Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores

Use of amphiphilic triblock copolymers to direct the organization of polymerizing silica species has resulted in the preparation of well-ordered hexagonal mesoporous silica structures (SBA-15) with uniform pore sizes up to approximately 300 angstroms. The SBA-15 materials are synthesized in acidic media to produce highly ordered, two-dimensional hexagonal (space group p6mm) silica-block copolymer mesophases. Calcination at 500 degrees C gives porous structures with unusually large interlattice d spacings of 74.5 to 320 angstroms between the (100) planes, pore sizes from 46 to 300 angstroms, pore volume fractions up to 0.85, and silica wall thicknesses of 31 to 64 angstroms. SBA-15 can be readily prepared over a wide range of uniform pore sizes and pore wall thicknesses at low temperature (35 degrees to 80 degrees C), using a variety of poly(alkylene oxide) triblock copolymers and by the addition of cosolvent organic molecules. The block copolymer species can be recovered for reuse by solvent extraction with ethanol or removed by heating at 140 degrees C for 3 hours, in both cases, yielding a product that is thermally stable in boiling water.     

A spectroscopic measurement of the coronal density of procyon

One of the open key issues in the astrophysics of stellar coronae is the determination of their spatial structure and density. From almost all previous measurements, one can infer merely the presence of a corona, which for the most energetic stellar coronae may exceed the solar x-ray output by as much as five orders of magnitude, but no information can be obtained on the densities and hence volumes and sizes of the hot x-ray emitting material. A direct spectroscopic measurement of the coronal density was obtained for the star Procyon with the spectrometer on board the Extreme Ultraviolet Explorer satellite; the ratio of two Fe XIV lines at 211.32 and 264.79 angstroms was used to determine a density of approximately 4 x 10(9) to 7 x 10(9) electrons per cubic centimeter, which is a factor of 2 to 3 higher than typical solar active region densities. From this value, we estimate that approximately 6 percent of the stellar surface is covered with approximately 7 x 10(4) coronal loops.     

Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.     

Capture of a single molecule in a nanocavity

We describe a heptameric protein pore that has been engineered to accommodate two different cyclodextrin adapters simultaneously within the lumen of a transmembrane beta barrel. The volume between the adapters is a cavity of approximately 4400 cubic angstroms. Analysis of single-channel recordings reveals that individual charged organic molecules can be pulled into the cavity by an electrical potential. Once trapped, an organic molecule shuttles back and forth between the adapters for hundreds of milliseconds. Such self-assembling nanostructures are of interest for the fabrication of multianalyte sensors and could provide a means to control chemical reactions.     

Porous, crystalline, covalent organic frameworks

Covalent organic frameworks (COFs) have been designed and successfully synthesized by condensation reactions of phenyl diboronic acid {C6H4[B(OH)2]2} and hexahydroxytriphenylene [C18H6(OH)6]. Powder x-ray diffraction studies of the highly crystalline products (C3H2BO)6.(C9H12)1 (COF-1) and C9H4BO2 (COF-5) revealed expanded porous graphitic layers that are either staggered (COF-1, P6(3)/mmc) or eclipsed (COF-5, P6/mmm). Their crystal structures are entirely held by strong bonds between B, C, and O atoms to form rigid porous architectures with pore sizes ranging from 7 to 27 angstroms. COF-1 and COF-5 exhibit high thermal stability (to temperatures up to 500 degrees to 600 degrees C), permanent porosity, and high surface areas (711 and 1590 square meters per gram, respectively).     

Interwoven metal-organic framework on a periodic minimal surface with extra-large pores

Interpenetration (catenation) has long been considered a major impediment in the achievement of stable and porous crystalline structures. A strategy for the design of highly porous and structurally stable networks makes use of metal-organic building blocks that can be assembled on a triply periodic P-minimal geometric surface to produce structures that are interpenetrating-more accurately considered as interwoven. We used 4,4',4"-benzene-1,3,5-triyl-tribenzoic acid (H(3)BTB), copper(II) nitrate, and N,N'-dimethylformamide (DMF) to prepare Cu(3)(BTB)(2)(H(2)O)(3).(DMF)(9)(H(2)O)(2) (MOF-14), whose structure reveals a pair of interwoven metal-organic frameworks that are mutually reinforced. The structure contains remarkably large pores, 16.4 angstroms in diameter, in which voluminous amounts of gases and organic solvents can be reversibly sorbed.     

Shock-wave compression and x-ray studies of titanium dioxide

The Hugoniot of the rutile phase of titanium dioxide has been determined to 1.25 megabars, and data show the existence of a phase change at about 0.33 megabar. The volume decrease associated with this transformation appears to be quite large (approximately 21 percent). Rutile, when recovered from shockloading in excess of the transformation pressure, is found to be irreversibly transformed to the orthorhombic lead dioxide structure (a distortion of the fluorite structure) with parameters a, 4.529; b, 5.464; and c, 4.905 angstroms and a calculated density of 4.374 grams per cubic centimeter. The new phase reverts to rutile at temperatures above 450 degrees C. It is suggested that the new phase may be another diagnostic indicator of meteorite impact on the earth's surface.     

Quantum-confined stark effect in single CdSe nanocrystallite quantum dots

The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state ( approximately 10(5) cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices.     

Crystal Structure and Optical Properties of Cd32S14(SC6H5)36. DMF4, a Cluster with a 15 Angstrom CdS Core

Recrystallization of the solid Cd(10)S(4)(SC(6)H(5))(12) from a solution of pyridine and N, N-di-methylformamide (DMF) results in the formation of the cluster Cd(32)S(14)(SC(6)H(5))(36)-DMF(4) as pale yellow cubes. The structure consists of an 82-atom CdS core that is a roughly spherical piece of the cubic sphalerite lattice approximately 12 angstroms in diameter. The four corners of the lattice are capped by hexagonal wurtzite-like CdS units, which results in an overall tetrahedral cluster approximately 15 angstroms in diameter. This cluster dissolves intact in tetrahydrofuran where its absorption spectrum reveals a sharp peak at 358 nanometers at room temperature and its emission spectra show a strong broad band at 500 nanometers.     

Antiprismatic Coordination about Xenon: The Structure of Nitrosonium Octafluoroxenate(VI)

The structure of nitrosonium octafluoroxenate(VI), 2NOF . XeF(6), has been determined by means of single-crystal x-ray counter methods (R-index = 0.046, weighted R-index = 0.042). The space group is Pnma, with a = 8.914(10) angstroms, b = 5.945(10) angstroms, and c = 12.83(2) angstroms (the numbers in parentheses are the standard deviations to the least significant digit or digits); the calculated density (rho) is 3.354 grams per cubic centimeter, and there are four formula units per unit cell. The material consists of well-separated NO(+) and (XeF(8))(2-) ions; the structural formula is thus (NO)(2) (XeF(8)). The anion configuration is that of a slightly distorted Archimedean antiprism. The observed distortion appears incompatible with a lone-pair repulsion model. Xenon-fluorine bond lengths of 1.971(7), 1.946(5), 1.958(7), 2.052(5), and 2.099(5) angstroms were found.     

Polymeric particles of protein insoluble at pH5 from rat liver

A procedure is given for approximating the volumes of small particles of protein insoluble at pH 5. Among particles smaller than 2.08 x 10(7) cubic angstroms the change in size was linear. The distribution of sizes indicated a polymeric relationship among the particles.     

Large-pore apertures in a series of metal-organic frameworks

We report a strategy to expand the pore aperture of metal-organic frameworks (MOFs) into a previously unattained size regime (>32 angstroms). Specifically, the systematic expansion of a well-known MOF structure, MOF-74, from its original link of one phenylene ring (I) to two, three, four, five, six, seven, nine, and eleven (II to XI, respectively), afforded an isoreticular series of MOF-74 structures (termed IRMOF-74-I to XI) with pore apertures ranging from 14 to 98 angstroms. All members of this series have noninterpenetrating structures and exhibit robust architectures, as evidenced by their permanent porosity and high thermal stability (up to 300°C). The pore apertures of an oligoethylene glycol-functionalized IRMOF-74-VII and IRMOF-74-IX are large enough for natural proteins to enter the pores.     

生物炭老化及其对重金属吸附的影响机制

生物炭具有丰富含氧官能团、多孔结构、阳离子交换量、芳香性结构等使其对重金属具有良好的固持作用,进而在重金属污染土壤修复中具有良好的应用前景。生物炭施入土壤中在与土壤接触过程中受物理、化学和生物作用而发生老化现象,致使生物炭特性发生改变。本文综述了原料来源、热解温度和老化方法对老化生物炭特性的影响,以及老化生物炭对重金属吸附的影响机制。老化作用对生物炭特性的改变主要体现在灰分、表面元素组成、含氧官能团、pH、形貌特征、孔隙结构及比表面积。老化生物炭表面含氧官能团、负电荷和CEC含量增加会促进其对重金属的吸附;而比表面积和pH的降低、酚羟基和芳香醚含量增加以及羧基数量减少则抑制其对重金属的吸附。     

The Framework Topology of ZSM-18, a Novel Zeolite Containing Rings of Three (Si,Al)-O Species

ZSM-18 is the first known aluminosilicate zeolite to contain rings of three (Si,Al)-O species (3-rings). Its framework topology has been determined by hypothetical model building, subsequent constrained distance and angle least-squares refinements of atomic coordinates, and x-ray powder diffraction pattern simulations. Its channel structure is characterized by a linear unidimensional 12-ring channel, with an approximate pore opening of 7.0 angstroms. In addition, the channels are lined with pockets that are capped by 7-rings with dimensions of 2.8 angstroms by 3.5 angstroms. An intraframework packing model of the organic moiety used in the synthesis suggests that a strong templating effect may be responsible for the formation of this unusual zeolite structure.     

The structure of an open form of an E. coli mechanosensitive channel at 3.45 A resolution

How ion channels are gated to regulate ion flux in and out of cells is the subject of intense interest. The Escherichia coli mechanosensitive channel, MscS, opens to allow rapid ion efflux, relieving the turgor pressure that would otherwise destroy the cell. We present a 3.45 angstrom-resolution structure for the MscS channel in an open conformation. This structure has a pore diameter of approximately 13 angstroms created by substantial rotational rearrangement of the three transmembrane helices. The structure suggests a molecular mechanism that underlies MscS gating and its decay of conductivity during prolonged activation. Support for this mechanism is provided by single-channel analysis of mutants with altered gating characteristics.     

Virus maturation involving large subunit rotations and local refolding

Large-scale conformational changes transform viral precursors into infectious virions. The structure of bacteriophage HK97 capsid, Head-II, was recently solved by crystallography, revealing a catenated cross-linked topology. We have visualized its precursor, Prohead-II, by cryoelectron microscopy and modeled the conformational change by appropriately adapting Head-II. Rigid-body rotations ( approximately 40 degrees) cause switching to an entirely different set of interactions; in addition, two motifs undergo refolding. These changes stabilize the capsid by increasing the surface area buried at interfaces and bringing the cross-link-forming residues, initially approximately 40 angstroms apart, close together. The inner surface of Prohead-II is negatively charged, suggesting that the transition is triggered electrostatically by DNA packaging.     

Crystal Structure of the High-Pressure Phase of Solid CO2

X-ray diffraction study of solid CO(2) at room temperature has shown that the powder pattern of the high-pressure phase, which supersedes the low-pressure cubic Pa3 phase at about 10 gigapascals, is consistently interpreted in terms of an orthorhombic Cmca structure. The orthorhombic cell at 11.8 gigapascals has dimensions of 4.330 +/- 0.015, 4.657 +/- 0.005, 5.963 +/- 0.009 angstroms for its a, b, and c faces, respectively, and a volume of 120.3 +/- 0.5 cubic angstroms. Four molecules contained in the unit cell are located at the base-centered positions with their molecular axes inclined at about 52 degrees with respect to the crystallographic c axis. The volume change associated with the Pa3-Cmca transition is close to zero. The structural dimensions obtained for the high-pressure crystalline phase of CO(2) are of great importance for a theoretical understanding of the role of intermolecular interactions, including quadrupole-quadrupole interactions, in molecular condensation.     

以二甲基-正丁基-磺化木质素基氯化铵为模板剂制备多孔钛材料

以碱木质素为原料合成二甲基-正丁基-磺化木质素基氯化铵两性表面活性剂,并以其为模板剂采用溶胶-凝胶法制备多孔钛材料。结果表明:二甲基-正丁基-磺化木质素基氯化铵含氮量为1.360%,含硫量1.860%;红外光谱和1H NMR证实产物中存在磺化和季胺的化学结构。溴酚蓝和亚甲基蓝实验表明二甲基-正丁基-磺化木质素基氯化铵具有两性化合物结构,溶于碱性、中性和稀酸溶液。随着模板剂与钛酸正丁酯的质量比增加,多孔钛材料的比表面积加大;继续增加模板剂的量,多孔钛材料的比表面积下降。多孔钛材料孔径分布范围较宽,为晶态,主要以锐钛矿形式存在。煅烧后的多孔材钛料中有TiO2特征吸收峰,也有少量残留的木质素。当二甲基-正丁基-磺化木质素基氯化铵与钛酸正丁酯的质量比为0.65∶1、煅烧温度为450 ℃时,可得到最大比表面积、锐钛矿晶态的多孔钛材料。      

Crystal structure of the human K2P TRAAK, a lipid- and mechano-sensitive K+ ion channel

TRAAK channels, members of the two-pore domain K(+) (potassium ion) channel family K2P, are expressed almost exclusively in the nervous system and control the resting membrane potential. Their gating is sensitive to polyunsaturated fatty acids, mechanical deformation of the membrane, and temperature changes. Physiologically, these channels appear to control the noxious input threshold for temperature and pressure sensitivity in dorsal root ganglia neurons. We present the crystal structure of human TRAAK at a resolution of 3.8 angstroms. The channel comprises two protomers, each containing two distinct pore domains, which create a two-fold symmetric K(+) channel. The extracellular surface features a helical cap, 35 angstroms tall, that creates a bifurcated pore entryway and accounts for the insensitivity of two-pore domain K(+) channels to inhibitory toxins. Two diagonally opposed gate-forming inner helices form membrane-interacting structures that may underlie this channel's sensitivity to chemical and mechanical properties of the cell membrane.     

The structure of ferrihydrite, a nanocrystalline material

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.