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Abstract
Recently, there has been a growing interest in adapting serial microcrystallography (SMX) experiments to existing storage ring (SR) sources. For very small crystals, however, radiation damage occurs before sufficient numbers of photons are diffracted to determine the orientation of the crystal. The challenge is to merge data from a large number of such 'sparse' frames in order to measure the full reciprocal space intensity. To simulate sparse frames, a dataset was collected from a large lysozyme crystal illuminated by a dim X-ray source. The crystal was continuously rotated about two orthogonal axes to sample a subset of the rotation space. With the EMC algorithm [expand-maximize-compress; Loh & Elser (2009). Phys. Rev. E, 80, 026705], it is shown that the diffracted intensity of the crystal can still be reconstructed even without knowledge of the orientation of the crystal in any sparse frame. Moreover, parallel computation implementations were designed to considerably improve the time and memory scaling of the algorithm. The results show that EMC-based SMX experiments should be feasible at SR sources.
Highlights
The advance of serial femtosecond microcrystallography (SFX) at X-ray free-electron lasers (XFELs) (Chapman et al, 2011; Boutet et al, 2012) allows structure determination with protein crystals whose sizes are too small for conventional crystallography experiments
Our calculation showed that an 8 mm3 crystal would have endured a 0.2 MGy radiation dose if it had scattered the same number of photons as our large hen egg white lysozyme (HEWL) crystal during this period
This dose is within the lifetime of protein crystals at room temperature if the radiation is delivered quickly (Owen et al, 2012), so the signal level in our study should be comparable to that in a real SMX experiment
Summary
The advance of serial femtosecond microcrystallography (SFX) at X-ray free-electron lasers (XFELs) (Chapman et al, 2011; Boutet et al, 2012) allows structure determination with protein crystals whose sizes are too small for conventional crystallography experiments. The tens of femtoseconds long pulse width enables the photon scattering process to outrun the radiation damage of the crystals, while the ultra-high brightness of the pulses results in a sufficient number of resolvable Bragg peaks collected by a fast-framing detector (Philipp et al, 2008) for indexing. Using this concept of ‘diffract before destroy’ (Neutze et al, 2000), a complete dataset can be obtained given enough indexed data frames.
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