In strongly correlated electron systems, it has been known that three degrees of freedom of electrons, that is, charge, spin and orbital play very important roles in their electronic and magnetic properties. In this report we show the resonant X-ray scattering is a very powerful tool to observe charge, spin, and orbital orderings. Synchrotron X-ray diffraction experiments in high pressure and magnetic field are introduced to reveal the mechanism of the orderings. It is also reported that the orbital excitation is studied by resonant inelastic X-ray scattering.
The interior of the Earth is under high-pressure and high-temperature conditions. High-pressure and -temperature experiments have been conducted by using the multi-anvil press and laser-heated diamond-anvil cell in order to determine the physical properties of minerals that constitute interior of the Earth. Since the sample volume in the high-pressure apparatus is very small, strong X-rays at synchrotron facilities are necessary for diffraction and imaging experiments. Here I introduce the recent studies on high-pressure and -temperature crystallography of the Earth and planetary materials.
Mutant proteins are useful for study on the mechanism of protein stability. However, it is difficult to evaluate the changes in stability due to mutation, because contributions of amino acid residues to the conformational stability differ depending on their locations even if the same residues are substituted. In order to evaluate each contribution of several factors responsible for protein stability, we determined the crystal structures and measured the protein stability for more than a hundred of mutant human lysozymes. Analyzing this structure/stability database, the parameters of each stabilization factor, such as hydrophobic effect and hydrogen bond, could be estimated. These parameters are available for understanding the thermostabilization mechanism of the proteins from hyperthermophiles and for the computer-aided drug design.
Human dipeptidyl peptidase IV (DPPIV) [EC 126.96.36.199] belongs to the prolyl oligopeptidase (POP) family, a member of the serine proteases. The crystal structure of DPPIV was determined at 2.6 Å resolution. The molecule consists of a unique eight-bladed β-propeller domain and a α/β-hydrolase domain. The large“cave”structure, which is thought to control the access of substrate, is found on the side of the β-propeller fold. Furthermore, the crystal structure of DPPIV in complex with diprotin A was determined at 2.2 Å resolution. The structure reveals the molecular interaction mechanism of diprotin A with DPPIV.
Peptidylarginine deimianse 4 (PAD4) is a Ca2+-dependent enzyme that catalyzes the conversion of both arginine and mono-methyl arginine in histones into citrullines, and regulates both histone argininine methylation level and gene activity. Its gene is susceptibility locus for rheumatoid arthritis (RA) . Here we present the crystal structure of Ca2+-free wild-type PAD4, which shows that the polypeptide chain adopts an elongated fold in which the N-terminal domain forms two immunoglobulin-like subdomains, and the C-terminal domain forms an α/β propeller structure. Five Ca2+-binding sites, none of which adopts an EF-hand motif, were identified in the structure of a Ca2+-bound inactive mutant with and without bound substrate. These structural data indicate that Ca2+binding induces conformational changes that generate the active site cleft. Our findings identify a novel mechanism for enzyme activation by Ca2+ions, and are important for understanding the mechanism of protein citrullination and for developing PAD-inhibiting drugs for the treatment of RA.
Dynamic properties of liquid Si are investigated by means of inelastic scattering using high-resolution inelastic X-ray scattering spectrometer at BL35XU/SPring-8. The S (Q, ω) spectra clearly demonstrate the existence of propagating short wavelength mode, which shows a positive deviation from the hydrodynamic value. A specific variation of the quasielastic line shape with increasing Q is observed close to the S (Q) maximum. This finding is related to a several-ten-femtosecond retaining of the covalent bonds, which has previously been suggested by an ab initio molecular dynamics simulation.
Accurate crystal structural analysis at high temperatures is of vital importance in various fields of science and engineering. A new electric furnace has been designed and fabricated to collect the high-resolution synchrotron radiation powder diffraction data from materials at high temperatures up to 1900 K in air (Yashima and Tanaka: J. Appl. Cryst. 37, 786 (2004) ) . We demonstrate that the present new furnace enables accurate determination of crystal structure and electron density distribution of the cubic calcium titanate perovskite at 1674 K in air.