Structure determination based on continuous diffraction from macromolecular crystals

Crystal structure via fluctuation scattering

Crystallography is a quintessential method for determining the atomic structure of crystals. The most common implementation of crystallography uses single crystals that must be of sufficient size, typically tens of micrometres or larger, depending on the complexity of the crystal structure. The emergence of serial data-collection methods in crystallography, particularly for time-resolved experiments, opens up opportunities to develop new routes to structure determination for nanocrystals and ensembles of crystals. Fluctuation X-ray scattering is a correlation-based approach for single-particle imaging from ensembles of identical particles, but has yet to be applied to crystal structure determination. Here, an iterative algorithm is presented that recovers crystal structure-factor intensities from fluctuation X-ray scattering correlations. The capabilities of this algorithm are demonstrated by recovering the structure of three small-molecule crystals and a protein crystal from simulated fluctuation X-ray scattering correlations. This method could facilitate the recovery of structure-factor intensities from crystals in serial crystallography experiments and relax sample requirements for crystallography experiments.

An electron counting algorithm improves imaging of proteins with low-acceleration-voltage cryo-electron microscope

Relative to the 300-kV accelerating field, electrons accelerated under lower voltages are potentially scattered more strongly. Lowering the accelerate voltage has been suggested to enhance the signal-to-noise ratio (SNR) of cryo-electron microscopy (cryo-EM) images of small-molecular-weight proteins (<100 kD). However, the detection efficient of current Direct Detection Devices (DDDs) and temporal coherence of cryo-EM decrease at lower voltage, leading to loss of SNR. Here, we present an electron counting algorithm to improve the detection of low-energy electrons. The counting algorithm increased the SNR of 120-kV and 200-kV cryo-EM image from a Falcon III camera by 8%, 20% at half the Nyquist frequency and 21%, 80% at Nyquist frequency, respectively, resulting in a considerable improvement in resolution of 3D reconstructions. Our results indicate that with further improved temporal coherence and a dedicated designed camera, a 120-kV cryo-electron microscope has potential to match the 300-kV microscope at imaging small proteins.

Implementation of wedged-serial protein crystallography at PROXIMA-1

An approach for serial crystallography experiments based on wedged-data collection is described. This is an alternative method for recording in situ X-ray diffraction data on crystalline samples efficiently loaded in an X-ray compatible microfluidic chip. Proper handling of the microfluidic chip places crystalline samples at geometrically known positions with respect to the focused X-ray interaction area for serial data collection of small wedges. The integration of this strategy takes advantage of the greatly modular sample environment available on the endstation, which allows access to both in situ and more classical cryo-crystallography with minimum time loss. The method represents another optional data collection approach that adds up to the already large set of methods made available to users. Coupled with the advances in processing serial crystallography data, the wedged-data collection strategy proves highly efficient in minimizing the amount of required sample crystals for recording a complete dataset. From the advances in microfluidic technology presented here, high-throughput room-temperature crystallography experiments may become routine and should be easily extended to industrial use.