Ionizing fragmentation of uracil and 5-bromouracil by electron impact in gas phase and hyperthermal Ar+ ion irradiation in condensed phase

The chemical composition and formation channels of charged fragments of uracil (U) and 5-bromouracil (BrU) produced by 70 eV electron impact in gas phase and 10–200 eV Ar+ ion irradiation in condensed phase are investigated in parallel with that of thymine (T). This is part of our effort to unravel possible different molecular level mechanisms of DNA damage caused by heavy ion radiation and the radiosensitivity of 5-BrU observed during UV, X- and γ-ray radiation. Abundant ionic fragments are produced from the three molecules by both electron impact and ion irradiation through similar bond-breaking channels due mainly to their very similar molecular structures. During electron impact major cation fragments are identified as [M − OCNH]+, NHCHCCH3+(T)/NHCHCH+(U)/NHCHCBr+(BrU), OCNH2+, HNCH+/CO+, CxHy+ (x = 1–3; y = 0–4), etc. and a parent cation [M]+ as well where M = T, U or 5-BrU. During ion irradiation some of these fragments partially or completely appear in their protonated forms, and an additional cation fragment [M − O]+ is also observed. Major anion fragments produced by ion irradiation are H−, O−, CN−, OCN− for all three molecules with trace amount of hydrocarbon anions. An additional Br− anion is produced for 5-BrU. The parent anions are discernable in their deprotonated forms only from very thin films. During Ar+ ion irradiation, fragment desorption begins at incident ion energies down to 20 eV. Anion fragment desorption appears at much higher energies at around 40 eV, and strongly depends on the film thickness.