Structure Without Crystals: From Biology to Aerosols

We are in the transition to an era where ultrafast and ultrabright x-ray pulses overcome resolution limitations in x-ray microscopy, imposed by x-ray induced damage to the sample, by “diffracting before destroying” the sample on sub-picosecond timescales. This will open unforeseen opportunities across a breadth of science.

X-rays have long been used to study the structure of crystallized proteins, producing many critical advances that have lead to improved human health. In contrast few, if any, methods existing for high resolution imaging of airborne particulate matter critical to Earth's climate, manufacturing and human health.

Radiation damage is caused by energy deposited into the sample during exposure, and this causes the sample to fall apart before enough signal is accumulated to form an image. Simulations based on molecular dynamics, hydrodynamic plasma models, and particle plasma models agree with each other, and predict that with a very short and very intense coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus, or a cell without the need for crystalline periodicity in the sample. A three-dimensional data set could be assembled from such patterns when copies of a reproducible sample are exposed to the beam one by one in random orientations, and damage can be distributed over many copies of the sample in a somewhat similar manner as in crystallography. Resolution can be further enhanced by signal averaging. The over-sampled diffraction pattern permits phase retrieval and hence structure determination. Neutze R et al, Nature 406, 752, 2000

LCLS will revolutionize the way X-rays are used to solve protein structures. Instead of having to shine X-rays on a crystal for a long period of time, LCLS will have enough X-rays in a single ultrafast pulse to collect a diffraction pattern from nanocrystals and maybe even just one protein molecule! Reducing barriers to structural determination will allow access to entire areas of structural biology that were previously difficult or inaccessible, such as membrane proteins. The advances motivated by solving protein structures have implications for studying non-periodic molecular structures in biology, or in any other area of science and technology where structural information with high spatial and temporal resolution is valuable. Chapman HN, Nature Materials 8, 299, 2009

In case of non-reproducible objects (e.g. cells and most aerosols), there is no chance to enhance the signal by averaging, and as a consequence, the maximum resolution is very strongly coupled to damage. Experimental strategies are, therefore, different for high-resolution studies on reproducible and non-reproducible objects, and influence the choice of wavelength and pulse parameters in flash diffraction experiments. Solem JC et al, Science 218, 229, 1982; Bergh M et al, Quar Rev Biophys 41, 181, 2008

Imaging Aerosol Morphology: At PULSE one of our research paths in single particle CXDI emphasizes aerosols. Aerosols are often unique non-periodic matter for which no high resolution imaging methodology is available in their airborne state, unlike the advanced state of development of existing biological imaging technology. For many lensless imaging algorithms used in CXDI it is convenient when the data satisfies an oversampling constraint that requires the sample to be an isolated object, i.e. an individual “free standing” portion of disordered matter delivered to the center of the x-ray focus. By definition, this type of matter is an aerosol.

We have implemented aerosol science methodologies for the validation of the “diffract before destroy” hypothesis and the execution of the first single particle CXDI experiments being developed for biological imaging. FLASH CXDI now enables the highest resolution imaging of single micron-sized or smaller airborne particulate matter to date while preserving the native substrate-free state of the aerosol. Electron microscopy offers higher resolution for single particle analysis but the aerosol must be captured on a substrate, potentially modifying the particle morphology. Thus FLASH is poised to contribute significant advancements in our knowledge of aerosol morphology and dynamics.

We have simulated CXDI of combustion particle (soot) morphology and introduced the concept of extracting radius of gyration of fractal aggregates from single pulse x-ray diffraction data. Recent upgrades to FLASH will enable higher spatial- and time-resolved single particle aerosol dynamics studies, filling a critical technological need in aerosol science and nanotechnology. The methodologies developed for FLASH will directly translate to use at hard x-ray free electron lasers such as the Linac Coherent Light Source here at the SLAC National Accelerator Laboratory. Bogan MJ et al, J Phys B: Atom, Mol, Optics 43, 194013, 2010

Coherent diffraction x-ray imaging (CXDI) overcomes the restrictions of limited-resolution X-ray lenses, offering a means to produce images of general non-crystalline objects at a resolution only limited in principle by the X-ray wavelength and by radiation-induced changes of the sample during exposure.

CXDI is elegant in its experimental simplicity: a coherent x-ray beam illuminates the sample and the far-field diffraction pattern of the object is recorded on an area detector. These measured diffraction intensities are proportional to the modulus squared of the Fourier transform of the wave exiting the object. An inversion of the diffraction pattern to an image in real space requires the retrieval of the phases of the diffraction pattern. This can be achieved by iterative transform algorithms if the object is isolated and the diffraction pattern intensities are adequately sampled (an approach known as oversampling). The shrinkwrap algorithm developed by our collaborators is particularly robust and practical. The algorithm reconstructs images ab initio which overcomes the difficulty of requiring knowledge of the high-resolution shape of the diffracting object. This lensless imaging technique can be scaled all the way to atomic resolution, but in practice the resolution of the image of a single object is restricted by x-ray damage to the sample.

 PULSE Research Nanoscale & Biomolecular Imaging • Publications • Scientific Staff