Beam-Induced Motion Mechanism and Correction for Improved Cryo-Electron Microscopy and Cryo-Electron Tomography

Beam induced motion (BIM) is a major cause of information loss in single-particle cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET). In this chapter we review the progress made in understanding the mechanism of BIM and methodologies developed to restore the information loss, in particular, in the context of cryo-ET. Observed data supports a doming model of BIM dominated by out-of-sample-plane motion and where heterogenous projections of local motions are observed. Sample tilting amplifies the apparent BIM as the out of plane motion becomes directly visible. This leads to more rapid motion at early doses as well as strong anisotropic motion in images recorded at high tilt angles. Based on the study of the BIM occurring within cryo-ET tilt series with very fine dose sampling, it was found that frozen hydrated samples, under repeated exposures, experience a “drumbeat” response, having both a repeated elastic component and a plastic deformation. The elastic deformation represents the portion that can be fully relaxed when the electron beam is turned off at the end of the exposure for each tilted image. By contrast, the plastic deformation decays exponentially with respect to the accumulated dose, causing progressive and permanent sample deformation that undermines the rigid-body hypothesis in cryo-ET. We propose that the repeated elastic component is likely due to radiolysis-induced pressure whereas the plastic deformation corresponds to sample buckling to relive stresses that can occur during freezing. In conventional cryo-EM, the elastic response is often hidden within the first frame and only the plastic response is observed. Finally, we described recent improvements in MotionCor2 to meet the need for efficient and accurate correction of the much greater anisotropic BIM in tilt image/series collected with higher frame rates for sampling the more rapid early motion.