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71.
Atomic ordering in network glasses on length scales longer than nearest-neighbour length scales has long been a source of controversy. Detailed experimental information is therefore necessary to understand both the network properties and the fundamentals of glass formation. Here we address the problem by investigating topological and chemical ordering in structurally disordered AX2 systems by applying the method of isotopic substitution in neutron diffraction to glassy ZnCl2. This system may be regarded as a prototypical ionic network forming glass, provided that ion polarization effects are taken into account, and has thus been the focus of much attention. By experiment, we show that both the topological and chemical ordering are described by two length scales at distances greater than nearest-neighbour length scales. One of these is associated with the intermediate range, as manifested by the appearance in the measured diffraction patterns of a first sharp diffraction peak at 1.09(3) A(-1); the other is associated with an extended range, which shows ordering in the glass out to 62(4) A. We also find that these general features are characteristic of glassy GeSe2, a prototypical covalently bonded network material. The results therefore offer structural insight into those length scales that determine many important aspects of supercooled liquid and glass phenomenology. 相似文献
72.
Hillier LW Graves TA Fulton RS Fulton LA Pepin KH Minx P Wagner-McPherson C Layman D Wylie K Sekhon M Becker MC Fewell GA Delehaunty KD Miner TL Nash WE Kremitzki C Oddy L Du H Sun H Bradshaw-Cordum H Ali J Carter J Cordes M Harris A Isak A van Brunt A Nguyen C Du F Courtney L Kalicki J Ozersky P Abbott S Armstrong J Belter EA Caruso L Cedroni M Cotton M Davidson T Desai A Elliott G Erb T Fronick C Gaige T Haakenson W Haglund K Holmes A Harkins R Kim K Kruchowski SS Strong CM Grewal N Goyea E 《Nature》2005,434(7034):724-731
Human chromosome 2 is unique to the human lineage in being the product of a head-to-head fusion of two intermediate-sized ancestral chromosomes. Chromosome 4 has received attention primarily related to the search for the Huntington's disease gene, but also for genes associated with Wolf-Hirschhorn syndrome, polycystic kidney disease and a form of muscular dystrophy. Here we present approximately 237 million base pairs of sequence for chromosome 2, and 186 million base pairs for chromosome 4, representing more than 99.6% of their euchromatic sequences. Our initial analyses have identified 1,346 protein-coding genes and 1,239 pseudogenes on chromosome 2, and 796 protein-coding genes and 778 pseudogenes on chromosome 4. Extensive analyses confirm the underlying construction of the sequence, and expand our understanding of the structure and evolution of mammalian chromosomes, including gene deserts, segmental duplications and highly variant regions. 相似文献
73.
74.
Hughes MD Xu YJ Jenkins P McMorn P Landon P Enache DI Carley AF Attard GA Hutchings GJ King F Stitt EH Johnston P Griffin K Kiely CJ 《Nature》2005,437(7062):1132-1135
Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products ( approximately 98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals. 相似文献
75.
Imagine a smart solvent that can be switched reversibly from a liquid with one set of properties to another that has very different properties, upon command. Here we create such a system, in which a non-ionic liquid (an alcohol and an amine base) converts to an ionic liquid (a salt in liquid form) upon exposure to an atmosphere of carbon dioxide, and then reverts back to its non-ionic form when exposed to nitrogen or argon gas. Such switchable solvents should facilitate organic syntheses and separations by eliminating the need to remove and replace solvents after each reaction step. 相似文献
76.
Ross AJ May-Simera H Eichers ER Kai M Hill J Jagger DJ Leitch CC Chapple JP Munro PM Fisher S Tan PL Phillips HM Leroux MR Henderson DJ Murdoch JN Copp AJ Eliot MM Lupski JR Kemp DT Dollfus H Tada M Katsanis N Forge A Beales PL 《Nature genetics》2005,37(10):1135-1140
The evolutionarily conserved planar cell polarity (PCP) pathway (or noncanonical Wnt pathway) drives several important cellular processes, including epithelial cell polarization, cell migration and mitotic spindle orientation. In vertebrates, PCP genes have a vital role in polarized convergent extension movements during gastrulation and neurulation. Here we show that mice with mutations in genes involved in Bardet-Biedl syndrome (BBS), a disorder associated with ciliary dysfunction, share phenotypes with PCP mutants including open eyelids, neural tube defects and disrupted cochlear stereociliary bundles. Furthermore, we identify genetic interactions between BBS genes and a PCP gene in both mouse (Ltap, also called Vangl2) and zebrafish (vangl2). In zebrafish, the augmented phenotype results from enhanced defective convergent extension movements. We also show that Vangl2 localizes to the basal body and axoneme of ciliated cells, a pattern reminiscent of that of the BBS proteins. These data suggest that cilia are intrinsically involved in PCP processes. 相似文献
77.
Zarnecki JC Leese MR Hathi B Ball AJ Hagermann A Towner MC Lorenz RD McDonnell JA Green SF Patel MR Ringrose TJ Rosenberg PD Atkinson KR Paton MD Banaszkiewicz M Clark BC Ferri F Fulchignoni M Ghafoor NA Kargl G Svedhem H Delderfield J Grande M Parker DJ Challenor PG Geake JE 《Nature》2005,438(7069):792-795
The surface of Saturn's largest satellite--Titan--is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing. 相似文献
78.
79.
Zusammenfassung Bei Reizung der helladaptierten Kaninchen-Netzhaut mit intermittierenden Lichtreizen werden elektroretinographisch Fusionsfrequenzen bis etwa 70/s, das heisst deutliche Zapfenfunktion, registriert. Die hierbei gemessene spektrale Netzhaut-Sensitivität zeigt im Gegensatz zu den unter gleichen Versuchsbedingungen an der Katzen-Netzhaut erhobenen Befunden keine Purkinje-Verschiebung, sondern folgt der Absorptionskurve des Sehpurpurs. 相似文献
80.