Developing electron crystallography for structure elucidation of organics and macromolecules

Abstract

Electron crystallography makes structure elucidation feasible when only nanometre-sized three-dimensional crystals of organics and macromolecules are available that are too small for structure determination by X-ray crystallography. An inherent drawback of electron diffraction is dynamic scattering that affects the measured intensities. These adverse effects dictate the use of only very thin crystals. Unfortunately, the small crystal volume limits the total diffracting power and thus results in a poor signal-to-noise ratio (SNR). The SNR can be improved by increasing the crystal exposure, which however leads to an increase in radiation damage. Therefore, fast and highly efficient data acquisition is required to outrun the radiation damage and improve the SNR. This can be achieved by collecting diffraction data at cryo-conditions using the rotation method with novel hybrid pixel detectors. This approach makes it feasible to solve structures of highly beam sensitive samples, such as organic pharmaceuticals and proteins, from nanometre-sized crystals. Existing crystallography software required several adaptations for successful data integration, structure solution and refinement. The resulting maps are of high quality and the model geometry is comparable to that of high-resolution X-ray structures. The results presented here suggest that electron crystallography is becoming a viable alternative for structure determination from nanometre-sized crystals of radiation sensitive samples.