  The emergence of ultra-fast X-ray free-electron lasers opens the possibility
of imaging single molecules in the gas phase at atomic resolution. The main
disadvantage of this imaging technique is the unknown orientation of the sample
exposed to the X-ray beam, making the three dimensional reconstruction not
trivial. Induced orientation of molecules prior to X-ray exposure can be highly
beneficial, as it significantly reduces the number of collected diffraction
patterns whilst improving the quality of the reconstructed structure. We
present here the possibility of protein orientation using a time-dependent
external electric field. We used ab initio simulations on Trp-cage protein to
provide a qualitative estimation of the field strength required to break
protein bonds, with 45 V/nm as a breaking point value. Furthermore, we
simulated, in a classical molecular dynamics approach, the orientation of
ubiquitin protein by exposing it to different time-dependent electric fields.
The protein structure was preserved for all samples at the moment orientation
was achieved, which we denote `orientation before destruction'. Moreover, we
find that the minimal field strength required to induce orientation within ten
ns of electric field exposure, was of the order of 0.5 V/nm. Our results help
explain the process of field orientation of proteins and can support the design
of instruments for protein orientation.
