Macromolecular Crystallography

The field of macromolecular crystallography (MX) has greatly benefitted from a number of methodological developments such as the adoption of cryo-crystallography and the increased brightness of X-ray sources. Simultaneously, advances in detector technology from film via image plates and CCD detectors to state-of-the-art single-photon-counting detectors have pushed the boundaries in MX research.

DECTRIS has pioneered the application of Hybrid Photon Counting (HPC) detectors in crystallography. Nowadays, HPC detectors are installed at MX beamlines around the world, and the high quality of data collected with them has led to thousands of structures deposited in the PDB in a very short time (Fig. 1). DECTRIS HPC detectors are the instrument of choice for synchrotron beamlines eager to provide the best data quality and collection efficiency. Furthermore, with DECTRIS HPC detectors specifically developed for laboratory applications, crystallographers can now obtain superb data at short acquisition times in their home laboratory.

 

Figure 1. Annual number of released PDB structures based on data collected with DECTRIS HPC detectors.


Figure 2. Absence of readout noise and dark current in PILATUS HPC detectors.
Dark images collected on a single PILATUS module without exposure to an X-ray source. a) After an acquisition time of 100 ms, all pixels have zero counts since no noise is added onto the readout of the image. b) After an acquisition time of 1 hour, most pixels still have zero counts since no dark current accumulates during long exposures and no noise is added during readout. High-energy cosmic radiation contributes only a single count per detected event, which is reflected in the low average count rate of 0.23 cts/h/pixel for 450 ┬Ám sensors.


The success of DECTRIS HPC detectors in MX is based on their unique characteristics:

  • No readout noise and dark current (Fig. 2)
  • Sharp point spread function of one pixel (Fig. 3)
  • High dynamic range of 20 bit for PILATUS (Fig. 3) and up to 32 bit for EIGER
  • Short readout times in the range of few milliseconds
  • High frame rates up to 250 Hz (PILATUS3 X 2M) and 750 Hz (EIGER X 4M)

In MX, scientists can benefit from the advantages of DECTRIS HPC detectors over previous detector technologies in numerous ways: The sharp point spread function ensures excellent resolution of closely spaced reflections over the entire dynamic range of the detector, minimizes overlap of diffraction intensities with scattering background and maximizes the signal-to-noise ratio. The high dynamic range virtually abolishes overloaded low-resolution reflections. The short readout time and high frame rates enable high-throughput data collection and optimize beamline efficiency. Furthermore, it allows collection of diffraction data with continuous rotation and eliminates the shutter as a source of error. Dark current and readout noise are completely absent, leading to better signal-to-noise ratio particularly for weak high-resolution reflections.

The unique characteristics of DECTRIS HPC detectors open the door to novel methods and advanced data collection strategies.

  • Fast readout and high frame rates allow rapid grid scanning and sample characterization [1].
  • Continuous-rotation data collection and the absence of readout noise permit to take full advantage of fine phi-slicing strategies. Compared to wide sliced data collection, scaling statistics improve substantially when using fine phi-slicing with rotation widths per image of only a fraction of the crystal's mosaicity [2].
  • The lack of noise makes it possible to collect highly redundant datasets in multiple low-intesity sweeps for experimental phasing by native SAD [3].


Figure 3. Superior dynamic range and point-spread function of PILATUS HPC detectors.

Details of diffraction images showing the same reflection of an insulin crystal. The images were acquired at a synchrotron beamline with identical parameters except for the detector distance which, according to the detector size, was adjusted to achieve the same resolution at the detector edge. (a) The 20-bit counter depth of the PILATUS HPC detector provides sufficient dynamic range to record 727,716 counts in the highest intensity pixel. Thanks to the excellent point-spread function, the spot is well confined to a small area. Furthermore, the sharp reflection profile of the low mosaicity crystal is accurately represented with a more than one-thousand-fold difference in intensity between neighboring pixels. (b) The same reflection recorded with a CCD contains many overloaded pixels. The reflection intensity is smeared out over a large area.

Literature

[1]   Aishima, J. et al. (2010) Acta Cryst. D66, 1032-5. doi:10.1107/S0907444910028192

[2]   Mueller, M. et al. (2011) Acta Cryst. D68, 42-56. doi:10.1107/S0907444911049833

[3]   Weinert, T. et al. (2015) Nat. Methods 12, 131-3. doi:10.1038/nmeth.3211