Design guidelines for an electron diffractometer for structural chemistry and structural biology

Abstract

3D electron diffraction has reached a stage where the structures of chemical compounds can be solved productively. Instrumentation is lagging behind this development, and to date dedicated electron diffractometers for data collection based on the rotation method do not exist. Current studies use transmission electron microscopes as a workaround. These are optimized for imaging, which is not optimal for diffraction studies. The beam intensity is very high, it is difficult to create parallel beam illumination and the detectors used for imaging are of only limited use for diffraction studies. In this work, the combination of an EIGER hybrid pixel detector with a transmission electron microscope to construct a productive electron diffractometer is described. The construction not only refers to the combination of hardware but also to the calibration of the system, so that it provides rapid access to the experimental parameters that are necessary for processing diffraction data. Until fully integrated electron diffractometers become available, this describes a setup for productive and efficient operation in chemical crystallography.

Discrimination of Aluminum from Silicon by Electron Crystallography with the JUNGFRAU Detector

“…Our data reliably distinguishes aluminum from silicon based on their scattering factors both during structure solution, and for the refined model. While a 200 keV TEM with a LaB 6 electron source is very well suited for electron diffraction studies [37], many TEMs are equipped with a field emission gun and are usually operated at 300 keV. For operation at 300 keV, one would choose a thicker silicon layer, e.g., 450 µm, to shield the ASIC.…”

3D-structured supports create complete data sets for electron crystallography

“…Without data set x10_5 for the data sets from coiled carbon film, and without the data set x11_11 from the nylon three-dimensional network, the respective sets miss 17 out of 4756 and 1 out of 4984 reflections, respectively, in the resolution range 1.2–1.0 Å. As fully integrated electron diffractometers are under current development in hardware and in software 33–35 , such outliers can be compensated with data collection of additional samples, as has been common practice in X-ray crystallography. Both types of sample supports are easy to produce.…”

“…Data were acquired and processed as described in 7,35 . In brief, data were collected at room temperature on a Tecnai F30 TEM (FEI, now ThermoFisher) equipped with a Schottky Emitter at an energy E = 200 keV, corresponding to the wavelength λ = 0.02508 Å. Diffraction data were collected in TEM bright field mode with a dose rate of ~0.01 e − Å −2 s −1 (The reading of the dose was displayed as 0.00–0.01 e − Å −2 s −1 with a precision of only two digits).…”

A shared vision for macromolecular crystallography over the next five years

“…Electron diffractometers are not commercially available. As their design requirements are clear (Heidler et al , 2019), they will probably be brought to the market fairly soon. Most algorithms for the processing of electron diffraction data and the refinement of the resulting models presume kinematic diffraction (i.e., each electron is diffracted only once, as in X-ray crystallography).…”