Proton irradiation of DNA nucleosides in the gas phase

Abstract

The four DNA nucleosides guanosine, adenosine, cytidine and thymidine have been produced in the gas phase by a laser thermal desorption source, and irradiated by a beam of protons with 5 keV kinetic energy. The molecular ions as well as energetic neutrals formed have been analyzed by mass spectrometry in order to shed light on the ionization and fragmentation processes triggered by proton collision. A range of 8-20 eV has been estimated for the binding energy of the electron captured by the proton. Glycosidic bond cleavage between the base and sugar has been observed with a high probability for all nucleosides, resulting in predominantly intact base ions for guanosine, adenosine, and cytidine but not for thymidine where intact sugar ions are dominant. This behavior is influenced by the ionization energies of the nucleobases (G < A < C < T), which seems to determine the localization of the charge following the initial ionization. This charge transfer process can also be inferred from the production of protonated base ions, which have a similar dependence on the base ionization potential, although the base proton affinity might also play a role. Other dissociation pathways have also been identified, including further fragmentation of the base and sugar moieties for thymidine and guanosine, respectively, and partial breakup of the sugar ring without glycosidic bond cleavage mainly for adenosine and cytidine. These results show that charge localization following ionization by proton irradiation is important in determining dissociation channels of isolated nucleosides, which could in turn influence direct radiation damage in DNA.

Ultrafast non-radiative decay of gas-phase nucleosides

“…We have previously reported mass spectra for each of the nucleosides using one-colour (1+1) resonanceenhanced multiphoton ionization (REMPI) at 267 nm with low interaction intensity (see supplementary material in reference [43]). These show that parent molecular ions make up more than 90% of the ion yield for thymidine and adenosine.…”

“…To produce gas-phase targets, nucleosides were desorbed directly from solid samples deposited on a 10 µm stainless steel foil which was integrated into the repeller electrode of a time-of-flight mass spectrometer [42] [43]. The samples (99% purity) were obtained from Sigma-Aldrich and used without further purification.…”

“…We have overcome these issues, however, through the use of a "soft" laser desorption method based on back-irradiation of thin foil targets coated with the molecular samples of interest [42] [43]. In the present study we have sought to remedy this situation by obtaining the first reported gas-phase measurements of ππ* excited state decay in all of the DNA nucleosides following UV excitation.…”

Ultrafast dynamics in the DNA building blocks thymidine and thymine initiated by ionizing radiation

“…The intact, isolated, and neutral molecules were produced by irradiating the reverse side of the foil with a continuous wave diode laser. The exact temperature of the desorbed molecules is hard to determine accurately, but previous measurements taken under similar desorption conditions have shown that there is no fragmentation of any of the molecules prior to the interaction with the laser pulses [8] [33].…”

Time-dependent density-functional-theory investigation of the collisions of protons and α particles with uracil and adenine

“…Also due to the size and di culty of isolation, measurements are currently limited with respect to biological molecules in their native (aqueous) environment. Currently most of our understanding comes from studies in the gas phase, where the e↵ects of proton and other ion collisions on small molecules have been investigated [5,8,[33][34][35][36][37][38][39][40][41]. The resulting fragments can then be used to determine the most common fragmentation channels for a given particle energy.…”