Macromolecular Serial Crystallography (Volume II)

The successful adaptation of the serial macromolecular crystallography approach at most 3rd generation synchrotron facilities allows a fruitful synergy between synchrotrons and XFELs that have accelerated the access and impact of this approach to an even larger community. The main goal of this Special Issue on “Macromolecular Serial Crystallography (Volume II)”, along with the previous Special Issue (Macromolecular Serial Crystallography), has been to gather research manuscripts from experts in the field to create an international platform to provide rich research and reference information on the latest advances and exciting discoveries in the technology of serial crystallography at XFELs and synchrotron radiation sources. This Special Issue contains four interesting manuscripts—three research manuscripts [1,2,3] and a review article [4].

The research article by Shelby and co-workers [1] reports two novel nanolipoprotein particles (NLPs) that can be used as scaffolds in future structural studies of membrane proteins. NLPs are disc-shaped particles composed of a patch of lipid bilayer surrounded by a belt of apolipoproteins, which are typically used in the structural characterization of membrane proteins by inserting them within the lipid bilayer. In this manuscript, the authors employ a novel approach to produce two NLPs by combining several techniques, including cell-free expression of the apolipoproteins A1 (ApoA1) and E4 (ApoE4), high-throughput crystallization, and the characterization of the particles using serial femtosecond crystallography (SFX) at an XFEL. This novel approach takes advantage of: (1) the cell-free expression in a detergent-free process, which makes it ideal for structural studies of detergent-sensitive membrane protein–nanodisc complexes; (2) SFX at XFELs with low sample consumption, low background, minimal crystal size requirements, and a lack of radiation damage effects on measurements make fixed-target SFX an ideal approach for the characterization of the diffuse NLP diffraction, while room temperature measurement enables future dynamic studies.

The research article by Shi and Liu [2] presents a new program called EM-detwin to overcome the common indexing ambiguity problem that is present in many serial crystallography data sets. Due to the nature of the serial crystallography experiments, the unknown orientation of crystals makes that more than one indexing solutions are equally good for a single diffraction pattern to satisfy peak position constraints. This mostly happens when the Bravais symmetry of a crystal lattice is higher than the space group symmetry, so a diffraction pattern from a crystal can be indexed either in the correct orientation or in its twinned orientation(s). Even if no physical twinning occurs to the crystals in serial crystallography experiments, merging the data may result in a twinned dataset for those crystals with indexing ambiguities. The newly developed detwinning program has been implemented within the CrystFEL pipeline.

The research article by Kabayiza et al. [3] introduces a modest educational game, XFEL Crystal Blaster (https://www.bioxfel.org/knowledge-transfer/234-xfel-educational-game, accessed on 22 May 2022), aimed at increasing the exposure of middle and high school students to advanced science concepts associated with XFEL science and techniques, particularly SFX and crystallography. The XFEL Crystal Blaster is available on multiple platforms, intuitive for gamers, and requires no prior knowledge of the game’s content. It employs retro-style graphics that demonstrate the SFX data collection process, the rationale for such experiments, and a brief explanation of real-life experimental results and their significance. The simple gameplay mechanics are appropriate for younger audiences and encourage gamers to play often to improve their scores. Overall, the assessment suggests that students who engage with the XFEL Crystal Blaster game are likely to develop some introductory knowledge of XFELs and X-ray crystallography and increase their interest in learning more about X-ray crystallography.

The review article by Golub et al. [4] discusses the principles and prototypical applications of small-angle neutron and X-ray scattering (SANS and SAXS, respectively) applied to the photosynthetic pigment–protein complexes phycocyanin (PC) and Photosystem I (PSI) as model systems for a water-soluble protein and a membrane protein, respectively.

To conclude, I would like to thank all authors who have contributed with their excellent manuscripts to this Special Issue, the reviewers who provided constructive and helpful feedback on all submissions, and the Editorial Office staff at Crystals for their fast and professional handling of all manuscripts during the submission process and the help provided throughout.