Fragmentation of singly charged adenine induced by neutral fluorine beam impact at 3 keV

Delayed fragmentation of isolated nucleobases induced by MeV ions

We evaluated the dissociation of isolated gas-phase nucleobase molecules induced by mega electron volt (MeV)-energy ions to gain fundamental insights into the reactions of nucleobases upon fast ion irradiation. We studied five nucleobase molecules-adenine, guanine, cytosine, thymine, and uracil-as gas-phase targets. We compared the fragmentation patterns obtained from carbon ion impacts with those obtained from proton impacts to clarify the effect of heavy ion irradiation. We also compared the results with electron impact and photoionization results. In addition, we identified several delayed fragmentation pathways by analyzing the correlation between fragment pairs generated from singly and doubly charged intermediate ions. To determine the lifetimes of delayed fragmentation from singly charged intermediate ions, we evaluated the detection efficiencies of the microchannel plate detector for the neutral fragment HCN as a function of kinetic energy using a new methodology. As the first demonstration of this method, we estimated the lifetimes of C5H5N5+ generated by 1.2-MeV C+ and 0.5-MeV H+ collisions to be 0.87 ± 0.43 and 0.67 ± 0.09 µs, respectively. These lifetimes were approximately one order of magnitude longer than those of the doubly charged intermediate ion C5H5N52+.

Excitation of L-valine molecules by electrons and photons

Excitation of L-valine molecules was studied by optical spectroscopy. Optical emission spectra of the L-valine molecule and optical excitation functions of molecular bands and the Hβ spectral line were measured in the gas phase using electron impact. In the spectra of optical emission in the wavelength range of 250-500 nm, intense emission bands were found at energies of incident electrons of 30, 50 and 70 eV. Analysis of structural features of the valine molecule suggested a fragmentation scheme with the formation of excited particles in collisions with electrons. A notable feature of the presented optical excitation functions is a different growth dynamics with an increase in the energy of exciting electrons and the presence of a number of features and kinks, which are especially pronounced for λ = 305 nm and λ = 311 nm. The excitation thresholds were determined from the initial sections of the excitation functions of the most intense spectral lines by the least-squares method. The photoluminescence spectra of L-valine were measured for the first time on a Shimadzu RF-6000 spectrofluorophotometer in the spectral range of 400-800 nm for excitation wavelengths of 250, 275, 333, 351, and 380 nm.

Fragmentation mechanisms of cytosine, adenine and guanine ionized bases

The different fragmentation channels of cytosine, adenine and guanine have been studied through DFT calculations. The electronic structure of bases, their cations, and the fragments obtained by breaking bonds provides a good understanding of the fragmentation process that can complete the experimental approach. The calculations allow assigning various fragments to the given peaks. The comparison between the energy required for the formation of fragments and the peak intensity in the mass spectrum is used. For cytosine and guanine the elimination of the HNCO molecule is a major route of dissociation, while for adenine multiple loss of HCN or HNC can be followed up to small fragments. For cytosine, this corresponds to the initial bond cleavage of N3-C4/N1-C2, which represents the main dissociation route. For guanine the release of HNCO is obtained through the N1-C2/C5-C6 bond cleavage (reverse order also possible) leading to the largest peak of the spectrum. The corresponding energies of 3.5 and 3.9 eV are typically in the range available in the experiments. The loss of NH3 or HCN is also possible but requires more energy. For adenine, fragmentation consists of multiple loss of the HCN molecule and the main route corresponding to HC8N9 loss is followed by the release of HC2N1.