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Single-particle imaging (SPI) at x-ray free-electron lasers is a technique to determine the three-dimensional structure of nanoscale objects like biomolecules from a large number of diffraction patterns of copies of these objects in random orientations. The technique has been limited to relatively low resolution due to background noise and heterogeneity of the target particles. A recently introduced reference-enhanced holographic SPI methodology uses strongly scattering holographic references to improve background tolerance, and, thus, the achievable resolution, at the cost of additional latent variables beyond orientation. Here, we describe an improved reconstruction algorithm based on maximum likelihood estimation, which scales better, enabling fine sampling of latent parameters to reach high resolutions, and much better performance in the low signal limit. Furthermore, we show that structural variations within the target particle are averaged in real space, significantly improving robustness to conformational heterogeneity in comparison to conventional SPI. With these computational improvements, we believe reference-enhanced SPI is capable of reaching sub-nanometer resolution biomolecule imaging.

We demonstrate that x-ray fluorescence emission, which cannot maintain a stationary interference pattern, can be used to obtain images of structures by recording photon-photon correlations in the manner of the stellar intensity interferometry of Hanbury Brown and Twiss. This is achieved utilizing femtosecond-duration pulses of a hard x-ray free-electron laser to generate the emission in exposures comparable to the coherence time of the fluorescence. Iterative phasing of the photon correlation map generated a model-free real-space image of the structure of the emitters. Since fluorescence can dominate coherent scattering, this may enable imaging uncrystallised macromolecules.

Scientific data 3, 160064 -(2016). doi:10.1038/sdata.2016.64, Single particle diffractive imaging data from Rice Dwarf Virus (RDV) were recorded using the Coherent X-ray Imaging (CXI) instrument at the Linac Coherent Light Source (LCLS). RDV was chosen as it is a well-characterized model system, useful for proof-of-principle experiments, system optimization and algorithm development. RDV, an icosahedral virus of about 70 nm in diameter, was aerosolized and injected into the approximately 0.1 μm diameter focused hard X-ray beam at the CXI instrument of LCLS. Diffraction patterns from RDV with signal to 5.9 Ångström were recorded. The diffraction data are available through the Coherent X-ray Imaging Data Bank (CXIDB) as a resource for algorithm development, the contents of which are described here., Published by Nature Publ. Group, London

Nature Communications 11(1), 4511 (2020). doi:10.1038/s41467-020-18156-7, Serial femtosecond crystallography (SFX) with X-ray free electron lasers (XFELs) allows structure determination of membrane proteins and time-resolved crystallography. Common liquid sample delivery continuously jets the protein crystal suspension into the path of the XFEL, wasting a vast amount of sample due to the pulsed nature of all current XFEL sources. The European XFEL (EuXFEL) delivers femtosecond (fs) X-ray pulses in trains spaced 100 ms apart whereas pulses within trains are currently separated by 889 ns. Therefore, continuous sample delivery via fast jets wastes >99% of sample. Here, we introduce a microfluidic device delivering crystal laden droplets segmented with an immiscible oil reducing sample waste and demonstrate droplet injection at the EuXFEL compatible with high pressure liquid delivery of an SFX experiment. While achieving ~60% reduction in sample waste, we determine the structure of the enzyme 3-deoxy-D-manno-octulosonate-8-phosphate synthase from microcrystals delivered in droplets revealing distinct structural features not previously reported., Published by Nature Publishing Group UK, [London]

The idea of using ultrashort X-ray pulses to obtain images of single proteins frozen in time has fascinated and inspired many. It was one of the arguments for building X-ray free-electron lasers. According to theory1, the extremely intense pulses provide sufficient signal to dispense with using crystals as an amplifier, and the ultrashort pulse duration permits capturing the diffraction data before the sample inevitably explodes2. This was first demonstrated on biological samples a decade ago on the giant mimivirus3. Since then a large collaboration4has been pushing the limit of the smallest sample that can be imaged5,6. The ability to capture snapshots on the timescale of atomic vibrations, while keeping the sample at room temperature, may allow probing the entire conformational phase space of macromolecules. Here we show the first observation of an X-ray diffraction pattern from a single protein, that ofEscherichia coliGroEL which at 14 nm in diameter7is the smallest biological sample ever imaged by X-rays, and demonstrate that the concept of diffraction before destruction extends to single proteins. From the pattern, it is possible to determine the approximate orientation of the protein. Our experiment demonstrates the feasibility of ultrafast imaging of single proteins, opening the way to single-molecule time-resolved studies on the femtosecond timescale.

With the development of X-ray free-electron lasers (XFELs), producing pulses of femtosecond durations comparable with the coherence times of X-ray fluorescence, it has become possible to observe intensity-intensity correlations due to the interference of emission from independent atoms. This has been used to compare durations of X-ray pulses and to measure the size of a focusedX-ray beam, for example. Here it is shown that it is also possible to observe the interference of fluorescence photons through the measurement of the speckle contrast of angle-resolved fluorescence patterns. Speckle contrast is often used as a measure of the degree of coherence of the incident beam or the fluctuations of the illuminated sample as determined from X-ray diffraction patterns formed by elastic scattering, rather than from fluorescence patterns as addressed here. Commonly used approaches to estimate speckle contrast were found to suffer when applied to XFEL-generated fluorescence patterns due to low photon counts and a significant variation of the excitation pulse energy from shot to shot. A new method to reliably estimate speckle contrast under such conditions, using a weighting scheme, is introduced. The method is demonstrated by comparing the speckle contrast of fluorescence observed with pulses of 3 fs to 15 fs duration.

We report on the use of multilayer Laue lenses to focus the intense X-ray Free Electron Laser (XFEL) beam at the European XFEL to a spot size of a few tens of nanometers. We present the procedure to align and characterize these lenses and discuss challenges working with the pulse trains from this unique x-ray source.

All macromolecular crystals contain some extent of disorder. The diffraction from such crystals contains diffuse scattering in addition to Bragg peaks and this scattering contains information about correlated displacements in the constituent molecules. While much work has been performed recently in decoding the dynamics of the crystalline ordering, the goal of understanding the internal dynamics of the molecules within a unit cell has been out-of-reach. In this article, we propose a general framework to extract the internal conformational modes of a macromolecule from diffuse scattering data. We combine insights on the distribution of diffuse scattering from short- and long-range disorder with a Bayesian global optimization algorithm to obtain the best fitting internal motion modes to the data. To illustrate the efficacy of the method, we apply it to a publicly available dataset from triclinic lysozyme. Our mostly parameter-free approach can enable the recovery of a much richer, dynamic structure from macromolecular crystallography.

We propose an encryption–decryption framework for validating diffraction intensity volumes reconstructed using single-particle imaging (SPI) with X-ray free-electron lasers (XFELs) when the ground truth volume is absent. This conceptual framework exploits each reconstructed volumes’ ability to decipher latent variables (e.g. orientations) of unseen sentinel diffraction patterns. Using this framework, we quantify novel measures of orientation disconcurrence, inconsistency, and disagreement between the decryptions by two independently reconstructed volumes. We also study how these measures can be used to define data sufficiency and its relation to spatial resolution, and the practical consequences of focusing XFEL pulses to smaller foci. This conceptual framework overcomes critical ambiguities in using Fourier Shell Correlation (FSC) as a validation measure for SPI. Finally, we show how this encryption-decryption framework naturally leads to an information-theoretic reformulation of the resolving power of XFEL-SPI, which we hope will lead to principled frameworks for experiment and instrument design.

Optica 8(1), 15 - 23 (2021). doi:10.1364/OPTICA.410851, Single particle imaging at x-ray free electron lasers (XFELs) has the potential to determine the structure and dynamics of single biomolecules at room temperature. Two major hurdles have prevented this potential from being reached, namely, the collection of sufficient high-quality diffraction patterns and robust computational purification to overcome structural heterogeneity. We report the breaking of both of these barriers using gold nanoparticle test samples, recording around 10 million diffraction patterns at the European XFEL and structurally and orientationally sorting the patterns to obtain better than 3-nm-resolution 3D reconstructions for each of four samples. With these new developments, integrating advancements in x-ray sources, fast-framing detectors, efficient sample delivery, and data analysis algorithms, we illuminate the path towards sub-nanometer biomolecular imaging. The methods developed here can also be extended to characterize ensembles that are inherently diverse to obtain their full structural landscape., Published by OSA, Washington, DC

Communications Physics 3(1), 97 (2020). doi:10.1038/s42005-020-0362-y, The emergence of high repetition-rate X-ray free-electron lasers (XFELs) powered by superconducting accelerator technology enables the measurement of significantly more experimental data per day than was previously possible. The European XFEL is expected to provide 27,000 pulses per second, over two orders of magnitude more than any other XFEL. The increased pulse rate is a key enabling factor for single-particle X-ray diffractive imaging, which relies on averaging the weak diffraction signal from single biological particles. Taking full advantage of this new capability requires that all experimental steps, from sample preparation and delivery to the acquisition of diffraction patterns, are compatible with the increased pulse repetition rate. Here, we show that single-particle imaging can be performed using X-ray pulses at megahertz repetition rates. The results obtained pave the way towards exploiting high repetition-rate X-ray free-electron lasers for single-particle imaging at their full repetition rate., Published by Springer Nature, London

Methods are described for processing XFEL data from individual crystalline fibrils. The methods are applied to data collected at the Linac Coherent Light Source from an amyloid-forming oligopeptide from the adenovirus shaft., Serial diffraction data collected at the Linac Coherent Light Source from crystalline amyloid fibrils delivered in a liquid jet show that the fibrils are well oriented in the jet. At low fibril concentrations, diffraction patterns are recorded from single fibrils; these patterns are weak and contain only a few reflections. Methods are developed for determining the orientation of patterns in reciprocal space and merging them in three dimensions. This allows the individual structure amplitudes to be calculated, thus overcoming the limitations of orientation and cylindrical averaging in conventional fibre diffraction analysis. The advantages of this technique should allow structural studies of fibrous systems in biology that are inaccessible using existing techniques.

X-ray single particle imaging involves the measurement of a large number of noisy diffraction patterns of isolated objects in random orientations. The missing information about these patterns is then computationally recovered in order to obtain a three-dimensional structure of the particle. While the method has promised to deliver room temperature structures at near-atomic resolution, there have been significant experimental hurdles in collecting data of sufficient quality and quantity to achieve this goal. This paper describes two ways to modify the conventional methodology which significantly ease the experimental challenges, at the cost of additional computational complexity in the reconstruction procedure. Both these methods involve the use of holographic reference objects in close proximity to the sample of interest, whose structure can be described with only a few parameters. A reconstruction algorithm to recover the unknown degrees of freedom is also proposed and tested with toy-model simulations., Comment: 9 pages, 5 figures

Supplementary Information

64th Annual Biophysical Society Meeting, San Diego, 15 Feb 2020 - 19 Feb 2020; Biophysical journal 118(3), 137a - 138a (2020). doi:10.1016/j.bpj.2019.11.877, Outrunning radiation damage, highly intense femtosecond pulses of X-ray free-electron lasers (XFELs) open up the possibility of structure determination of macromolecules to viruses at room temperature, by aggregating diffraction patterns recorded from uncrystallized single-particles. A key challenge in XFEL single-particle diffractive imaging (SPI) is to either constrain the orientation of the particle or to determine it from each of the very noisy weak diffraction patterns. Here we report a unique approach to address these challenges using structural DNA nanotechnology. A DNA-origami ���rigid tail��� is site-specifically attached to the macromolecule to flow-align it in a thin liquid jet, and also provides a strong holographic reference. In a proof-of-principle study, the computational design and production of the DNA-origami-target construct has been achieved and the experimental results obtained from the Linac Coherent Light Source (LCLS), USA show the alignment of the target single-particle, consistent with simulations, at extremely low concentrations approaching single-molecule in the interaction region with the nanofocus hard X-ray laser beam, in a sum of as few as a thousand single-shots. The results open up the possibility of single-molecule diffractive imaging in solution with XFEL pulses. Acknowledgements: The Human Frontier Science Program (RGP0010/2017)., Published by Soc., Bethesda, Md.

Optics express 27(26), 37816 - (2019). doi:10.1364/OE.27.037816, An outstanding question in X-ray single particle imaging experiments has been thefeasibility of imaging sub 10-nm-sized biomolecules under realistic experimental conditionswhere very few photons are expected to be measured in a single snapshot and instrumentbackground may be significant relative to particle scattering. While analyses of simulated datahave shown that the determination of an average image should be feasible using Bayesian methodssuch as the EMC algorithm, this has yet to be demonstrated using experimental data containingrealistic non-isotropic instrument background, sample variability and other experimental factors.In this work, we show that the orientation and phase retrieval steps work at photon counts dilutedto the signal levels one expects from smaller molecules or with weaker pulses, using data fromexperimental measurements of 60-nm PR772 viruses. Even when the signal is reduced to afraction as little as 1/256, the virus electron density determined using ab initio phasing is ofalmost the same quality as the high-signal data. However, we are still limited by the total numberof patterns collected, which may soon be mitigated by the advent of high repetition-rate sourceslike the European XFEL and LCLS-II., Published by Soc., Washington, DC

The application of a new generation of x-ray sources called X-ray Free Electron Lasers (XFELs) to diffractive imaging has allowed structural studies of specimens not previously accessible. Specimens of reduced crystallinity are of particular interest, including fibrous nano-crystals and single fibrous molecules. Diffractive imaging experiments using XFELs generate large datasets of diffraction frames from specimens with random, unknown orientations. The orientation of each diffraction frame needs to be determined from features in the pattern in order to register and merge the dataset for subsequent structural analysis. Certain sample delivery techniques simplify this process by limiting the range of orientations a specimen may take. In this paper we consider two sample delivery techniques: a liquid jet and a fixed target on a silicon wafer. Orientations determined from diffraction patterns from each delivery method are classified in order to investigate the type of orientation present. This information also helps to characterize the quality of sample preparations and provides feedback valuable for designing future experiments.

[Ebook] X-ray Free Electron Lasers : A Revolution in Structural Biology / Boutet, Sébastien ; Fromme, Petra ; Hunter, Mark S. , Cham : Springer International Publishing, 2018,; Cham : Springer Nature 253-300 (2018). doi:10.1007/978-3-030-00551-1_9, The coherent diffraction pattern of a non-periodic finite object does not consist of Bragg peaks but is continuously and smoothly varying. Such patterns do not suffer from the well-known phase problem of crystallography. In this case, robust iterative algorithms exist to determine the electron density of the object from the diffraction pattern alone. Continuous diffraction is accessible from ensembles of aligned molecules, including disordered protein crystals. We discuss the application of the concepts of coherent diffractive imaging to such cases and describe the experimental considerations to adequately measure the weak continuous diffraction signals., Published by Springer Nature, Cham

The application of high-powered X-ray Free Electron Lasers (XFELs) in Serial Femtosecond Crystallography (SFX) has led to an increasing number of crystal structure determinations. Achieving the same success for systems of diminished crystallinity, such as fibrous systems and single particles, is inhibited by low signal strength and difficulties in orientating the data. We present methods developed for analyzing serial diffraction data from fibrous systems. The data are processed to identify cases where a single XFEL pulse intersects a single fibril. The fibril orientations are then determined by analysis of detected features within the diffraction data. With the fibril orientation determined, serial diffraction data is merged in 3D reciprocal space, allowing the individual structure amplitudes to be calculated. This allows structural studies of previously inaccessible fibrous systems to be performed, and represents a step closer towards the long-term goal of imaging single particles.

Physical review letters 119(5), 053401 (2017). doi:10.1103/PhysRevLett.119.053401, Established x-ray diffraction methods allow for high-resolution structure determination of crystals, crystallized protein structures, or even single molecules. While these techniques rely on coherent scattering, incoherent processes like fluorescence emission—often the predominant scattering mechanism—are generally considered detrimental for imaging applications. Here, we show that intensity correlations of incoherently scattered x-ray radiation can be used to image the full 3D arrangement of the scattering atoms with significantly higher resolution compared to conventional coherent diffraction imaging and crystallography, including additional three-dimensional information in Fourier space for a single sample orientation. We present a number of properties of incoherent diffractive imaging that are conceptually superior to those of coherent methods., Published by APS, College Park, Md.

Scientific data 4, 170079 (2017). doi:10.1038/sdata.2017.79, Single-particle diffraction from X-ray Free Electron Lasers offers the potential for molecular structure determination without the need for crystallization. In an effort to further develop the technique, we present a dataset of coherent soft X-ray diffraction images of Coliphage PR772 virus, collected at the Atomic Molecular Optics (AMO) beamline with pnCCD detectors in the LAMP instrument at the Linac Coherent Light Source. The diameter of PR772 ranges from 65–70 nm, which is considerably smaller than the previously reported ~600 nm diameter Mimivirus. This reflects continued progress in XFEL-based single- particle imaging towards the single molecular imaging regime. The data set contains significantly more single particle hits than collected in previous experiments, enabling the development of improved statistical analysis, reconstruction algorithms, and quantitative metrics to determine resolution and self-consistency., Published by Macmillan, London

Single-particle imaging (SPI) with X-ray free-electron lasers has the potential to change fundamentally how biomacromolecules are imaged. The structure would be derived from millions of diffraction patterns, each from a different copy of the macromolecule before it is torn apart by radiation damage. The challenges posed by the resultant data stream are staggering: millions of incomplete, noisy and un-oriented patterns have to be computationally assembled into a three-dimensional intensity map and then phase reconstructed. In this paper, theDragonflysoftware package is described, based on a parallel implementation of the expand–maximize–compress reconstruction algorithm that is well suited for this task. Auxiliary modules to simulate SPI data streams are also included to assess the feasibility of proposed SPI experiments at the Linac Coherent Light Source, Stanford, California, USA.

Nature 530(7589), 202 - 206(2016). doi:10.1038/nature16949, The three-dimensional structures of macromolecules and their complexes are mainly elucidated by X-ray protein crystallography. A major limitation of this method is access to high-quality crystals, which is necessary to ensure X-ray diffraction extends to sufficiently large scattering angles and hence yields information of sufficiently high resolution with which to solve the crystal structure. The observation that crystals with reduced unit-cell volumes and tighter macromolecular packing often produce higher-resolution Bragg peaks suggests that crystallographic resolution for some macromolecules may be limited not by their heterogeneity, but by a deviation of strict positional ordering of the crystalline lattice. Such displacements of molecules from the ideal lattice give rise to a continuous diffraction pattern that is equal to the incoherent sum of diffraction from rigid individual molecular complexes aligned along several discrete crystallographic orientations and that, consequently, contains more information than Bragg peaks alone. Although such continuous diffraction patterns have long been observed—and are of interest as a source of information about the dynamics of proteins—they have not been used for structure determination. Here we show for crystals of the integral membrane protein complex photosystem II that lattice disorder increases the information content and the resolution of the diffraction pattern well beyond the 4.5 $\mathring{A}$ limit of measurable Bragg peaks, which allows us to phase the pattern directly. Using the molecular envelope conventionally determined at 4.5 $\mathring{A}$ as a constraint, we obtain a static image of the photosystem II dimer at a resolution of 3.5 $\mathring{A}$. This result shows that continuous diffraction can be used to overcome what have long been supposed to be the resolution limits of macromolecular crystallography, using a method that exploits commonly encountered imperfect crystals and enables model-free phasing., Published by Nature Publ. Group, London [u.a.]

Schemes for X-ray imaging single protein molecules using new x-ray sources, like x-ray free electron lasers (XFELs), require processing many frames of data that are obtained by taking temporally short snapshots of identical molecules, each with a random and unknown orientation. Due to the small size of the molecules and short exposure times, average signal levels of much less than 1 photon/pixel/frame are expected, much too low to be processed using standard methods. One approach to process the data is to use statistical methods developed in the EMC algorithm (Loh & Elser, Phys. Rev. E, 2009) which processes the data set as a whole. In this paper we apply this method to a real-space tomographic reconstruction using sparse frames of data (below $10^{-2}$ photons/pixel/frame) obtained by performing x-ray transmission measurements of a low-contrast, randomly-oriented object. This extends the work by Philipp et al. (Optics Express, 2012) to three dimensions and is one step closer to the single molecule reconstruction problem., 10 pages, 5 figures

Single-particle imaging experiments of biomolecules at x-ray free-electron lasers (XFELs) require processing hundreds of thousands of images that contain very few x-rays. Each low-fluence image of the diffraction pattern is produced by a single, randomly oriented particle, such as a protein. We demonstrate the feasibility of recovering structural information at these extremes using low-fluence images of a randomly oriented 2D x-ray mask. Successful reconstruction is obtained with images averaging only 2.5 photons per frame, where it seems doubtful there could be information about the state of rotation, let alone the image contrast. This is accomplished with an expectation maximization algorithm that processes the low-fluence data in aggregate, and without any prior knowledge of the object or its orientation. The versatility of the method promises, more generally, to redefine what measurement scenarios can provide useful signal.

New sources and detectors are allowing scientists to look at matter with finer spatial and temporal resolutions. These experiments can produce data that are a series of severely Poisson limited snap-shots of randomly oriented samples. An extreme case of this is destructive imaging of single particles with an x-ray free-electron laser – many frames are needed for a reconstruction, but there is no a priori information associated with the frames about particle orientation. We use Cornell's Pixel Array Detectors (PADs) to examine the practical limits of an expectation maximization (EM) algorithm designed to deal with extremely low-fluence data, having just a few photons per frame. We demonstrate image reconstruction of a high-contrast sample using hundreds of thousands of randomly oriented frames with an average x-ray photon occupancy as low as 2.5 photons per frame. Practical aspects of reducing low-fluence data, such as thresholding and noise limits, will be discussed for high- and low-contrast samples; and data collected in the presence of significant background signal.

Single Particle Imaging (SPI) with intense coherent X-ray pulses from X-ray free-electron lasers (XFELs) has the potential to produce molecular structures without the need for crystallization or freezing. Here we present a dataset of 285,944 diffraction patterns from aerosolized Coliphage PR772 virus particles injected into the femtosecond X-ray pulses of the Linac Coherent Light Source (LCLS). Additional exposures with background information are also deposited. The diffraction data were collected at the Atomic, Molecular and Optical Science Instrument (AMO) of the LCLS in 4 experimental beam times during a period of four years. The photon energy was either 1.2 or 1.7 keV and the pulse energy was between 2 and 4 mJ in a focal spot of about 1.3 μm x 1.7 μm full width at half maximum (FWHM). The X-ray laser pulses captured the particles in random orientations. The data offer insight into aerosolised virus particles in the gas phase, contain information relevant to improving experimental parameters, and provide a basis for developing algorithms for image analysis and reconstruction., Measurement(s) X-ray diffraction data Technology Type(s) free electron laser Sample Characteristic - Organism Enterobacteria phage PR772 Sample Characteristic - Location contiguous United States of America Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.13070546