Fast event-based electron counting for small molecule structure determination by MicroED

Electron counting helped realize the resolution revolution in single particle cryoEM and is now accelerating the determination of MicroED/3DED structures. Its advantages are best demonstrated by new direct electron detectors capable of fast (kilohertz) event-based electron counting (EBEC). This strategy minimizes the inaccuracies introduced by coincidence loss (CL) and promises fast, accurate structures. We use the Direct Electron Apollo camera to leverage EBEC technology for MicroED data collection. Given its ability to count single electrons, the Apollo collects high quality MicroED data from organic small molecule crystals illuminated with incident electron beam flux values as low as 0.01–0.045 e-/Å2/s. Under even the lowest flux (0.01 e-/Å2/s) condition, fast EBEC data produced ab initio structures of a salen ligand (268 Da) and biotin (244 Da). Each structure was determined from 100–degree wedge of data collected from a single crystal in as few as 50 seconds, with a delivered fluence of only ~0.5 e-/Å2. Fast EBEC data collected with a fluence of 2.25 or 3.33 e-/Å2 also facilitated a 1.5Å structure of thiostrepton (1,665 Da). While refinement of those structures appeared unaffected by CL, a CL-adjustment applied to gain-normalized EBEC data further improved the distribution of intensities measured from salen ligand and biotin crystals. However, CL-adjustment only marginally improved the refinement of their corresponding structures, signaling the already high counting accuracy of detectors with counting rates in the kilohertz range. Overall, by delivering low-dose structure-worthy data, fast EBEC collection strategies open new possibilities for high-throughput MicroED.