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Fourier Coefficient, Protein Crystal, Atomic Resolution, Structure Research, Specimen Temperature
Abstract:
Cryoelectron microscopy is now proven to be an advantageous tool for structure research of protein crystals. Using a helium-cooled superconducting electron microscope, the following protein crystals have been imaged with high resolution in one projection: crotoxin complex 3.5 Â, purple membrane 2.8 Â, matrix porin OmpF 3.5 Â, surface protein of sulfolobus spec. B12 10 Â. Fine-structure details within the unit cell were resolved, although even at 4.5 K specimen temperature the crystal still suffers considerable radiation damage. While the tolerable dose is only 20 e/Â2, for atomic resolution a dose at least 100 times higher is needed. The ladder to these successes had two steps: First, the image was taken in a special way with a minimum of pre-exposure. Second, the low-dose images were evaluated by a sophisticated image-processing procedure, the main part of which is the cross-correlation averaging. The high resolution structure is in all cases revealed by averaging thousands of low-dose-imaged unit cells.
