Bond- and energy-selective carbon abstraction from D-ribose by hyperthermal nitrogen ions.

The interaction of energetic, highly charged heavy particles with biological media bears significance in both heavy-ion cancer therapy and space radiation risks for mission crews. Heavy-ion traversal of living cells produces extremely high concentrations of secondary electrons, ions, and excited neutral species along the tracks, which can cause severe damage to the medium, including the nuclear DNA. The initial energetic secondary species are rapidly thermalized and solvated through multiple scattering energy-loss events, during and after which they interact with the DNA molecules, leading to cell death or mutations. Such genotoxic interactions have been extensively studied on the long time scales of thermally diffusion-limited chemical processes. However, knowledge of the nascent (picosecond timescale) events immediately following primary ion irradiation, which link the initial physical processes and the later thermal diffusion-limited radiation chemistry, is still rudimentary, and it leads to major uncertainties in space radiation risk estimates to astronauts and prohibits the development of global models for radiation damage. Early secondary electron damage to DNA occurs even at subionization electron energies by dissociative electron attachment (DEA), resulting in sugar–phosphate cleavage, base loss, singleand double-strand breaks, 10] and so forth, while hydrogen loss by DEA was found to be both bondand site-selective. Regarding the formation of hyperthermal secondary species, heavy-particle radiation is distinct from conventional, sparsely ionizing photon or electron radiation in the sense that it imparts much higher kinetic energies to the secondary ion and neutral fragments. Recent measurements show that a primary heavy ion (MeV range) can produce secondary C, O, and N ions (n = 1–3) from DNA bases with hyperthermal energies up to several 100 eV, 16] which is not observed for photon impact even at high energies. These hyperthermal ion fragments can cause further complex damage to DNA components in subsequent scattering events by collisional (physical) pathways at energies down to approximately 10 eV, and reactive scattering (physicochemical) pathways at energies down to approximately 1 eV. For example, the N ion efficiently abstracts hydrogen atoms from DNA components at energies down to about 1 eV. Hydrogen abstraction is found to be bondand chargestate-selective. However, herein we show for the first time that the more severely damaging carbon abstraction from within the ring structure of d-ribose molecules by hyperthermal nitrogen ions exhibits a strong atom-site and energy selectivity, and we demonstrate that this selectivity is determined by the local chemical bond strengths. Our experiments were conducted with a hyperthermal ion beam system. It delivers a well focused, massand energy-selected, positive ion beam in the 1–100 eV energy range onto molecular films condensed on a polycrystalline platinum substrate in an ultrahigh-vacuum reaction chamber. Beams of approximately 30 nA Ar, 70 nA N and N2 , focused at the target to a 2–4 mm spot, are used. The ion beams have an energy spread of approximately 1 eV full width at half maximum over the entire energy range. Multilayer molecular films are prepared by in vacuo evaporation and subsequent condensation onto atomically clean Pt substrates, which are cleaned by resistive heating to 1000 8C prior to each film deposition. Its cleanliness is verified by 200 eV Ar secondary ion mass spectrometry (SIMS). The desorbing ionic reaction products during primary ion irradiation of molecular films are monitored by a quadrupole mass spectrometer (QMS, Hiden Analytical Ltd.) installed perpendicular to the ion-beam axis. The QMS measures desorbed ions with in vacuo energies of 0–5 eV. The sample surface is at approximately 608 with respect to the QMS axis during experiments. d-ribose was purchased from Sigma Aldrich with purity of 99.5%, and d-[5-C]ribose was purchased from Cambridge Isotope Laboratories with isotopic purity of 98 %. During experiments, the molecules were evaporated from a miniature oven at 60–70 8C and condensed on the substrate at room temperature. Film deposition rates were monitored with a quartz crystal microbalance and characterized in nanograms per square centimeter per minute. Film thickness is given by the known deposition rate and time. Since d-ribose films suffer some evaporative loss in high vacuum, only thick films (ca. 1000 ngcm 2 or 20 monolayers) were used. We find that both N and N2 + ions efficiently abstract carbon atoms from d-ribose to form CN, which desorbs from the film as a CN ion. Figure 1a shows anion mass spectra between 23 and 28 amu produced by 50 eV Ar, N, and N2 +