Protein nanocrystals pose a formidable challenge for macromolecular crystallography. Their small size makes them difficult to locate and their small number of unit cells produces much weaker Bragg scattering for a given flux. Electron microscopy can overcome some of these complications. Nanocrystals can be easily observed in the TEM and the larger scattering cross section of electrons relative to X-rays allows meaningful signal to be measured from far fewer crystalline repeats. At the Rosalind Franklin Institute, we have been taking advantage of this by developing electron microscopy methods to determine structures from these tiny crystals. Additionally, we are developing workflows to study their formation and composition. 3DED, also known as microED, can be used to collect complete or near-complete datasets for individual nanocrystals. We have implemented 3DED on a cryoARM300 for routine data collection from protein nanocrystals. By incorporating both energy filtering and electron counting we ensure accurate detection of as much scattered signal as possible whilst maintaining a low electron flux. For the most challenging cases, we are developing scanning diffraction (4DSTEM) methodologies to record data from crystals and crystalline regions that are < 100 nm. Finally, we are employing tomographic techniques to understand the growth of nanocrystals in complex environments.

​