Low energy electron stimulated desorption from DNA films dosed with oxygen

Abstract

Desorption of anions stimulated by 1-18 eV electron impact on self-assembled monolayer (SAM) films of single DNA strands is measured as a function of film temperature (50-250 K). The SAMs, composed of 10 nucleotides, are dosed with O 2 . The OH − desorption yields increase markedly with exposure to O 2 at 50 K and are further enhanced upon heating. In contrast, the desorption yields of O − , attributable to dissociative electron attachment to trapped O 2 molecules decrease with heating. Irradiation of the DNA films prior to the deposition of O 2 shows that this surprising increase in OH − desorption, at elevated temperatures, arises from the reaction of O 2 with damaged DNA sites. These results thus appear to be a manifestation of the so-called "oxygen fixation" effect, well known in radiobiology.

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

“…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.…”

“…6 Below 20 eV, the dehydrogenation of the DNA producing RCH x−1 • radicals appears to proceed via the production of dissociative TNIs.…”

“…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.…”

“…The maximum yield from O 2 /DNA reaction products has been reported at 7 eV. 6 The duration of ESD measurement was set to 3 s, ensuring that electron fluence was no more than 1% of that during pre-irradiation. The procedure (i-iii) was repeated on the same sample, while E p during (i) was incremented from 4.4 to 20.4 eV.…”

“…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…”