Estimation of attainable structural resolution by computer simulation for single biomolecule imaging with X-ray Free Electron Laser

Tokuhisa, Atsushi*; Kai, Takeshi   ; Taka, Junichiro*; Kono, Hidetoshi; Go, Nobuhiro*

XFEL can potentially offer a new mean for single biomolecule 3D-imaging. Measured intensity, which is a 2D diffraction pattern per one observation without phase information is extremely weak even using XFEL because sample is a single molecule in a single shot and incident intensity of XFEL cannot make stronger than a certain intensity due to suppress molecule decay. Under such experimental conditions, we consequently have to use extremely weak signals effectively to reconstruct a 3D molecule structure using multiply measured many 2D diffraction patterns. We have developed a two-step algorithm for reconstructing a 3D diffraction intensity data from many experimentally measured, quantum-noise limited 2D patterns of single molecule with unknown orientation. Once a 3D diffraction intensity data of a molecule obtained, the 3D structure can be obtained by applying oversampling method to the data. The developed algorithm enables us to detect as low as 0.1 photons per an effective pixel as signal for the classification in such a noisy experimental photon-count data. Theoretically, wave-number of a pixel giving such a limiting photon-count defines the attainable structural resolution.