Electron stimulated desorption of anions containing Oxygen and Nitrogen from self-assembled monolayers of DNA

Abstract

Abstract. The electron stimulated desorption of anions from self-assembled monolayers of double stranded DNA is reported. Desorption of the oxygen and nitrogen containing anions O¯, OH¯, CN¯, OCN¯, and OCNH¯ is induced by the impact of 0.1-20 eV electrons. The anion desorption yields, measured as a function of incident electron energy exhibit pronounced maxima that can be attributed to dissociative electron attachment (DEA) to basic DNA units. Above 15 eV, desorption is attributed to dipolar dissociation (DD). This study further indicates that electrons with energy as low as 2.5 ± 0.3 eV can not only cause damage to DNA but also produce fragments with considerable kinetic energy. IntroductionAlthough the vast majority of investigations have focused on interactions of high energy radiation with living cells and DNA molecules, it is well established that the genotoxic effects of high energy particles are effectuated by the secondary species generated by the primary particles. These secondary species can cause mutagenic, recombinogenic, and other potentially lethal DNA lesions such as single and double strand breaks [1]. The most abundant of these species are secondary electrons with typical energies below 30 eV [2]. Therefore, understanding the mechanisms of low-energy electron (LEE) induced damage in DNA is highly relevant to radiobiology. Boudaïfa et al.[3] investigated strand break formation in plasmid DNA by 3-20eV electrons. Single and double strand breaks were observed even at electron energies well below ionization thresholds (7.5 eV to 10 eV) [4]. Thresholds for these breaks were measured to be near 3-5 eV with a maximum at about 10 eV. The authors concluded that these breaks occur as a result of electron attachment to DNA subunits to form a transient negative ion (TNI), and subsequent bond dissociation. The threshold for strand-breaks in their work differs from that for damage induced by photons, where the onset lies at about 7 eV [5].To better understand the mechanisms underlying DNA damage through electron attachment, studies have examined interactions of LEE with various components of DNA such as the bases [6-10], deoxyribose analogues [ 11,12], and the phosphate group [13] of the backbone in gas and solid phases. For all of the DNA components [6][7][8][9][10][11][12][13], the formation of resonances (i.e., TNI) that dissociate into a stable anionic and radical fragments was observed. One of the remaining challenges is to identify, in a long DNA chain, which TNI state of the different basic constituents ( the bases, sugar and phosphate

Low energy electron stimulated desorption from DNA films dosed with oxygen

“…1 as the solid black lines in panels (a) and (b), respectively. The data are consistent with those published earlier, 19,22,26 where the structures peaking at 8.5 and 7.5 eV in the respective O − and OH − yield functions are due to DEA to DNA and the linearly increasing signal above ~12 eV to DD. 19 The threshold energies for desorption of O − and OH − are estimated as being 3.9 and 2.9 eV, respectively.…”

“…The results are essentially the same as those previously recorded. 19,22,24,26 With the exception of O − and OH − , the yield functions of these anions did not change significantly with film exposure to 0.75 to 3.5 ML of O 2 at 50 K. Since H − , CH 2 − , and CH 3 − can only arise from the bases and sugar ring and NH − and CN − only from the bases, either adsorption of O 2 on these subunits did not change anion yields or O 2 adsorbs only on the phosphate group. Below 12 eV, the phosphate group is thought to be responsible for O − and OH − ESD via DEA to pure DNA; 2 i.e., (1) or (2) where the DNA phosphate charge is neutralized by protonation as in the present experiment.…”

“…This system, which has been previously described in detail, 19 is housed in an UHV system reaching a base pressure of 5 × 10 −10 Torr. Recently, a custom load-lock has been coupled to the TOF mass analyzer to allow samples, prepared under ambient conditions, to be introduced into UHV.…”

“…Recently, a custom load-lock has been coupled to the TOF mass analyzer to allow samples, prepared under ambient conditions, to be introduced into UHV. 19 Once transferred into the analysis chamber, the temperature of the samples can be controlled between 10 and 300 K with a closed-cycle He cryostat and resistive heating elements. Samples were degassed within the load-lock chamber for at least 4 hours prior to their transfer into the main chamber, where they were bombarded with a pulsed electron beam.…”

“…With a recently improved system, 19 we measure the electron stimulated desorption (ESD) of anions from self-assembled monolayers (SAMs) of DNA molecules 20,21 exposed to monolayer quantities of O 2 at various film temperatures in the range 50-250 K, under ultrahigh vacuum (UHV). We inspect any modifications in the DEA and dipolar dissociation (DD) processes as a result of oxygen deposition onto and within SAM-DNA samples.…”