Abstract
X-ray free electron lasers (XFELs) now routinely produce millijoule level pulses of x-ray photons with tens of femtoseconds duration. Such x-ray intensities gave rise to the idea that weakly scattering particles—perhaps single biomolecules or viruses—could be investigated free of radiation damage. Here, we examine elements from the past decade of so-called single particle imaging with hard XFELs. We look at the progress made to date and identify some future possible directions for the field. In particular, we summarize the presently achieved resolutions as well as identifying the bottlenecks and enabling technologies to future resolution improvement, which in turn enables application to samples of scientific interest.
Highlights
Structural biology, the endeavor to obtain atomic resolution spatial structures of biological systems, operates under the principle that form determines function
The majority of protein structures deposited to the protein data bank (PDB)1 have been obtained using x-ray crystallography, where the protein structure is determined from the scattering of x rays interacting with a crystal consisting of an orderly arrangement of millions of individual proteins
Each particle presented to the x-ray free electron laser (XFEL) beam can only be measured once, in a single orientation
Summary
Structural biology, the endeavor to obtain atomic resolution spatial structures of biological systems, operates under the principle that form determines function. The deposited radiation energy is hundreds of times higher for x rays compared to electrons for the same scattering strength.3 This fundamental property, along with recent technological developments, has enabled high-resolution single-particle cryo-EM (cryogenic electronmicroscopy),7–9 where the structure is obtained by electron imaging of samples cryo-cooled to liquid nitrogen temperatures in order to reduce radiation damage.
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