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Biologische Systeme und Medizin Poster: Mi., 14:00–16:30 M-P213
Computational studies of radiation damage in protein crystals
Melanie Zehnder 1 , Ivan Vartaniants 1 , Edgar Weckert 1
1 HASYLAB at DESY, Notkestrasse 85, 22607 Hamburg
Radiation damage limits the achievable resolution and data quality received from protein
crystals in X-ray structure analysis. For a better quantitative understanding the
damage produced by inelastic interaction of photons in the crystal is studied. Inelastic
scattering of photons and the produced electrons are investigated by a Monte - Carlo
approach.
Two types of inelastic interactions are possible, photoelectric effect and Compton
scattering. For small energies up to 30 keV the photoelectric effect is dominant. For
higher X-ray energies Compton scattering becomes dominant. The photo electron
carries nearly all the energy of the incoming photon, which is absorbed in the crystal.
In most cases a low energetic Auger electron is created afterwards. For Compton
scattering an outer shell interaction takes place and most of the energy is carried out
from the crystal by the high-energetic photon. Typically the Compton electron has a
much lower energy comparing to that of the photo electron. Electrons produce a higher
damage comparing to photons due to a higher inelastic cross section. The Monte Carlo
approach simulates the very early stages of the damage (first few picoseconds). In the
simulation programme the ionization and excitation of ions are simulated, but no bond
braking or recombination is studied.
Different energies (from the keV range up to 100 keV) and crystal sizes (from the
nanometer scale up to hundred micrometer scale) were studied by this approach. The
investigations were done to explore the best ratio between diffracted intensity and
damage produced for the given chemical composition. It is evaluated from the number
of ions produced by the incoming photon and the subsequent cascade. This value can
be calculated either per inelastic photon or per elastic photon. The simulation gives
also information on the spatial distribution, the number of excited atoms in the crystal
and the time of interaction.