Note: Transient negative ions as initiators of oxygen fixation in <20 eV electron-irradiated DNA

Abstract

Low-energy electrons (LEEs) can cause biologically significant damage to DNA. 1,2 Most experiments on LEE-DNA interactions have been performed under ultra-high vacuum (UHV) conditions with energy-selected beams of LEEs incident on pure, dry DNA films. 2 Such conditions are unlike those within the cell and recent experiments have sought to identify the effect on LEE-induced DNA damage of such commonly encountered species as organic ions, 3 proteins, 4 and water. 5 Recently, we investigated interactions between O 2 , DNA, and LEEs in electron stimulated desorption (ESD) experiments. 6 By "pre-irradiating" DNA films with LEEs prior to dosing with 18 O 2 , we observed the new desorption signals of 18 O − and 18 OH − which were due to reactions of 18 O 2 with LEE-induced damage sites in the film; the new desorption signals were fragments formed by LEE-initiated dissociation of the 18 O 2 -DNA reaction products. It appears then that the binding of O 2 to damage sites in DNA is dependent on the interactions of LEEs with the sugar rings or bases, which contain hydrocarbons. It was proposed that in both DNA 6 and alkanethiol 7 samples, an initial damage to RCH x site by LEE irradiation forms a RCH x−1• radical which then reacts with O 2 to form a RCH x−1 OO • radical, from which O − can be desorbed by resonant (dissociative electron attachment (DEA)) or direct (dipolar dissociation (DD)) processes. 8 The previously observed phenomena are thus related to "oxygen fixation" 9 which describes the rendering permanent (or "fixing") of radiation damage in biological materials by prompt exposure to O 2 . Such fixation may underlie the long observed radio-sensitization properties of O 2 . Here, we use ESD to monitor the yield of O 2 /DNA reaction products as a function of the energy of electron pre-irradiation (E p ). Such measurements are compared to measurements of H − desorption from pristine DNA-SAM (Self-Assembled Monolayer) films to confirm if the binding of 18 O 2 to DNA SAMs is an electron-induced process involving dehydrogenation. 6 For these experiments, SAMs of 5′AAAGGACAAA-3′ phosphothiolated single-stranded DNA oligonucleotides (from University of Calgary) were chemically grafted onto clean Au/Si substrates via the terminal thiol. Prior to deposition, the Au substrates (~1.2 × 1.2 cm 2 ) were rinsed with methanol and Millipore water (R > 18 MΩ), and cleaned by three ozone/UV treatments of 20 min. After each ozone/UV treatment, the substrates were rinsed with Millipore water and dried with N 2 . Upon cleaning, 20 μl of DNA solution (~16 μM in Millipore water) was dropped on the Au substrate. After 20 h, the SAMs were rinsed and immersed multiple times in Millipore water. The SAMs were dried before introduction into Figure 1(a) shows the yield function of the anion H − from a pristine DNA SAM at a temperature of 150 K. Here, "yield function" describes the variation of a particular anion yield with E i . Consistent with earlier ESD measurements on DNA SAMs, 13 the H − signal exhibits a desorption threshold at ~4.5 eV, a...