Magnetic structure of Sr₂IrO₄, a new class of Mott insulator, has been determined only by means of resonant X-ray magnetic diffraction. Spectrum of resonant X-ray magnetic scattering also provides information on the ground state electron configurations. Here, we describe in detail how we determined the magnetic structure. The result of the X-ray magnetic scattering study establishes a better understanding of the unconventional insulating nature in this 5d transition metal oxide.

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X-ray crystallography can routinely determine the atomic structure of crystalline materials. The method can be extended to non-crystalline materials by using coherent X-ray diffraction. In X-ray diffraction microscopy, coherent X-ray diffraction patterns are sampled finely enough to satisfy the oversampling condition for solving the phase problem, and the iterative phase retrieval method is used for the sample image reconstruction. Recently, we succeeded in three-dimensional visualization of an unstained human chromosome by X-ray diffraction microscopy. It is the first hard X-ray tomography for cellular organelles. The reconstructed images revealed the internal axial structure, demonstrating an excellent image-contrast of the method.

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Peroxisome is an organelle in eukaryotic cells, which functions in various metabolisms such as β-oxidation of very long fatty acids. Peroxisomal matrix proteins synthesized in cytosol are imported into the peroxisome by a dynamic system consisting of over a dozen peroxins, Pex1p to Pex26p. Pex14p is a central component of the peroxisomal matrix protein import machinery. Until now, any structural information of Pex14p has not been elucidated at all. We describe here the crystal structure of the conserved domain of mammalian Pex14p at 1.8 Å resolution. A hydrophobic surface is composed of the conserved residues, of which two phenylalanine residues (Phe35 and Phe52) protrude to the solvent. Consequently, two putative binding pockets suitable for recognizing the helical WxxxF/Y motif of Pex5p are formed on the surface by the two phenylalanine residues accompanying with positively charged residues. Other structural studies for peroxins are also reviewed in this report.

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Iodine is a good anomalous scatter for radiations from in-house X-ray generators (Cu/CrKα). Non-natural amino acid, 3-iodo-L-tyrosine, is able to be site-specifically incorporated into proteins with amber suppresser tRNA and mutated TyrRS from M. jannaschii in the E. coli expression system. To determine the crystal structure of acetyl transferase from T. thermophilus, iodotyrosine-containing proteins were prepared and crystallized. Structure determination was successfully conducted with the protein variant with iodotyrosine at position 111. Anomalous signals from iodotyrosine with Cu/CrKα radiations were both sufficient to calculate clear electron density map. In the crystal structure, iodotyrosine did not significantly disturb the native structure.

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Experimental strategy and setup for the 100 ps time-resolved X-ray absorption spectroscopy (TR-XAS) is presented. X-ray positional active feedback combined with a figure-of-merit scan of the laser beam position was employed as a key technique. It is shown that the pump-probe TR-XAS technique using the X-ray pulse structure of synchrotron radiation sources is a powerful tool to investigate dynamics of the electronic state and molecular structure of non-crystalline samples. A TR-XAS measurement of a photo-induced spin crossover reaction of the tris(1,10-phenanthrorine)iron(II) complex in water is presented.

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