hydration for Inducing Decomposition of 2-Deoxy-d-ribose by Ionization of Oxygen K-Shell Electrons.

To experimentally investigate the role of hydration in the initial process of the decomposition of 2-deoxy-d-ribose (dR), which is a major component of the DNA backbone, we used mass spectrometry to monitor the ions desorbing from hydrated dR films exposed to monochromatic soft X rays (560 eV). The X-ray photons preferentially ionize the K-shell electrons of the oxygen atoms in DNA. Hydrated dR samples were prepared under vacuum by exposing a cooled (∼150 K) dR film deposited on a Si substrate to water vapor. Using a quadrupole mass spectrometer, we observed the desorption of ions such as H+, CH x+, C2H x+, CH xO+, C3H x+ and C2H xO+ ( x = 1, 2, 3 and 4). In addition, the desorption of H2O+ or H3O+ was observed in the mass spectra of hydrated dR films. Except for H+, the yields of these ions decreased when one layer of water molecules was deposited onto the film. These ions are produced by C-C or C-O bond scission. This result suggests that the water molecules act as a quencher, suppressing Coulomb repulsion and thus the extensive molecular decomposition of dR. Ab initio molecular dynamics simulations were performed to rationalize the fragments observed in the experiments. The results of the dynamical process of a hydrated dR molecule after oxygen K-ionization revealed elongation of a C-O bond of dR and the O-H bonds of both dR and water molecules prior to the Auger process, resulting in the ejection of H+ ions. These results strongly suggest that the very early process contributes to reducing the dR fragmentation, producing the H3O+ and H+ detected from the hydrated dR films. These desorbed ions may be involved in the induction of other types of damage, such as oxidatively generated base lesions, concomitantly produced with a strand break when produced in DNA.

Double-strand break induction and repair in V79-4 hamster cells: the role of core ionisations, as probed by ultrasoft X-rays.

PURPOSE: To compare the induction of double-strand breaks (DSB) in cells irradiated by 250 and 350 eV ultrasoft X-rays and assess the residual yield of breaks 2 hours post irradiation in order to unravel the correlation between the sharp increase in cell-killing efficiency of ultrasoft X-rays above versus below the carbon-K threshold (284 eV) and the induction of core events in DNA atoms. MATERIALS AND METHODS: V79-4 hamster cells were irradiated with synchrotron ultrasoft X-rays at isoattenuating energies of 250 eV and 350 eV. DSB were quantified using pulse field gel electrophoresis. RESULTS: A significant increase in DSB induction was observed for 350 eV ultrasoft X-rays above the carbon-K threshold, compared to 250 eV below the threshold, per unit dose to the cell. The DSB induced by the 350 eV ultrasoft X-rays were less repaired 2 h after irradiation. CONCLUSION: The increased DSB induction at 350 eV is attributed to the increase in the relative proportion of photon interactions in DNA resulting in significant dose inhomogeneity across the cell with a local increase in dose to DNA. It results from an increase in carbon-K shell interactions and the short range of the electrons produced. Core ionisations in DNA, through core-hole relaxation in conjunction with localised effects of spatially correlated low-energy photo- and Auger-electrons lead to an increase in number and the complexity of DSB.

Induction and repairability of DNA damage caused by ultrasoft X-rays: role of core events.

PURPOSE: To investigate the severity of damage induced in plasmid DNA by ultrasoft X-rays at different energies, in order to unravel the correlation between the sharp increase in cell-killing efficiency of ultrasoft X-rays above versus below the carbon K-threshold and the induction of core events in DNA atoms. MATERIALS AND METHODS: Bluescript (pBS, tight packing) and pSP189 (pSP, loose packing) plasmids were exposed to ultrasoft X-rays at 250, 380 and 760 eV energies, respectively, above phosphorus L-, carbon K- and oxygen K-thresholds. Complex DNA lesions were assayed by the repair protein Formamidopyrimidine DNA glycosylase (Fpg) and by in vitro repair assay using whole cell-free extracts. RESULTS: Clustered damage, as revealed by Fpg-induced double strand breaks, was observed at low level, but at similar rate at the three energies. Damage induced at 380 eV may be slightly less efficiently repaired by cell extracts than those produced at 250 eV. 760 eV photons which yield longer range electrons than 250 and 380 eV photons, induced more total damages which were more efficiently repaired, and thus likely more dispersed. CONCLUSION: It is demonstrated that ultrasoft X-rays induce complex damage, which do not exhibit the same ability to be repaired, depending on the energy and on DNA packing.