Crystallography characteristic of fine particles prepared by the gas evaporation technique is described. The crystal habits observed for fcc and bcc metals, truncated octahedron and truncated rhombic dodecahedron, respectively, are explained by a requirement of the surface energy minimum, that is, Wulff polyhedron. Characteristic crystal structures caused by the small size for fcc and bcc metals are shown as multiply twinned particles and A15 structure, respectively. Multiply twinned particles observed for Wurzite type of structure such as GaN, AIN, CdS are also mentionned. Finally, crystal structures and habits of fine particles accompanying the phase transformation during the growth are described for Ti, Be, Fe and Mn particles.
The established concept of quasicrystals is described, in which quasicrystals are defined as the structure having (1) a long-range quasiperiodicity and (2) a crystallographically-forbidden rotational symmetry. It is shown that the quasicrystalline structure is regarded as a section of a higher-dimensional crystalline structure. The concepts of phason strain, random tiling model and approximant crystal are introduced according to the higher-dimensional description of the quasicrystalline structure. Experimental investigations made so far on the structure of the quasicrystalline phases are discussed in the light of these concepts. Important roles of approximant crystals in the research of quasicrystals are emphasized.
Various types of topochemical [2+2] photocycloaddition behaviors are described focusing on the recent results of a series of alkyl pyridylethenylcinnamate derivatives. Highly stereoregular polymerization and“absolute”asymmetric synthesis were achieved by using crystal lattice controlled [2+2] photocycloaddition. Product control by employing co-crystallization of solvent molecules and an extremely low temperature photoirradiation technique is demonstrated in the topochemical photoreactions of diolefin molecules. A methodology for the synthesis of highly strained compounds, [2.2] paracyclophane derivatives, in the crystalline state is also described.
Flavodoxins are electron transfer proteins which contain one FMN molecule as the redox active component. The article reviews the structures of flavodoxins from bacteria and algae determined at high resolution and their functional implications. Each flavodoxin has a core of five parallel β-strands surrounded by α-helices. Although the FMN binds to similar position in the protein to each other, the environment of the isoalloxazine ring of FMN differs. It appears that there are various modes of structural changes associated with conversion of the redox states. The relationship between the environment of the isoalloxazine ring and the potential for the oxidized/semiquinone couple is discussed.
A review of the authors' studies of alkali-metal adsorption on the Si (001) surface has been presented. Experimental techniques used are angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and X-ray photoelectron diffraction (XPD) . We showed from ARUPS and XPD that alkalimetals on the Si (001) at its saturation do not form one dimensional chains of alkali metal but form what we call a double-layer of alkali atoms. Negative electron affinity (NEA) surfaces which are formed by exposing the alkali-saturated Si (001) 2×1 surfaces to oxygen are also studied by XPD and ARUPS. A model of the NEA surfaces is presented in which the double-layer arrangement of alkali atoms is preserved.
The three dimensional structure of cytochrome c oxidase, a integral mitochondrial membrane protein complex, is critical to elucidation of the mechanism of the enzyme reactions. The crystallization of the enzyme isolated from bovine hearts has been tried under various conditions (exp. ionic strength, detergents, precipitants) . At low inoic strength (1-3mM Na-Pi buffer, pH 7.4), increasing the detergent (Brij-35) concentration was successful in producing the hexagonal bipyramidal crystals with typical dimension of 0.5 mm which diffracted X-rays at a resolution of 7 Å. On the other hand, at high ionic strength (100mM Na-Pi buffer, pH 7.4, 33% sat. AmSO4) the tetragonal crystals were obtained, which contain BL8SY as the detergent. The tetragonal crystals diffracted X-rays at 5-6Å resolution under anaerobic conditions.
Low-temperature 4-circle X-ray diffractometer is briefly discussed. The following important problems are described: the balance of weight during x-axis rotation, the movement of the center of the specimen position with temperature, the adhesion of specimen to the low-temperature holder and troubles in using the closed-cycle cryogenic refrigerator etc.