Highly crystalline lanthanum titanium oxynitride LaTiO2N has been determined to have an Imma perovskite-type structure with a0b—b— tilt system by the electron, neutron and synchrotron diffraction experiments (Yashima et al., Chem. Comm., 46, 4704-4706 (2010)). The refined crystallographic parameters agree well with those of the optimized structure obtained by density functional theory-based calculations. Experimental and theoretical electron densities indicate the covalent bonding between Ti cation and N,O anions. The covalent bonding and existence of N atoms are responsible for the reduced band gap, which leads to the visible-light response of LaTiO2N photocatalyst.
Dark-operative protochlorophyllide reductase (DPOR) catalyses the reduction of protochlorophyllide (Pchlide) to chlorophyllide a, which is a key step in the chlorophyll biosynthesis pathway. DPOR is a nitrogenase-like enzyme consisting of two components, BchL and BchNB, which are structurally related to nitrogenase NifH and NifDK, respectively. We determined the crystal structure of the catalytic component of DPOR, BchNB, in Pchlide-bound and Pchlide-free forms. BchNB has a novel FeS cluster (NB-cluster) coordinated uniquely by one aspartate and three cysteines. NB-cluster is located at the spatial position corresponding to an electron mediating FeS cluster, P-cluster, in nitrogenase NifDK. A Pchlide molecule found in the Pchlide-bound form is accommodated in the cavity surrounded by many hydrophobic residues. We propose a unique trans-specific reduction mechanism by comparison between the Pchlide-bound and the Pchlide-free forms. The spatial arrangement of the NB-cluster and Pchlide is almost identical to that of the P-cluster and FeMo-cofactor in nitrogenase NifDK, suggesting that a common architecture exists to reduce chemically stable multibonds of porphyrin and dinitrogen.
Correlations between magnetic and crystallographic domains in an ordered double perovskite, Ba2FeMoO6, were investigated by transmission electron microscopy. We found that magnetic domain walls perfectly coincide with crystallographic antiphase domain boundaries. This suggests a pinning effect on the magnetic domains at the antiphase boundaries. In addition, we observed a magnetic nanodomain structure derived from coupling between magnetic and structural ordering domains where Fe/Mo short-range ordering was developed. The magnetic domain structure in the ordered double perovskite is significantly affected by the crystallographic structures, i.e., the antiphase boundary and the short-range ordering, due to their strong mutual coupling.
Enzymes are widely used for the synthesis of pharmaceuticals, agrochemicals, and food additives because they can catalyze high enantioselective transformations. In order to construct selective enzymes by protein engineering, it is important to understand the molecular basis of enzyme-substrate interactions that contribute to enantioselectivity. The haloalkane dehalogenase DbjA showed high enantioselectivity for two racemic mixtures: α-bromoesters and β-bromoalkanes. Thermodynamic analysis, protein crystallography, and computer simulations indicated that DbjA carries two bases for the enantiodiscrimination of each racemic mixture. This study helps us understand the molecular basis of the enantioselectivity and opens up new possibilities for constructing enantiospecific biocatalysts through protein engineering.
Slow lattice dynamics in the relaxor ferroelectric 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 were investigated by neutron spin echo technique. At much higher temperature than TC (400 K), we observed oscillating normalized intermediate scattering functions I(Q,t)/I(Q,0) for the [1-10] direction, which is ascribed to low frequency vibrational mode with ω∼40 μeV. Such low frequency mode suggests that flat energy surfaces exist in the free energy potential along the [1-10] direction, which could be associated with degeneracy between the rhombohedral and tetragonal ground states at morphotropic phase boundary. On cooling towards TC, the oscillation in I(Q,t)/I(Q,0) decreases and I(Q,t)/I(Q,0) becomes time-independent below TC. This indicates that the low frequency vibrational mode change into the slow relaxational mode showing giant responses.
Zeolites are microporous crystalline aluminosilicates and have ordered micropores of a few Å in the structure. Zeolites are widely used in petrochemistry and fine-chemical synthesis because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously, and thus produce nanosheet-type MFI zeolite that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The nanosheet-type MFI zeolite showd a significantly increased surface area, high catalytic activities, and excellent thermal and hydrothermal stability.
By synchrotron X-ray diffraction measurement, we found that V trimerization occurs in various vanadates with V ions on quasi-triangular lattice, AV10O15(bilayer), A2V13O22(trilayer), and AV13O18(infinite layer), where A = Ba, Sr. We also found that this V trimerization substantially affects the electrical resistivity and magnetic susceptibility of these compounds. These results can be explained by the bond-ordered-type orbital ordering of V t2g states.