Electron-induced chemical processes : From Quantum Astrochemistry to the biological medium
Prof.Francesco A. Gianturco
(Universitá La Sapienza, Roma)
Low –energy electrons provide a very flexible and detailed probing tool for the study of molecular
processes and transformations at the nanoscopic level , and therefore they have been employed in
a broad variety of experiments covering gas-phase and condensed media containing molecular partners.
One of the interesting areas for such studies is provided by quantum processes in the Interstellar Medium (ISM) , where , in particular, the group in Rome is directly studying the mechanisms that could provide a better understanding of the formation of increasingly larger Polycondensed Aromatic Hydrocarbons (PAHs) starting with expected cornerstones like benzene and benzyne molecules that are forming metastable anionic states leading to ring-opening reactions.
A further class of compounds that are also connected with the formation of metastable and stable anions are the linear carbon chains terminated by either H or N ,that have been recently observed in the Interstellar medium : C6H- , C4H-, C8H- , NC4N- , etc. They are all object of analysis and simulation by the Rome group in order to isolate the most likely paths leading electron attachment.
The damaging effect of ionizing radiation has been known for a long time, and it has been an accepted fact that high-energy tracks formed by a, b, g particles and atomic ions can ionize cell components along the radiation track, thereby leading to various dissociation channels and to the formation of damaging radicals . Recent observations have indeed discovered that not only photons, but also electrons at energies below the ionization thresholds can generate damaged DNA strands by forming mutagenic, lethal DNA and RNA lesions which lead to single and double strand breaks .. Their data showed that electron attachment within an energy range between a few and less than 20 eV can indeed cause the above breaking processes , following the formation and ensuing decay of a broad variety of Transient Negative ions (TNIs) of the various components of the initial double helix structures.
The corresponding theoretical description, at the molecular level, of the rather complex energy redistributions which can lead to the final break-ups into stable fragment anions has been attempted by the calculations carried out in our group in Rome , where we analyse the quantum scattering process of low-energy electrons off the molecular components of DNA. In this approach we devise an essentially an initio model which allows us to locate and classify several low-energy resonances which then decay by different pathways of energy redistribution into the molecular nuclear networks. In particular, we shall show that detailed spatial analysis of the metastable electron wavefunctions at the initial geometry, and the study of the resonance changes following specific bond deformations, can allow to identify rather specifically he most likely dissociation pathways and the chemical nature of the final, neutral and ionic fragments to be compared with experiments .