Achieving a Good Crystal System for Crystallographic X-ray Fragment Screening

The XChem facility at Diamond Light Source offers fragment screening by X-ray crystallography as a general access user program.  The main advantage of X-ray crystallography as a primary fragment screen is that it yields directly the location and pose of the fragment hits, whether within pockets of interest or merely on surface sites:  this is the key information for structure-based design and for enabling synthesis of follow-up molecules. Extensive streamlining of the screening experiment at XChem has engendered a very active user programme that is generating large amounts of data:  in 2017, 36 academic and industry groups generated 35,000 datasets of uniquely soaked crystals.  It has also generated a large number of learnings concerning the main remaining bottleneck, namely obtaining a suitable crystal system that will support a successful fragment screen.  Here we discuss the practicalities of generating screen-ready crystals that have useful electron density maps, and how to ensure they will be successfully reproduced and usable at a facility outside the home lab.