EM-detwin: A Program for Resolving Indexing Ambiguity in Serial Crystallography Using the Expectation-Maximization Algorithm

Abstract

Serial crystallography (SX), first used as an application of X-ray free-electron lasers (XFELs), is becoming a useful method to determine atomic-resolution structures of proteins from micrometer-sized crystals with bright X-ray sources. Because of unknown orientations of crystals in SX, indexing ambiguity issue arises when the symmetry of Bravais lattice is higher than the space group symmetry, making some diffraction signals wrongly merged to the total intensity in twinned orientations. In this research, we developed a program within the CrystFEL framework, the EM-detwin, to resolve this indexing ambiguity problem based on the expectation-maximization algorithm. Testing results on the performance of the EM-detwin have demonstrated its usefulness in correctly indexing diffraction data as a valuable tool for SX data analysis.