Computing methods for icosahedral and symmetry-mismatch reconstruction of viruses by cryo-electron microscopy

Three-dimensional(3 D) reconstruction of icosahedral viruses has played a crucial role in the development of cryoelectron microscopy single-particle reconstruction, with many cryo-electron microscopy techniques first established for structural studies of icosahedral viruses, owing to their high symmetry and large mass. This review summarizes the computational methods for icosahedral and symmetry-mismatch reconstruction of viruses, as well as the likely challenges and bottlenecks in virus reconstruction, such as symmetry mismatch reconstruction, contrast transformation function(CTF)correction, and particle distortion.     

Measurement of Specimen Thickness by Using Electron Holography and Electron Dynamic Calculation with a Transmission Electron Microscope

A method of transmission-electron microscopy for accurate measurement of specimen thickness has been proposed based on off-axis electron holography along with the dynamic electron diffraction simulation.The phase shift of the exit object wave with respect to the reference wave in vacuum,resulting from the scattering within the specimen,has been simulated versus the specimen thickness by the dynamic electron diffraction formula.Offaxis electron holography in a field emission gun transmission-electron microscope has been used to determine the phase shift of the exit wave.The specimen thickness can be obtained by match of the experimental and simulated phase shift.Based on the measured phase shift of the [110] oriented copper foil,the thickness can be determined at a good level of accuracy with an error less than-10%.     

Three-Dimensional Reconstruction of Icosahedral Virus by Symmetry-Adapted Functions

A three-dimensional （3D） reconstruction of icosahedral virus is carried out by the icosahedral symmetry-adapted function （ISAF） method in spherical coordinates. In order to reduce the influence of noise, it is better to use the basis functions that have identical symmetry with the object reconstructed. It is verified that the ISAF method has stronger ability to reduce the influence of noise to grain the resolution better than that of the conventional method by the simulation of 3D reconstruction.