Estimation of high-order aberrations and anisotropic magnification from cryo-EM data sets in RELION-3.1.

Jasenko Zivanov,Sjors H W Scheres,Takanori Nakane

doi:10.1107/s2052252520000081

Jasenko Zivanov, Sjors H W Scheres + Show 1 more

Open Access

https://doi.org/10.1107/s2052252520000081

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Abstract

Methods are presented that detect three types of aberrations in single-particle cryo-EM data sets: symmetrical and antisymmetrical optical aberrations and magnification anisotropy. Because these methods only depend on the availability of a preliminary 3D reconstruction from the data, they can be used to correct for these aberrations for any given cryo-EM data set, a posteriori. Using five publicly available data sets, it is shown that considering these aberrations improves the resolution of the 3D reconstruction when these effects are present. The methods are implemented in version 3.1 of the open-source software package RELION.

Highlights

Structure determination of biological macromolecules using electron cryo-microscopy is primarily limited by the radiation dose to which samples can be exposed before they are destroyed
In order to increase contrast, cryo-EM images are typically collected out of focus, which introduces a phase shift between the scattered and unscattered components of the electron beam. This phase shift varies with spatial frequency and gives rise to the contrast-transfer function (CTF)
Since the electronscattering potential of the sample corresponds to a real-valued function, its Fourier-space representation exhibits Friedel symmetry: the amplitude of the complex structure factor at spatial frequency k is the complex conjugate of the structure factor at frequency Àk

Summary

Introduction

Structure determination of biological macromolecules using electron cryo-microscopy (cryo-EM) is primarily limited by the radiation dose to which samples can be exposed before they are destroyed. In order to increase contrast, cryo-EM images are typically collected out of focus, which introduces a phase shift between the scattered and unscattered components of the electron beam. This phase shift varies with spatial frequency and gives rise to the contrast-transfer function (CTF). Imperfections of the optical system can, produce asymmetrical phase shifts that break the Friedel symmetry of the scattered wave. The effect of this is that the CTF has to be expressed as a complex-valued function, which affects the amplitudes of the structure factors and their phases

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