Proposal of Bragg Diffraction Imaging on Protein Crystals with Precession-Electron Annular-Bright-Field Microscopy

Abstract

In this theoretical study, the author firstly discusses the wave interference of Bragg diffraction inside 3D crystal, followed by quantum mechanical interpretation on the diffraction process, and proves that the interference fringe between Bragg diffraction and the incident beam is identical to the lattice plane. By introducing the beam expander concept, we may explain the image projection mechanism of Bragg diffractions propagating through TEM: Transmission Electron Microscope. In practice, we will take projection images at zone axes, because of its symmetric high-density diffractions associated with flat Ewald sphere and minimum overlap with the neighboring molecules. By precessing the illuminating beam around the zone axis, and introducing annular objective aperture, we select only the kinematic Bragg diffractions which correctly contribute to the real image reconstruction on the 2D electron detector. The outcome should be a positive contrast, clean image with no dynamical diffractions nor Moir\'e patterns. Importantly, the image should be insensitive to the defocus, and the spatial resolution may not be limited by the spherical and the chromatic aberrations of objective lens. Therefore, the imaging of small molecules, such as, the drug structure will be straightforward. By taking multiple projection images at the different zone axes from many micro-crystals, or from surrounding thin edges of a large crystal, we may reconstruct 3D structure of the larger protein complex in atomic resolution.