  The emerging technique of serial femtosecond X-ray crystallography (SFX) can
be used to study the structure and dynamics of biological macromolecules to
high spatial and temporal resolutions. An ongoing challenge for SFX is the
damage caused by the ultrabright X-ray free electron laser pulse. Though it is
often assumed that sufficiently femtosecond pulses `outrun' radiation damage,
in reality electronic damage processes commence during exposure. We model the
electronic damage to protein nanocrystals using a plasma model that tracks the
continuous changes to the energy distribution of the unbound electrons.
Tracking the continuous energy distribution is of particular importance for
distinguishing the influence of differing elements on secondary damage
processes. Heavy atoms have a ubiquitous but small presence in protein targets
- typically as integral components of the macromolecule and as salts in the
solvent. We find that these atoms considerably influence the simulated
ionization and scattering behavior of realistic targets due to their rapid
seeding of secondary ionization processes. In lysozyme, even the presence of
native sulfur atoms significantly contributes to theoretical measures of
damage-induced noise for >= 6 keV, 15 fs pulses. Contributing to the effect is
that heavy atoms seed `intermediate' energy electron cascades that are
particularly effective at ionizing the target on the femtosecond timescale. In
addition, the disproportionate effect of heavy atoms means the damage to a
protein crystal can be sensitive to their presence in the solvent. Outside of
reducing the concentration of heavy atoms in the target, these results suggest
the dose limits of SFX targets will be higher where the ionization of deep >~ 6
keV absorption edges is minimized, or, to a lesser extent, when such edges are
only ionized with X-rays >> 2 keV above their binding energy.
