Dissociative photoionization of adenine following valence excitation

Abstract

We present results on the valence level excitation, ionization and dissociation of adenine, using time-of-flight mass spectrometry and synchrotron radiation, in the vacuum ultraviolet (VUV) range of 12-21 eV. The measurements were performed using a gas-phase (Ne) harmonics filter in order to eliminate contributions from higher-order harmonics. Mass spectra were obtained using the photoelectron-photoion coincidence technique (PEPICO). The relative abundances for each ionic fragment and their mean kinetic energy release have been determined from the analysis of the corresponding peak shapes in the mass spectra. Comparison with the available photoelectron spectra and previous measurements allowed the assignment of the main features in the spectra. A discussion on the dissociative photoionization channels of this molecule has also been included. Due to our harmonics-free incident photon beam we were able to propose new appearance energy (AE) for the most important ionic channels in this energy range. The precursor ion, C(5)H(5)N(5)+, is the most abundant species (40% at 15 eV and 20% at 20 eV), which confirms the high stability of adenine upon absorption of VUV photons. We have observed other intense fragment ions such as: C(4)H(4)N(4)+, C(3)H(3)N(3) (+), C(2)H(2)N(2)+ and HCNH+. The production of the neutral HCN fragment represents up to 40% of the dissociative channels for this molecule as induced by VUV photons.

Electron impact fragmentation of adenine: partial ionization cross sections for positive fragments

“…Table 1 compares the appearance energies for the parent ion and a number of the fragment ions obtained by electron impact and by photoionisation using synchrotron ionisation. We have obtained 8.0 ± 0.2 eV for the parent ion, which is significantly lower than the recent values obtained by electron impact by Minaev et al [16] and Dawley et al [17], but is in agreement with the recent values obtained using synchrotron radiation by Jochims et al [18] Regarding the appearance energies of the fragment ions, we note that all our appearance energies are in agreement with those of Jochims et al [18] (only slightly outside the range of quoted error bars for 108 u), but there are several disagreements with the results of Dawley et al [17] and Pilling et al [19]. Pilling et al [19] attribute the disagreement of their results with those of Jochims et al [18] to the presence of higher harmonics in the synchrotron beam in reference [18].…”

“…In combination with thermochemical data they have clarified fragmentation pathways. Pilling et al [19] have studied photoionization of adenine using 12-21 eV synchrotron radiation in 1 eV steps and a time-of-flight mass spectrometer. They have obtained mass spectra by measuring photoelectrons and photoions in coincidence, and have determined the appearance energies of 5 fragment ions.…”

Survival of gas phase amino acids and nucleobases in space radiation conditions

“…As pointed out by Pilling et al (2007b), in the case of adenine, the neutral HCN may represent as much as 40% of its photodissociative channels. The ion HCNH + is another largely photoproduced fragment from both amino acids and nucleobases.…”

“…Briefly, the radiation (∼10 12 photons s −1 ) from the beamline perpendicularly intersect the vapor-phase sample at the center of the ionization region inside a high vacuum chamber (Boechat-Roberty et al 2005, Pilling et al 2006. Mass spectra were obtained using the photoelectron photoion coincidence (PEPICO) technique (Pilling et al 2007b, c and references therein).…”

“…The goal of this work is to review some experimental gas-phase photoionization and photodissociation studies of amino acids and nucleobases induced by vacuum ultraviolet (VUV) and soft X-ray photons, obtained recently by our group (Lago et al 2004, Coutinho et al 2005, Marinho et al 2006, Pilling et al 2007b. A possible direction on the different survivability of these biomolecules on astrophysical environments are given.…”