Single-particle cryoEM: Theory and practice

Radiation damage is the fundamental limitation to the useful exposure in single-particle electron cryomicroscopy (cryoEM), so the goal of the field is to record near-perfect images, to extract as much information as possible from them, and then use this information to determine the three-dimensional structure of the desired biological molecule or molecular complex. This review presents a theoretical estimate of the number of single-particle images required to reach any desired resolution assuming perfect performance of all aspects of the imaging process. This target is then compared with the current state of cryoEM in practice, as represented by published structures in the Electron Microscopy Data Bank (EMDB). Considering recent structures, it becomes clear that the best structures in the 2.5-3.0 Å resolution range come within a factor of 3 of theoretical expectations. More typical datasets still require 100 times more images in practice than necessary in theory. This also applies to the higher resolution structures where larger datasets are essential. The reasons for this discrepancy are discussed alongside possible strategies to bring experimental structures closer to theoretical perfection. Finally, even the best cryoEM structures have significant radiation damage, which can only be ameliorated by recovering information from the best possible images at the beginning of the exposure at a fraction of the total dose currently used.