Development of serial X-ray fluorescence holography for radiation-sensitive protein crystals

X-ray fluorescence holography (XFH) is a powerful atomic resolution technique capable of directly imaging the local atomic structure around atoms of a target element within a material. Although it is theoretically possible to use XFH to study the local structures of metal clusters in large protein crystals, the experiment has proven difficult to perform, especially on radiation-sensitive proteins. Here, the development of serial X-ray fluorescence holography to allow the direct recording of hologram patterns before the onset of radiation damage is reported. By combining a 2D hybrid detector and the serial data collection used in serial protein crystallography, the X-ray fluorescence hologram can be directly recorded in a fraction of the measurement time needed for conventional XFH measurements. This approach was demonstrated by obtaining the Mn Kα hologram pattern from the protein crystal Photosystem II without any X-ray-induced reduction of the Mn clusters. Furthermore, a method to interpret the fluorescence patterns as real-space projections of the atoms surrounding the Mn emitters has been developed, where the surrounding atoms produce large dark dips along the emitter-scatterer bond directions. This new technique paves the way for future experiments on protein crystals that aim to clarify the local atomic structures of their functional metal clusters, and for other related XFH experiments such as valence-selective XFH or time-resolved XFH.

Processing and analysis of X-ray photoelectron diffraction data using IGOR Pro

A software package is presented for nearly real-time display of diffractograms during X-ray photoelectron diffraction (XPD) data acquisition and for processing and analysis after an experiment. During the experiment, the package is able to automatically read X-ray photoelectron spectroscopy (XPS) data, perform initial data processing and project intensity values as XPD diffractograms. Four display modes are supported. After the experiment, the package is able to open, process and analyze XPD patterns. The processing functions include rotation, cropping, creating a full 2π pattern using symmetry operations, smoothing and converting a pattern to an image suitable for publication. The analysis functions include displaying polar angles, azimuthal angles, intensity, the core level spectrum at a selected angular data point, and azimuthal and radial profiles. The package also integrates fitting functions for core level spectra. The package is developed using the IGOR Pro scripting language. A graphical user interface has been developed to allow all the operations just by mouse clicking. The package is designed to interface directly with an XPD system at the Saclay center of the French Atomic Energy and Alternative Energies Authority, but the algorithms are generally applicable and can be readily adapted to other XPD systems.