Orientation Reconstruction of Proteins Using Coulomb Explosions

We solve the orientation recovery of a tumbling protein in the gas phase from single-event measurements of the spatial positions of its ions after an x-ray laser-induced explosion. We simulate diffracted x-ray signal and ion dynamics under experimental conditions and compare our method to conventional orientation recovery in single-particle imaging with x-ray free-electron lasers using only diffraction data. We reconstruct 3D diffraction intensities using orientations recovered from the ion signatures and retrieve the electron density with established phase-retrieval algorithms. We test our orientation recovery procedure on 56 proteins ranging from 14 to 52 kDa (1800 to 6500 atoms), achieving an angular error of around 5°. The resulting 3D electron-density reconstructions are compared to ground-truth volumes simulated at the same nominal resolution and achieve the resolution at the edge of the detector in conditions similar to current single-particle imaging setups. We investigate the reconstruction quality and demonstrate that ion data can be used for reliable orientation recovery of particles in single-particle imaging, achieving orientation on par with or better than currently used recovery techniques. This Letter shows the potential of ion detection for retrieving additional information from the sample fragmentation and boosting single-particle imaging with x-ray lasers in the cases where the diffraction signal is a limiting factor.