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+, CHx+, C2Hx+, CHxO+, C3Hx+ and C2HxO+ (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 an...