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Institut de Biologie StructuraleGrenoble / France

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Electron Diffraction

A new hybrid pixel detector, Cheetah from Amsterdam Scientific Instruments, has been installed on the Tecnai cryo-electron microscope F20 of the IBS electron microscopy platform. This detector is dedicated to the development of electron diffraction for protein structure determination, a promising technique that is unique in its capability to make use of nanosized crystals and to provide information of charge states of metal ions in protein molecules (bioRxiv : 10.1101/2020.07.08.191049).

The new detector uses the Medipix 3 chip developed by the Medipix Collaborations, initially set up to meet the challenge of particle tracing at CERN LHC. (The same Medipix3 chip is also used in the latest generation LAMBDA detector found in synchrotrons.) It has single electron sensitivity (DQE(0) > 0.9) and high frame-rate (zero dead time, 2kHz). Thanks to a thresholding feature built into the chip, near noiseless data acquisition is achieved. Importantly, the detector is radiation hard and has a high dynamic range, allowing diffraction patterns with weak intensities, such as from protein crystal diffraction, to be acquired without a beam stopper for the direct beam.
Preliminary data have been collected on the Tecnai F20 microscope using the new detector. Continuous rotation data on a molybdenum oxide crystal at room temperature as well as a lysozyme crystal at liquid nitrogen temperature, have been collected.

Cliché de diffraction d’un cristal de Lysozyme

This mode of data collection allows the electron diffraction data to be analyzed using the standard programs developed for X-ray macromolecular crystallography (e.g. CCP4 suite).
The two crystals used in these data sets exemplifies some of the suitable types of crystals for electron crystallography experiments. The MoO crystal used here was an extended sheet, which facilitated the data collection as it stayed in the field of view during the sample rotation. The lysozyme crystal was broken up from an urchin type crystal, which is not suitable for X-ray crystallography but often works well in electron crystallography. As is evident from the preliminary data, protein samples are still a challenge due to the limited number of unit cells available in a nano-crystals and their radiation sensitivity. If you have any robust nanosized protein crystals, please get in touch !

Contact : Wai-Li Ling, Dominique Housset

(This new detector was partly financed by the sale of the POLARA cryo-microscope (FEI, The Netherlands) of the IBS Electron Microscopy PF and also by a contribution from the ILL).