Unit-cell determination from randomly oriented electron-diffraction patterns

Abstract

An algorithm is described that calculates the most likely primitive unit cell given a set of randomly oriented electron-diffraction patterns with unknown angular relationships.

A Medipix quantum area detector allows rotation electron diffraction data collection from submicrometre three-dimensional protein crystals

“…4). In the past we found orthorhombic nanocrystals of lysozyme to have P2 1 2 1 2 1 symmetry, with a unit cell of around a = 31.5, b = 52.5, c = 89 Å (Jiang et al, 2009). However, we could not index the rotation data with the latter unit-cell parameters.…”

“…Prior to installing the Medipix2 detector in the electron microscope, we collected diffraction data on an image plate, film and CCD, but with these detectors we were never able to collect multiple diffraction patterns from single protein crystals to high resolution because of beam damage (Jiang et al, 2009). The Medipix2 detector has an efficiency at low count rates that is at least an order of magnitude higher than that of an image plate (Faruqi & McMullan, 2011;Georgieva et al, research papers 2011).…”

“…Three-dimensional crystals with an equivalent volume would measure approximately 150 Â 150 Â 150 nm. Recent data demonstrate that useful high-resolution electron diffraction data (up to 2.5 Å resolution) can be obtained from nanosized three-dimensional protein crystals, where synchrotron X-rays fail (Jiang et al, 2009). …”

Protein structure determination by MicroED

“…Electron diffraction of 3D protein crystals in the TEM has been attempted over the years but none yielded a refined structure [25][26][27][28]. One of the major hurdles was the beam damage associated with data collection, which typically allowed only one diffraction pattern to be collected per crystal [28].…”

“…One of the major hurdles was the beam damage associated with data collection, which typically allowed only one diffraction pattern to be collected per crystal [28]. By only having one pattern per crystal it is difficult to properly index the reflections and determine the crystallographic orientation especially as the data is affected by the shape function.…”