Effect of impurities and post-experimental purification in SAD phasing with serial femtosecond crystallography data

In serial crystallography (SX) with either an X-ray free-electron laser (XFEL) or synchrotron radiation as the light source, huge numbers of micrometre-sized crystals are used in diffraction data collection. For a SAD experiment using a derivative with introduced heavy atoms, it is difficult to completely exclude crystals of the native protein from the sample. In this paper, simulations were performed to study how the inclusion of native crystals in the derivative sample could affect the result of SAD phasing and how the post-experimental purification proposed by Zhang et al. [(2015), Acta Cryst. D71, 2513-2518] could be used to remove the impurities. A gadolinium derivative of lysozyme and the corresponding native protein were used in the test. Serial femtosecond crystallography (SFX) diffraction snapshots were generated by CrystFEL. SHELXC/D, Phaser, DM, ARP/wARP and REFMAC were used for automatic structure solution. It is shown that a small amount of impurities (snapshots from native crystals) in the set of derivative snapshots can strongly affect the SAD phasing results. On the other hand, post-experimental purification can efficiently remove the impurities, leading to results similar to those from a pure sample.