: a reference-based auto-indexing algorithm for sparse serial crystallography data.

(sparse-pattern indexing) is an auto-indexing algorithm for sparse snapshot diffraction patterns ('stills') that requires the positions of only five Bragg peaks in a single pattern, when provided with unit-cell parameters. The capability of is demonstrated for the orientation determination of sparse diffraction patterns using simulated data from microcrystals of a small inorganic molecule containing three iodines, 5-amino-2,4,6-triiodoisophthalic acid monohydrate (I3C) [Beck & Sheldrick (2008 ▸), E, o1286], which is challenging for commonly used indexing algorithms. , integrated with [White (2012 ▸), , 335-341], is then shown to improve the indexing rate and quality of merged serial femtosecond crystallography data from two membrane proteins, the human δ-opioid receptor in complex with a bi-functional peptide ligand DIPP-NH and the NTQ chloride-pumping rhodopsin (CIR). The study demonstrates the suitability of for indexing sparse inorganic crystal data with smaller unit cells, and for improving the quality of serial femtosecond protein crystallography data, significantly reducing the amount of sample and beam time required by making better use of limited data sets. is written in Python and is publicly available under the GNU General Public License from https://github.com/LiuLab-CSRC/SPIND.