Heavy-element damage seeding in proteins under XFEL illumination

Area of Science:

• Structural biology
• Biophysics
• Materials science

Background:

• Serial femtosecond X-ray crystallography (SFX) uses X-ray free-electron lasers (XFELs) to determine biomolecular structures.
• XFEL pulses outrun conventional radiation damage but are limited by rapid electronic damage from ionization.
• Understanding ionization dynamics is key to mitigating damage in XFEL crystallography.

Purpose of the Study:

• To differentiate the impact of various atomic species on protein crystal ionization.
• To quantify the contribution of heavy atoms to radiation damage in SFX.
• To identify optimal X-ray energies for minimizing ionization cascades.

Main Methods:

• Utilized a plasma code to simulate electron energy distributions and track ionization.
• Modeled ionization cascades initiated by photoelectrons from different atomic species.
• Analyzed the effect of heavy atoms (Z > 10), sulfur, and solvated salts on light-atom ionization.

Main Results:

• Trace amounts of heavy atoms (Z > 10) significantly seed electron ionization cascades, contributing substantially to global radiation damage.
• Sulfur atoms and solvated salts in protein crystals induce considerable light-atom ionization.
• Global ionization peaks at approximately 2 keV above inner-shell absorption edges, initiating brief ionization cascades.

Conclusions:

• Heavy elements, even in small quantities, critically influence radiation damage in XFEL experiments.
• Minimizing heavy atom content or selecting appropriate X-ray energies can reduce damage.
• This research provides insights for optimizing experimental parameters in XFEL crystallography.