Abstract
In recent years, the success of serial femtosecond crystallography and the paucity of beamtime at X-ray free-electron lasers have motivated the development of serial microcrystallography experiments at storage-ring synchrotron sources. However, especially at storage-ring sources, if a crystal is too small it will have suffered significant radiation damage before diffracting a sufficient number of X-rays into Bragg peaks for peak-indexing software to determine the crystal orientation. As a consequence, the data frames of small crystals often cannot be indexed and are discarded. Introduced here is a method based on the expand-maximize-compress (EMC) algorithm to solve protein structures, specifically from data frames for which indexing methods fail because too few X-rays are diffracted into Bragg peaks. The method is demonstrated on a real serial microcrystallography data set whose signals are too weak to be indexed by conventional methods. In spite of the daunting background scatter from the sample-delivery medium, it was still possible to solve the protein structure at 2.1 Å resolution. The ability of the EMC algorithm to analyze weak data frames will help to reduce sample consumption. It will also allow serial microcrystallography to be performed with crystals that are otherwise too small to be feasibly analyzed at storage-ring sources.
Highlights
X-ray free-electron lasers (XFELs) have catalyzed several novel methods in biostructural science
Data frames with more than 20 candidate peaks were discarded to show that the EMC algorithm is able to reconstruct the three-dimensional crystal intensity from the sparse data frames, where normal indexing methods, including the one used by Martin-Garcia et al (2017), would fail
From the estimated X-ray beam size, the diameter of the lipidic cubic phase (LCP) gel column (50 mm) and the reconstructed crystal volumes (Fig. 4b), we can estimate the total number of photons scattered by LCP to be tens to thousands of times more than that scattered by the crystal in each data frame
Summary
X-ray free-electron lasers (XFELs) have catalyzed several novel methods in biostructural science. Despite the construction of XFELs worldwide, available beamtime in the near future will still be scarce compared with that provided by existing storage-ring synchrotron sources. This has inspired the development of serial microcrystallography experiments at current storage-ring sources (Gati et al, 2014; Stellato et al, 2014; Heymann et al, 2014; Gruner & Lattman, 2015; Botha et al, 2015; Nogly et al, 2015; Roedig et al, 2016; Martin-Garcia et al, 2017).
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