Interaction of hydantoin with solar wind minority ions: O6+ and He2.

The laboratory study of prebiotic molecules interacting with solar wind ions is important to understand their role in the emergence of life in the complex context of the astrochemistry of circumstellar environments. In this work, we present the first study of the interaction of hydantoin (C3N2O2H4, 100 a.m.u.) with solar wind minority multi-charged ions: O6+ at 30 keV and He2+ at 8 keV. The fragmentation mass spectra as well as correlation maps resulting from the interaction are presented and discussed in this paper. Prompt and delayed dissociations from metastable states of the ionized molecule have been observed and the corresponding lifetimes measured. Experimental results are completed by quantum Density Functional Theory (DFT) calculations for energies, structures and dynamics (Internal Reaction Coordinates and Dynamic Reaction Coordinates) of the molecule for its different reachable charge states and the major observed fragmentation pathways. These calculations show that the molecule can only support two charges before spontaneously dissociating in agreement with the experimental observations. Calculations also demonstrate that hydantoin's ring opens after double ionization of the molecule which may enhance its reactivity in the background of biological molecule formation in a cirmcumstellar environment. For the major experimentally observed fragmentations (like 44 a.m.u./56 a.m.u. dissociation), Internal Reaction Coordinate (IRC) calculations were performed pointing out for example the important role of hydrogen transfer in the fragmentation processes.

Proton-induced direct and indirect damage of plasmid DNA.

Clustered DNA damage induced by 10, 20 and 30 MeV protons in pBR322 plasmid DNA was investigated. Besides determination of strand breaks, additional lesions were detected using base excision repair enzymes. The plasmid was irradiated in dry form, where indirect radiation effects were almost fully suppressed, and in water solution containing only minimal residual radical scavenger. Simultaneous irradiation of the plasmid DNA in the dry form and in the solution demonstrated the contribution of the indirect effect as prevalent. The damage composition slightly differed when comparing the results for liquid and dry samples. The obtained data were also subjected to analysis concerning different methodological approaches, particularly the influence of irradiation geometry, models used for calculation of strand break yields and interpretation of the strand breaks detected with the enzymes. It was shown that these parameters strongly affect the results.

Dehalogenation of 5-halo-uracil molecules induced by 100 keV proton collisions.

Neutral and cationic halogen loss of singly and doubly ionised 5-X-uracil (X = F, Cl, Br, I) after collisions with 100 keV protons have been studied in the gas phase. The rates of these dissociation channels are strongly dependant on the nature of the halogen substituent. It is very weak in the case of fluorine but is a dominant channel for iodine. Dissociation mechanisms are proposed for a number of significant channels associated to dehalogenation. It is suggested that some final ion products originate from specific processes. For instance, ion products of mass 38, 39 and 40 amu are very sensitive to the nature of the primarily ejected halogen and result from pathways associated to dehydrogenation and transient formation of the dehalogenated uracil cation.

Energetics and metastability of the adenine dication observed in proton-adenine collisions.

We present here a study that deals with the correlated fragmentation of a doubly charged adenine molecular target induced by a 100 keV proton beam. We have elucidated part of the dissociation dynamics for several channels and have obtained the corresponding kinetic energy released values. We have extracted activation energies by combining our experimental data with computations using the ab initio GAMESS code. We have observed metastability patterns against fragmentation, for which we have extracted the temporal mechanism (one or two steps). Subsequently, we have obtained lifetimes in the 100-200 ns range. In the simplest case of two-body fragmentation with the emission of mass 28, the determination of transition states and reaction paths has showed that emission of the H-C-N-H fragment is preferred to that of C-N-H(2). From the calculated activation barriers and lifetimes, we have deduced an equivalent temperature of the dication that we have compared with the existing models.