Accounting for electron-beam-induced warping of molecular nanocrystals in MicroED structure determination

Abstract

Here we identify and characterize the warping of molecular crystal lattices induced by electron beam exposure during microcrystal electron diffraction (MicroED/3DED) data collection. We find changes to consensus crystal lattice orientation that are often dramatic, and appear ubiquitous in small organic molecule crystals. This evidence highlights the relevance of crystal bending or warping as a consequence of radiation-induced damage on molecular specimens, and points to it as a fundamental source of error in MicroED/3DED data collection and structure determination.

High-energy electrons induce sample damage and motion at the nanoscale to fundamentally limit the determination of molecular structures by electron diffraction. Using a fast event-based electron counting (EBEC) detector, we characterize beam-induced, dynamic, molecular crystal lattice reorientations (BIRs). These changes are sufficiently large to bring reciprocal lattice points entirely in or out of intersection with the sphere of reflection, occur as early events in the decay of diffracted signal due to radiolytic damage, and coincide with beam-induced migrations of crystal bend contours within the same fluence regime and at the same illuminated location on a crystal. These effects are observed in crystals of biotin, a series of amino acid metal chelates, and a six-residue peptide, suggesting that incident electrons inevitably warp molecular lattices. The precise orientation changes experienced by a given microcrystal are unpredictable but are measurable by indexing individual diffraction patterns during beam-induced decay. Reorientations can often tilt a crystal lattice several degrees away from its initial position before irradiation, and for an especially beam-sensitive Zn(II)-me­thio­nine chelate, are associated with dramatic crystal quakes prior to 1 e⁻ Å⁻² electron beam fluence accumulates. Since BIR coincides with the early stages of beam-induced damage, it echoes the beam-induced motion observed in single-particle cryoEM. As with motion correction for cryoEM imaging experiments, accounting for BIR-induced errors during data processing could improve the accuracy of MicroED data.