Femtosecond bioimaging

X-ray crystallography is remarkably successful at yielding atomic-resolution structures of proteins and other biological molecules. But that success relies on growing macroscopic crystals. Unfortunately, some molecules crystallize with difficulty or not at all. A decade ago, researchers predicted that the femtosecond pulses from an x-ray laser would be short enough and intense enough to produce a useful diffraction pattern from an uncrystallized biomolecular cluster before vaporizing it. An international collaboration of more than 80 scientists has now used SLAC’s Linac Coherent Light Source free-electron laser to perform two proof-of-concept demonstrations of the feat. In one study, the researchers squirted a suspension of nanocrystals (the photosynthetic protein photosystem I) across the 1.8-keV x-ray beam, recorded the two-dimensional diffraction pattern each time a crystal intersected the beam, and then combined 15 000 single-crystal snapshots to form the 3D projection shown here. From the data, the team reconstructed the protein’s structure at the near-atomic resolution of 8.5 Å, just over the x-ray wavelength. In the second study, the team injected an aerosol stream of 0.45-µm-diameter noncrystalline mimivirus particles across the beam. Thanks to the beam intensity—nearly 10¹³ photons per pulse—the collaboration was able to transform the diffraction pattern of a single virus particle into a real-space projection of its interior, though at the more modest resolution of 32 nm. The studies represent a step toward making molecular movies at atom-by-atom detail using harder and shorter pulses. (H. N. Chapman et al., Nature 470, 73, 2011 ; M. M. Seibert et al., Nature 470, 78, 2011 .)—R. Mark Wilson