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Investigation of electron-impact-induced fragmentation processes of DNA components


The first step in the development of biological damage due to ionizing radiation is the fragmentation of molecular components of the DNA after ionization. The relevant literature has so far reported on numerous experiments to investigate fragmentation processes of molecules after a bombardment with electrons. Thereby, the major part of these experiments relates to the determination of the relative frequency distribution of the fragment masses. Only few data sets exist with regard to the absolute cross sections for the fragmentation of biomolecules after the interaction with electrons and ions, although these are important for the quantification of the primary physical radiation damage of the DNA due to ionizing radiation. Therefore, an apparatus has been set up whose aim is to determine these data sets for the DNA components as exactly as possible within the scope of the EMRP-JRP BioQuaRT project [1].

The experimental determination of the fragmentation cross sections is achieved by means of a supersonic molecular beam and a time-of-flight spectrometer in which those charged fragments of molecules are detected that are ionized by an electron beam which crosses the molecular beam. The uncertainty of the measurement results significantly depends on the accuracy with which the detection probability of the detector and the time and place of the interaction between the molecules and the projectiles can be determined.

Test measurements carried out on propane and methanol have shown that the deviation of the moment of interaction from the start signal of the time-of-flight spectrometer depends on the primary energy of the electrons, on the type of the molecule, and on the energy transmitted during the interaction. Individual corrections are, therefore, required.

The influence of the transferred energy can be illustrated by means of fragments of the DNA model molecules tetrahydrofuran and pyrimidine.

Figure 1: Measured frequency of fragment ions of the two model molecules after bombardment with electrons as a function of the time difference between the electron pulse and the start signal of the time-of-flight spectrometer (Δt).

The time dependencies shown in Fig. 1 can be explained by energy transfers from the electrons to the molecule. After ionization and dissociation, the charged fragments move away from each other with an initial velocity in an exactly straight and uniform line and, thus, veer away from the place of interaction.

Figure 2: On the left, the time dependency of the measured frequencies of THF fragment ions is shown after the bombardment with 50 eV electrons (shown as a measurement signal on the time-of-flight spectrometer, standardized to the gas pressure and to the charge emitted by the electron gun). For comparison purposes, the right side shows model calculations for which Maxwell's velocity distribution for particles of different mass was used as a basis.

In Fig. 2, the measurement data for THF are compared with the results of the model calculations, whereby in the first approach, Maxwell's velocity distribution of the molecules was used as a basis. The energy of the primary electrons amounted to 50 eV.

The knowledge gained in these experiments can be used for a more exact determination of the moment of interaction and, consequently, of the electron-impact-induced fragmentation cross sections of biomolecules.


  1. www.ptb.de/emrp/bioquart.html