The X-ray structures of myoglobin and hemoglobin initiated structural biology around 1960. By exploiting synchrotron radiation and gene engineering, protein crystallography has become a powerful tool in structural biology. To date, the atomic coordinates of more than 35, 000 proteins and nucleic acids are known, which has expanded the fundamental knowledge of protein and nucleic acid structures. Current protein crystallography would not be possible without synchrotron radiation. However, structural studies of biological macromolecular assemblies, including tentative protein complexes, must be accelerated to establish a collaborative relationship with system biology, a novel biological field. Another important structural study in the coming decade is the highly accurate structural determination in combination with other methods to elucidate reaction mechanism in anisotropic reaction fields of proteins.
Recently, we have found a new ferroelectric barium dititanate, BaTi2O5, with a spontaneous polarization of 7μC/cm2at room temperature. The crystals, grown by a cooling method, show a ferroelectric phase transition at the Curie temperatureTcof 700-750 K, and the space group changes from monoclinicC2/mto monoclinicC2 on cooling. The dielectric constant εb' along the b-axis measured at 1 MHz reaches very large values of about 30000 atTc, while the dielectric loss tanδ remains small at values below 0.1. Thus, BaTi2O5is very promising for use in dielectric and piezoelectric devices operating at high temperatures. In this review, we would like to describe the synthetic method, the crystal structure and the physical property on the barium dititanate.
Barium titanates have been widely studied due to their useful dielectric properties; however, the phase diagram of the BaO-TiO2system has not been well understood. This paper briefly reviewed the study on the phase diagram for the BaO-TiO2system particularly in the BaTi2O5 (BT2) -related region. BT2has been long understood as a paraelectric phase belonging to the space group ofC2/m. However, BT2single crystal prepared by a floating-zone method exhibits significant ferroelectricity in theb-direction, contradicting to the symmetricC2/m. The space group of BT2could beC2with monoclinic lattice parameters ofa=1.6899nm, b=0.39350nm, c=0.94105 nm and β= 103.103°.
A new method to analyze the thickness of a very thin film without any substrates, which is called here“nanofilm”, has been developed. When it is composed of the stacking of more than about 10 atomic layers, the thickness can easily be determined by counting the number of lattice fringes, appearing in high-resolution transmission electron microscope (HRTEM) images of its cross section, which arise at puckered sites. For further thinner films, however, this is not the case, since they are so flexible that the diffraction condition quickly varies depending on sites and HRTEM images do not necessarily reflect the exact number of stack-ing layers. The flexibility is closely related to the thickness. This means that the thickness can be determined by the quantitative analysis of electron diffraction intensity, which takes an apparent temperature factor into account. We have applied this method to a carbon nanofllm (CNF) and succeeded in clarifying the number of stacking layers unambiguously. It has been proved that a single sheet of graphene, which is a carbon hexagonal-ring plane, exists in a CNF as one of component films.
By neutron diffraction measurements of B-DNA duplex, we have succeeded in determining the D positions in 27 D2O molecules. Moreover, the complex water network in the minor groove has been observed in detail. By a combined structural analysis using X-ray and neutron data, it is clear that the spine of hydration is built up, not only by a simple hexagonal hydration pattern, but also by many other water bridges hydrogen-bonded to the DNA strands. The complexity of the hydration pattern in the minor groove is derived from an extraordinary variety of orientations displayed by the water molecules.
The structures of the orthorhombic room-temperature phase of Cu8GeS6 (phase II) and the monoclinic low-temperature phase of Ag7TaS6 (phase II) have been successfully refined based on X-ray diffraction data from 12-fold twinned (Cu8GeS6II) and 24-fold twinned (Ag7TaS6II) crystals. Respectively among 6 major and 6 minor twin domains of Cu8GeS6II, or among 12 major and 12 minor twin domains of Ag7TaS6II, the argyrodite-type frameworks, GeS6or TaS6, can be superposed to each other in principle, and only Cu-Cu or Ag-Ag network directions differ. At higher temperature, the crystals were considered to be 2-fold twinned crystals of superionic-conductor phase I with a space groupF43m. On cooling, each domain transforms into 6 domains of orthorhombic Cu8GeS6II or 12 domains of monoclinic Ag7TaS6II. Superposed projections along 6 directions of the structure of Cu8GeS6II and along 12 directions of the structure of Ag7TaS6II seem to show approximate expressions for Cu-ion and Ag-ion conduction paths in superionic-conductor phases, Cu8GeS6I and Ag7TaS6I.
Factor VIIa/tissue factor is a key serine protease for initiation of blood coagulation. It is seen as promising target for the developing new anticoagulant drugs. X-ray crystal structures of human factor VIIa/tissue factor in complex with peptide mimetic inhibitors revealed important interactions for the selective inhibition of factor VIIa/tissue factor. These observations should provide some important information for a design of new factor VIIa selective inhibitors.
Anti-terminator proteins control gene expression by recognizing control signals on cognate mRNAs and preventing transcriptional termination. HutP ofBacillus subtilisregulates thehutoperon by an anti-termination mechanism. Recently, crystal structures of HutP [apo-form of HutP, HutP-HBN (histidine analog), HutP-L-histidine-Mg2+, and HutP-L-histidine-Mg2+-RNA] have been reported. These structures and functional studies on HutP showed how the protein undergoes conformational changes in response to two key components : L-histidine and Mg2+ions, before binding to the cognate terminator RNA.