Electron interaction with nano-sized biomolecular systems

The topic of natural and artificial irradiation of biological systems is comprehensive and involves processes of different natures and timescales starting from the physical stage in <10 -16 sec up to the biological stage lasting up to months and longer. The physical stage involves interactions of the primary energetic ionizing radiation with biomolecular cell compounds leading to the formation of secondary species like electrons, ions and neutral radicals. Thereby electrons with a maximum kinetic energy of few tens of eV are important due to their formation as very abundant secondary species in the irradiation of cellular compounds. They loose their kinetic energy in a series of inelastic collisions before they thermalize and finally reach the hydrated stage. In the present proposal we will investigate the damaging effect of such electrons with kinetic energies up to 50 eV in the interaction with complex DNA building blocks like nucleosides and nucleotides which are surrounded by water molecules. In general, electron ionization at electron energies above the ionization threshold is a hard ionization process leading often to substantial fragmentation when an ionized molecule is isolated in the gas phase. By the hydration of the DNA compounds we achieve here a more realistic environment than the isolated situation. The aim is to unravel if hydration can buffer electron induced fragmentation of DNA building blocks by fast energy dissipation. Our intention is to study biomolecules with a known specific grade of hydration which will be achieved by mass selection of the electrospray-ionization generated biomolecule-water ions before the electron collision. In the second of the part project we will then use electrons with kinetic energies below the ionization threshold where an electron can attach to a molecule leading to a transient negative ion state. The latter may decay by emission of the captured electron but also undergo dissociation of the molecule. Severe DNA damage like strand breaks was shown to occur when a film of plasmid DNA was irradiated by low energy electrons. The underlying molecular reaction pathway to a strand break is still not fully understood; however there exist few theoretical predictions to be experimentally verified. Here we will explore electron attachment to nucleotides as a function of the electron energy and monitor any charged reaction product formed. Using a double focusing mass spectrometer for mass selection of the fragment anions formed we will elucidate the efficiency of sugar-phosphate bond cleavage which corresponds to a strand break.

The radiation damage of biological tissue by ionizing radiation is based on several processes with different nature. These processes occur on highly differing time scales. For example, at the so-called physical stage the interaction of the primary radiation with cellular tissue takes place in less than 10-16 s, while processes at the biological stage last up to month or longer (which includes the growth of cancer tissue). The primary radiation produces a waste amount of secondary particles (electrons, ions, and neutral radicals). Thereby low-energy electrons with a maximum energy of about few tens of electronvolt play an important role. In the frame of this completed project we studied the interaction of such low-energy electrons with biomolecules.In the course of this project two main groups of biomolecules relevant for cells have been investigated in the gas phase, (i) building blocks of DNA, and (ii) building blocks of proteins. Thereby it turned out that the stepwise increase of the molecular complexity provides valuable knowledge concerning the stability of biomolecules against irradiation with low-energy electrons. This was particularly showing up in the case of amino acids, where we investigated single amino acids, the dipeptide dialanine (consisting of two condensed alanine units) and the tripeptide trialanine (three condensed units). It turned out that the interpretation of the results for the condensed systems was highly supported by the knowledge gained for the electron interaction for the single building blocks. In general, we demonstrated by the present investigations that the high selectivity in cleavage of molecular bonds by low-energy electrons is also retained for the more complex systems. These results are crucial for the description of the radiation damage on the molecular level by secondary electrons. In case of nucleobases we also observed reaction pathways, which are highly remarkable from the point of molecular physics. However, these reactions turned out to be so slow, that they will not play a considerable role in radiation damage.In summary, the project provided substantial new insight into the decay of biomolecular ions, which are formed by interaction with low-energy electrons and the step from single molecules towards more complex condensed systems was achieved.