Limiting the effects of radiation damage in MicroED through dose selection during data processing

Microcrystal electron diffraction (MicroED), also known as three-dimensional electron diffraction (3D ED), allows the collection of diffraction data from submicrometre-sized crystals under low electron-dose conditions. Despite having several advantages over conventional X-ray crystallographic techniques, susceptibility to radiation damage is a great challenge that remains to be solved in MicroED. Similar to X-ray crystallography, radiation damage to the macromolecular crystal structures in MicroED manifests in two forms: global damage that affects the overall order of the crystal lattice and site-specific damage that affects highly sensitive residues and moieties in macromolecules. Traditionally, the unit e^- Å^-2 has been used for electron-dose estimations, which does not consider the interaction between the incident electron beam and the sample. In this study, we clarify the terminology for describing `dose' in electron crystallography, including the procedure for converting values from e^- Å^-2 to grays (Gy). Furthermore, we investigated data-processing strategies that could be used to limit the effects of radiation damage to the crystal. During MicroED data collection, radiation damage increases with the number of acquired ED frames because the accumulated electron dose increases. Data collected from several crystals and processed in this way can be merged to increase the completeness and subsequently be used for structure refinement. According to our results, this approach improves the resolution of the data, the data statistics, the structure determination and the quality of the final structure.