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.     

Hindered diffusion in microporous membranes with known pore geometry

The hindrance effect on the aqueous diffusion rate of solutes within membrane pores of molecular size has been accurately determined. Mica membranes, 3 to 5 micrometers thick, were prepared with uniform, straight pores from 90 to 600 angstroms in diameter. With these membranes a direct estimation was possible of the interaction between pore size and molecular diffusion rates. There were no uncertainties due to wide pore size distributions or nonuniform tortuous channels as in previously used model microporous materials such as dialysis tubing or gels. Aqueous diffusion rates through these mica membranes were measured for a series of compounds with molecular diameters from 5.2 to 43 angstroms and were corrected for "liquid film resistances" adjacent to the membrane-solution interface to obtain estimates of molecular diffusivities within the micropores of the membrane. Definite evidence is presented showing that, even when molecular size is a small fraction of pore size, diffusion rates decrease markedly. The apparent reduction in solute diffusivity in the microporous membrane can be quantitatively estimated by means of the Renkin equation for hindered diffusion.     

Crystalline transfer RNA: the three-dimensional Patterson function at 12-angstrom resolution

An orthorhombic form of crystalline formylmethionine transter RNA has been obtained which contains one molecule as the asymmetric unit of the unit cell. Three-dimensional x-ray diffraction data have been collected up to a resolution of 12 angstroms, and from this a Patterson function has been calculated. The function contains an elongated ridge of interatomic vectors parallel to the c-axis of the crystal. Analysis of the function suggests that the molecules are elogated and dimerized in an overlapping antiparrael fashion along the c-axis. The dimer has a length near 109 angstroms and a width of 35 angstroms in one direction. The individual molecular length is approximately 80 angstroms with an irregular cross section measuring 25 by 35 angstrms.     

Three-dimensional structure at 0.86 A of the uncomplexed form of the transmembrane ion channel peptide gramicidin A

The crystal structure of the uncomplexed orthorhombic form of gramicidin A has been determined at 120 K and at 0.86 angstrom resolution. The pentadecapeptide crystallizes as a left-handed antiparallel double-stranded helical dimer with 5.6 amino acid residues per turn. The helix has an overall length of 31 angstroms and an average inner channel diameter of 4.80 angstroms. The channel of this crystalline form is void of ions or solvent molecules. The channel diameter varies from a minimum of 3.85 angstroms to a maximum of 5.47 angstroms and contains three pockets where the cross-channel contacts are 5.25 angstroms or greater. The range of variation seen for the phi and psi torsion angles of the backbone of the helix suggests that these potential ion binding sites can be induced to travel the length of the channel in a peristaltic manner by cooperatively varying these angles. The indole rings of the eight tryptophan residues of the dimer are overlapped in three separate regions on the outer surface of the helix when viewed down the barrel of the channel. This arrangement would permit long-chained lipid molecules to nest parallel to the outer channel surface between these protruding tryptophan regions and act like molecular splines to constrain helical twist deformations of the channel.     

DNA structure: evidence from electron microscopy

The contour lengths of phiX174 DNA duplex and RNA-DNA hybrid molecules were measured by several commonly used electron microscopic techniques. The countour length of the hybrid molecules corresponds to a rise of 2.5 to 2.6 angstroms per base pair, as expected for the A conformation, while the length of phiX174 duplex DNA similarly measured corresponds to a 2.9-angstrom rise, very different from 3.4 angstroms of the classic B form. Thus any chromatin structure parameter based on electron microscopy and a rise of 3.4 angstroms must be reappraised. The possibility that DNA in dilute solution also has a rise of 2.9 angstroms and a screw of 10.5 base pairs per turn is discussed.     

Structure of langmuir-blodgett films of disk-shaped molecules determined by atomic force microscopy

Monolayer Langmuir-Blodgett films of a discotic mesogen have been studied with atomic force microscopy (AFM). These measurements confirm the "edge on" arrangement for the disk-shaped molecules suggested by surface pressure-area isotherms and show that the molecules form columns that are separated by 17.7 angstroms +/- 10 percent. Column alignment is found to be predominantly along the film deposition direction, with an angular spread of 35 degrees . The AFM images also show that the mean disk separation within the columns is 5.1 +/- 1.3 angstroms, in good agreement with x-ray diffraction (XRD) results. Roomtemperature XRD measurements on bulk samples of the same material indicate a disordered-hexagonal liquid crystalline mesophase, with a column-to-column spacing of 19.9 +/- 0.2 angstroms.     

Problem of sieve-tube slime

It is proposed that sieve tubes contain a stationary, living, filamentous reticulum that persists throughout their functioning life. The filaments are about 100 to 150 angstroms in diameter and often striated. By swelling they block the pores of old sieve plates after slime has been lost from the elements. In Cucurbita, slime may be loosely bound to the filaments; in many species it is hydrolyzed, swept out by the assimilate stream, and eventually metabolized.     

Direct determination of ionic solvation from neutron diffraction

Much information on ionic solvation in electrolyte solutions has been inferred from macroscopic thermodynamic and transport properties and from spectroscopy. These ion-water interactions can now be probed directly and unambiguously by neutron diffraction. Such measurements have been done with neodymium trichloride solutions in heavy water that are identical in every respect except the isotopic state of the neodymium ions; these experiments yield in a straightforward manner the distribution of oxygen and deuterium atoms from the water molecules in the first hydration sphere of the neodymium ion. Each ion is surrounded by 8.6 oxygen atoms at a distance of 2.48 angstroms and 16.7 deuterium atoms at 3.13 angstroms indicating a well-defined first hydration sphere of water molecules, the deuterium atoms pointing away from the cation.     

Calibrated membranes with coated pore walls

Extremely uniform small-radius pores, formed in thin mica membranes, have been coated with monoand multimolecular layers of fatty acids. Stearate monolayers orient themselves on the pore walls in layers 25 to 28 angstroms thick. The resulting membranes, with radii of the order 100 angstroms or less, may have applications in simulating certain features of biological membranes and in fabricating highly selective membranes for differential dialysis or ultrafiltration.     

NMR studies of simple molecules on metal surfaces

In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possible to detect progressively smaller numbers of nuclei. Experiments and studies previously thought to be impractical can now be undertaken, for example, the study of phenomena at surfaces. Nuclear magnetic resonance has been applied to study simple molecules (carbon monoxide, acetylene, and ethylene) adsorbed on metal surfaces (ruthenium, rhodium, palladium, osmium, iridium, and platinum). The metals, in the form of clusters 10 to 50 angstroms in diameter, supported on alumina, are typical of real catalysts. The experiments provide information about the bonding of the molecules to the metal, the structures the molecules assume after adsorption, the motion of molecules on the surface, the breakup of molecules induced by heating, and the products of such breakup.     

Photolytic cleavage of sulfonamide bonds

It has been found that the sulfonamide bond is relatively susceptible to photolytic cleavage. The breakdown was effected either by irradiation with a source having a continuous emission above the wavelengths of 1800 angstroms or by another source emitting principally at 2537 angstroms. Less destruction of the amino acids was seen with the latter relative to the sulfonamide bond cleavage. The cleavage was not effected by irradiation at wavelengths greater than about 3000 angstroms. Side reactions were noted involving decarboxylation, demination, and destruction of certain susceptible amino acids such as tryptophan. In only one case was a product found that arose from cleavage of a carboxamide bond; glycyltyrosine gave glycine and tyrosine upon irradiation. A yield of 75 percent of the corresponding amino acid has been obtained by irradiation of tosylhistidine; yields of 75 to 100 percent have been obtained from sulfamic acid (NH(2)SO(3)H). A qualitative method for identifying sulfonylated amino acids is described.     

Structure of a dinucleotide: thymidylyl-(5'-3')-thymidylate-5' (pTpT)

The crystal and molecular structure of the naturally occurring deoxyribose dinucleotide sodium thymidylyl-(5'-->3')-thymidylate-5' has been determined by x-ray diffraction. There are four molecules of dinucleotide and 52 water molecules in an orthorhombic unit cell of dimensions (in angstroms) a = 16.06, b = 15.13, c = 15.65, space group P2(1)2(1)2. There is a very high degree of conformational consistency between the two halves of the molecule when the dinucleotide is viewed as the combination of two 5'-mononucleotides. The planes of the two thymines are not parallel, but are tilted 38 degrees with respect to each other. An extensive system of hydrogen bonding exists involving the bases, waters, phosphates, and sodiums; no base-base hydrogen bonding occurs. The dinucleotide structural parameters should be of assistance in interpreting DNA fiber diagrams in terms of possible structures.     

Structure of an oxygen-carrying cobalt complex

Bis(3-fluorosalicylaldehyde) ethylenediimine cobalt(II), a reversible oxygen-carrying compound, has been crystallized as a tetramer with two oxygen molecules bound to the four cobalt atomns, each oxygen molecule bridging between two metal atoms. These dimers are further linked by two bonds between the oxygen atom of the salicylaldehyde and the cobalt atom. The oxygen-to-oxygen distance, 1.21 angstroms in molecular oxygen, has increased to 1.308(28) angstroms (where the number in parentheses is the estimated standard deviation) in this compound, the shortest distance yet observed in sutch a bridging arrangement.     

Protein, DNA, and virus crystallography with a focused imaging proportional counter

A set of programs has been developed for rapid collection of x-ray intensity data from protein and virus crystals with a commercially available two-dimensional focused geometry electronic detector. The detector is compact and portable, with unusually high spatial resolution comparable to that used in oscillation photography. It has allowed x-ray data collection on weakly diffracting crystals with large unit cells, as well as more conventional "diffractometer-quality" crystals. The quality of the data is compared with that from oscillation photography and automated diffractometry in the range of unit cells from 96.3 to 383.2 angstroms. Isomorphous and anomalous difference Pattersons, based on detector data, are shown for a variable surface glycoprotein mercury derivative and for a repressor-DNA bromine derivative, which has been solved at 7 angstroms with detector data only.     

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.     

Formaldehyde absorption coefficients in the vacuum ultraviolet (650 to 1850 angstroms)

The absorption spectrum of formaldehyde has been measured photo-electrically from 650 to 1850 angstroms. A broad continuum observed at wavelengths shorter than 1570 angstroms will be the major spectral feature contributing to the formaldehyde lifetime in the interstellar radiation field.     

Palladium: preparation and catalytic properties of particles of uniform size

A method has been developed for the preparation of uniform palladium particles of diameter from 55 to 450 angstroms. Uniform particles of gold layered on palladium were also synthesized. Hydrothermal treatment of aluminum hydroxide sol was used to prepare rods of alumina with uniform cross section from 100 to 500 angstroms and of varying lengths. The palladium was adsorbed as individual particles on alumina rods, both present in aqueous suspension. Then the suspension was dried to give a catalyst containing metal particles of uniform size dispersed in open pores produced by the intermeshing of the alumina rods. This procedure guaranteed the absence of diffusion control in the rate of reactions observed experimentally. All stages of the preparation were monitored with the electron microscope. The kinetics of the ethylene-hydrogen reaction were examined by means of a pulse technique. The number of active sites determined by carbon monoxide titration of the surface was equal to the number of surface atoms as determined by the calculation of the quantities of compounds involved in the synthesis and electron microscope examination. Furthermore, the activity per site depended on the method of preparation, being four times smaller when sodium formate was used as a reducing agent instead of sodium citrate. This may be due to the fact that the shape of particles makes certain crystallographic planes more favorable. Decrease in the size of particles to 56 angstroms produced no effect on catalytic activity beyond that due to the increase in the number of surface atoms. The activity of commercial 5 percent palladium on alumina diluted 100-fold with alumina gave 80.4 percent conversion with propylene and 82.7 percent conversion with ethylene. Thus there was little difference in the behavior of the two olefins.     

Ultrastructure of secretory and high-polymer serum immunoglobulin A of human and rabbit origin

Electron micrographs of immunoglobulins A from human and rabbit colostrum, which were purified on tall agarose columns, revealed Y-shaped molecules (125 by 140 angstroms). The linear dimensions of the arms were 55 to 75 by 25 to 30 angstroms. A molecular model is postulated in which two immunoglobulin A monomers are superimposed on each other in a close-packed state with the secretory piece inserted in the constant region of the alpha-chains. High-polymer (11 or 13S) immunoglobulin A molecules (total span 100 to 110 angstroms) from human serum were composed of four arms (50 to 55 by 20 angstroms) joined at a contrast-rich center.     

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.     

Gas adsorption sites in a large-pore metal-organic framework

The primary adsorption sites for Ar and N2 within metal-organic framework-5, a cubic structure composed of Zn4O(CO2)6 units and phenylene links defining large pores 12 and 15 angstroms in diameter, have been identified by single-crystal x-ray diffraction. Refinement of data collected between 293 and 30 kelvin revealed a total of eight symmetry-independent adsorption sites. Five of these are sites on the zinc oxide unit and the organic link; the remaining three sites form a second layer in the pores. The structural integrity and high symmetry of the framework are retained throughout, with negligible changes resulting from gas adsorption.