Abstract
The proliferation of extremely intense synchrotron sources has enabled ever higher-resolution structures to be obtained using data collected from smaller and often more imperfect biological crystals (Helliwell, 1984). Synchrotron beamlines now exist that are capable of measuring data from single crystals that are just a few micrometres in size. This provides renewed motivation to study and understand the radiation damage behaviour of small protein crystals. Reciprocal-space mapping and Bragg coherent diffractive imaging experiments have been performed on cryo-cooled microcrystals of hen egg-white lysozyme as they undergo radiation damage. Several well established metrics, such as intensity-loss and lattice expansion, are applied to the diffraction data and the results are compared with several new metrics that can be extracted from the coherent imaging experiments. Individually some of these metrics are inconclusive. However, combining metrics, the results suggest that radiation damage behaviour in protein micro-crystals differs from that of larger protein crystals and may allow them to continue to diffract for longer. A possible mechanism to account for these observations is proposed.
Highlights
Over the past 50 years macromolecular crystallography has been the primary method for establishing the structure of proteins
The results presented here summarize a series of experiments investigating radiation damage in micrometre-sized crystals illuminated with micrometre-sized beams
In these studies the aim has been to shed some light on the key scientific question of whether the radiation damage behaviour observed under these conditions matches the behaviour seen in macroscopic crystals
Summary
Over the past 50 years macromolecular crystallography has been the primary method for establishing the structure of proteins. Progress with optimizing and improving protein crystallography beamlines has been such that the collection of partial datasets from single crystals that are just a few micrometres in size is possible. Theoretical predictions based on radiation damage behaviour suggest that a complete dataset could be collected from a protein crystal as small as 1.2 mm (Holton & Frankel, 2010). Complete datasets from samples this small can be collected provided many different crystals are measured in the beam, the quality of individual crystals is sufficient to observe diffraction, and the crystal lattices are sufficiently homogeneous that partial datasets can be merged. Following successful proof-of-principle experiments, serial synchrotron X-ray crystallography (SSX) is rapidly becoming established as a viable route to protein structure determination (Stellato et al, 2014; Gati et al, 2014)
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