Electron Microscopy Study of Gold Nanoparticles Deposited on Transition Metal Oxides

Many researchers have investigated the catalytic performance ofgold nanoparticles (GNPs) supported on metal oxides for various catalytic reactions of industrial importance. These studies have consistently shown that the catalytic activity and selectivity depend on the size of GNPs, the kind of metal oxide supports, and the gold/metal oxide interface structure. Although researchers have proposed several structural models for the catalytically active sites and have identified the specific electronic structures of GNPs induced by the quantum effect, recent experimental and theoretical studies indicate that the perimeter around GNPs in contact with the metal oxide supports acts as an active site in many reactions. Thus, it is of immense importance to investigate the detailed structures of the perimeters and the contact interfaces of gold/metal oxide systems by using electron microscopy at an atomic scale.This Account describes our investigation, at the atomic scale using electron microscopy, of GNPs deposited on metal oxides. In particular, high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are valuable tools to observe local atomic structures, as has been successfully demonstrated for various nanoparticles, surfaces, and material interfaces. TEM can be applied to real powder catalysts as received without making special specimens, in contrast to what is typically necessary to observe bulk materials. For precise structure analyses at an atomic scale, model catalysts prepared by using well-defined single-crystalline substrates are also adopted for TEM observations. Moreover, aberration-corrected TEM, which has high spatial resolution under 0.1 nm, is a promising tool to observe the interface structure between GNPs and metal oxide supports including oxygen atoms at the interfaces. The oxygen atoms in particular play an important role in the behavior of gold/metal oxide interfaces, because they may participate in catalytic reaction steps. Detailed information about the interfacial structures between GNPs and metal oxides provides valuable structure models for theoretical calculations which can elucidate the local electronic structure effective for activating a reactant molecule. Based on our observations with HRTEM and HAADF-STEM, we report the detailed structure of gold/metal oxide interfaces

Biological Crystallography Crystallization and preliminary X-ray study of alkaline mannanase from an alkaliphilic Bacillus isolate

An alkaline mannanase (EC 3.2.1.78) from the alkaliphilic Bacillus sp. strain JAMB-602 was cloned and sequenced. The deduced aminoacid sequence of the enzyme suggested that the enzyme consists of a catalytic and unknown additional domains. The recombinant enzyme expressed by B. subtilis was crystallized using the hanging-drop vapour-diffusion method at 277 K. X-ray diffraction data were collected to 1.65 A Ê . The crystal belongs to space group P2 1 2 1 2 1 , with unit-cell parameters a = 70.7, b = 79.5, c = 80.4 A Ê . The asymmetric unit contains one protein molecule, with a corresponding V M of 2.26 A Ê 3 Da À1 and a solvent content of 45.6%. Molecular replacement for initial phasing was carried out using the three-dimensional structure of a mannanase from Thermomonospora fusca as a search model, which corresponds to the catalytic domain of the alkaline mannanase. It gave suf®cient phases to build the unknown domain

Crystal structure of the catalytic domain of a GH16 β-agarase from a deep-sea bacterium, <i>Microbulbifer thermotolerans</i> JAMB-A94

<div><p>A deep-sea bacterium, <i>Microbulbifer thermotolerans</i> JAMB-A94, has a β-agarase (<i>Mt</i>AgaA) belonging to the glycoside hydrolase family (GH) 16. The optimal temperature of this bacterium for growth is 43–49 °C, and <i>Mt</i>AgaA is stable at 60 °C, which is one of the most thermostable enzymes among GH16 β-agarases. Here, we determined the catalytic domain structure of <i>Mt</i>AgaA. <i>Mt</i>AgaA consists of a β-jelly roll fold, as observed in other GH16 enzymes. The structure of <i>Mt</i>AgaA was most similar to two β-agarases from <i>Zobellia galactanivorans, Zg</i>AgaA, and <i>Zg</i>AgaB. Although the catalytic cleft structure of <i>Mt</i>AgaA was similar to <i>Zg</i>AgaA and <i>Zg</i>AgaB, residues at subsite −4 of <i>Mt</i>AgaA were not conserved between them. Also, an α-helix, designated as α4′, was uniquely located near the catalytic cleft of <i>Mt</i>AgaA. A comparison of the structures of the three enzymes suggested that multiple factors, including increased numbers of arginine and proline residues, could contribute to the thermostability of <i>Mt</i>AgaA.</p></div