The structures of biomacromolecules are conventionally characterized by crystallography and cryogenic electron microscopy . The requirements of sample preparation limit the understanding of the specimens in their native states. Small-angle x-ray scattering (SAXS) has the capability of obtaining structural information from biological specimens in solution. However, resolving the structure from the acquired one-dimensional (1D) diffraction data requires the prior knowledge of the sample, and no unique solution can be guaranteed. Coherent diffraction imaging (CDI) provides excellent uniqueness in 2D/3D phase retrieval while the resolution is restricted by the poor signal-to-noise ratio at high-angle scattering. Here we combine CDI and SAXS to directly image a 19-nm-sized nodavirus particle in solution and determine the core-shell density distribution at a 1.3 nm pixel resolution. With 77,170 diffraction patterns summarized from randomly distributed nodavirus particles, the structural information can be obtained from the diffraction intensity alone without preknowledge. The hollow density distribution of a nodavirus particle revealed by our reconstruction is consistent with the structural determinations from crystallography and cryogenic electron microscopy. We believe this work represents a new protocol for characterizing the structures of macromolecules in solution from accumulated x-ray scattering data.
Coherent diffraction microscopy (CDM) is a potential approach to image micromaterials at atomic resolution without crystals. Due to the lack of high-angle scattering, the achieved resolution is limited to several nanometers. Small-angle scattering allows researchers to reveal high-resolution 3D structures of specimens by fitting 1D diffraction signals. However, prerequisite 3D models and non-unique solutions restrict the potential to image general specimens. Under the assumption of an ensemble containing large amounts of identical specimens with the same orientation, the intensity distribution of the diffraction pattern of the whole ensemble is approximated to the form factor of a single specimen multiplied by the number of identical specimens. Since the diffraction intensities are contributed from the whole ensemble, the signal can be significantly extended to high-frequency regions. The feasibility of ensemble diffraction microscopy (EDM) was demonstrated by a designed sample using both totally and partially coherent X-ray sources at Taiwan Photon Source (TPS). The reconstructed images show excellent consistency with the image of a scanning electron microscope. This work represents a new protocol for directly characterizing the structures of nanomaterials, or potentially biomacromolecules, from accumulated X-ray scattering data.
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