We demonstrate that electrons at energies below the threshold for electronic excitation (<3 eV) effectively decompose gas phase uracil generating a mobile hydrogen radical and the corresponding closed shell uracil fragment anion (U-H)(-). The reaction is energetically driven by the large electron affinity of the (U-H) radical. This observation has significant consequences for the molecular picture of radiation damage, i.e., genotoxic effects or damage of living cells due to the secondary component of high energy radiation.
We have measured the electron energy dependence for production of a great variety of anion fragments, induced by resonant attachment of subionization electrons to thymine (T) and cytosine (C) within femto-second time scales. At the lowest electron energies we also observe stable molecular anions of these bases, viz., T− and C−. Our measurements suggest that this resonant mechanism may relate to critical damage of irradiated cellular DNA by subionization electrons prior to thermalization.
At the very early time of irradiation, ballistic secondary electrons are produced as the most abundant of the radiolytic species directly within DNA or its environment. Here, we demonstrate the propensity of such low-energy (<3 eV) electrons to damage DNA bases via an effective loss of hydrogen located at the specific nitrogen positions. Since this site is directly implicated in the bonding of nucleobases within DNA and since dehydrogenation of the nucleic acid bases has been observed to be the predominant dissociative channel, the present findings foreshadow significant implications for the initial molecular processes leading to genotoxicity in living cells following unwanted or intended exposure to ionizing radiation (e.g., sunbathing, air travel, radiotherapy, etc.).
Excess charge deposited on gas-phase thymine (T) and uracil (U) by resonant attachment of low-energy (0-3 eV) electrons induces the loss of hydrogen, which exclusively takes place from the N positions. This bond selectivity can be made site selective by properly adjusting the electron energy. While electrons at 1 eV result in loss of hydrogen from N1, the reaction can be switched to loss of hydrogen from N3 by tuning the electron energy to 1.8 eV. We find that any energy (and charge) transfer is completely blocked when the NÀH bond is replaced by NÀCH 3 . The present results have significant consequences for the exploration of the initial molecular processes leading to DNA damage, specifically in relation to recent observations of strand breaks in plasmid DNA induced by very low energy (0-4 eV) electrons.[1]Recent gas-phase experiments on the isolated nucleobases (NBs) thymine (T), cytosine (C), adenine (A), guanine (G), and uracil (U) have demonstrated that they all effectively capture low-energy electrons in the range below 3 eV . [2][3][4][5][6] The generated transient negative ion (TNI) subsequently decomposes by the loss of a neutral hydrogen atom. The overall dissociative electron attachment (DEA) reaction can be expressed as Equation (1), in which NB À# is the TNI of thecorresponding nucleobase and (NBÀH) À is the closed-shell anion formed by the ejection of a neutral hydrogen radical whereby the excess charge remains on the nucleobase. The reaction is effective already at energies below the threshold for electronic excitation (at subexcitation energies) and driven by the appreciable electron affinity of the (NBÀH) radicals, which is in the range between 3 and 4 eV, dependent on the site from which the hydrogen atom is ejected [2,6] (see below). Experiments with partly deuterated thymine [5] demonstrated that hydrogen abstraction occurs exclusively from the N sites, although H loss from the C sites is energetically accessible within that energy range. In the present contribution we demonstrate by means of methylated thymine and uracil that by properly adjusting the electron energy, the loss of hydrogen can be made even site selective with respect to the N1 and N3 positions. In light of strong efforts to induce cleavage of particular bonds by coherent laser control using tailored ultrafast pulses [7] the present result is very remarkable.In addition to these basic aspects, our findings have direct implications for the molecular description of radiation damage in biological systems, more specifically, for DNA in living cells. It is well accepted that the main biological effect is usually not produced by the primary interaction of the highenergy quanta with the complex molecular network within a living cell, but rather by the action of the secondary species generated along the ionization track.[8] The interaction of these secondary species (ions, electrons, radicals) with DNA and its surrounding can cause mutagenic, genotoxic, and other potentially lethal DNA lesions such as single-strand breaks (SSBs) and do...
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