However, the small scattering cross-section of biological samples remains a big obstacle to extending the resolution of electron density maps with the currently available photon flux density of XFELs.Īnother serious problem in CXDI is the quality and incompleteness of experimental diffraction data. Thus far, XFEL-CXDI has visualized a large virus 9 and a macromolecular assembly 10, an organelle 11 and a bacterium 12 at resolutions of 30–60 nm. X-ray free-electron laser (XFEL) sources launched recently 6, 7 have the potential to solve this contrary problem, since the femto-second pulse duration and the high photon flux density of XFELs allow diffraction data collection before sample destruction 8. Thereby, we can obtain a projection map of sample objects within a given spatial resolution, where the curvature of the Ewald sphere can be regarded as a flat plane perpendicular to the incident X-ray beam (projection approximation 2).īiological samples are extremely sensitive to radiation even at cryogenic temperatures 5, yet need to be imaged with significant doses of X-rays due to their small scattering cross-section. When the diffraction pattern is sampled at a spacing finer than the Nyquist interval on the detector (oversampling OS) 3, iterative phase retrieval (PR) algorithms 4 can recover phase information of the object directly from the diffraction pattern. In CXDI experiments, spatially coherent X-rays irradiate a sample object and the Fraunhofer diffraction pattern of the object on the Ewald sphere 2 is recorded on an area detector ( Fig. Thus, CXDI fills a gap among other techniques, since it could resolve finer structures of samples that are too thick for electron microscopy beyond the resolution limit of optical microscopy. The high penetrating power of X-rays allows visualization of internal structures of thick objects in micrometer to sub-micrometer dimensions at nanometer resolution. An understanding of their physicochemical function requires visualization of internal structures of whole cells and/or organelles as close to the native state as possible.Ĭoherent X-ray diffraction imaging (CXDI) 1 is a promising technique to study such non-crystalline objects. A set of calculations based on current experiments demonstrates that resolution is improved by a factor of two or more.īiological cells comprise spatially hierarchical and highly functionalized components from organelles measured in micrometers to macromolecules of nanometer sizes. The positions of the gold particles determined by Patterson analysis serve as the initial phase and this dramatically improves reliability and convergence of image reconstruction by iterative phase retrieval. The weak diffraction signals from biological objects are enhanced by interference with strong waves from dispersed colloidal gold particles. Here, we propose a method to extend spatial resolution by enhancing diffraction signals and by robust phasing. The challenge is to recover correct phase information from experimental diffraction patterns that have a low signal-to-noise ratio and unmeasurable lowest-resolution data. However, the spatial resolution is limited to several tens of nanometers due to the poor scattering power of biological samples. ![]() In this decade coherent X-ray diffraction imaging has been demonstrated to reveal internal structures of whole biological cells and organelles.
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