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Biomedical subjects

G D Stucky

Publications and source records attributed to G D Stucky.

12 recordsLinked to original sources
PubMed2003

Atomic force microscopy study of living diatoms in ambient conditions.

We present the first in vivo study of diatoms using atomic force microscopy (AFM). Three chain-forming, benthic freshwater species -Eunotia sudetica, Navicula seminulum and a yet unidentified species - are directly imaged while growing on glass slides. Using the AFM, we imaged the topography of the diatom frustules at the nanometre range scale and we determined the thickness of the organic case enveloping the siliceous skeleton of the cell (10 nm). Imaging proved to be stable for several hours, thereby offering the possibility to study long-term dynamic changes, such as biomineralization or cell movement, as they occur. We also focused on the natural adhesives produced by these unicellular organisms to adhere to other cells or the substratum. Most man-made adhesives fail in wet conditions, owing to chemical modification of the adhesive or its substrate. Diatoms produce adhesives that are extremely strong and robust both in fresh- and in seawater environments. Our phase-imaging and force-pulling experiments reveal the characteristics of these natural adhesives that might be of use in designing man-made analogues that function in wet environments. Engineering stable underwater adhesives currently poses a major technical challenge.

Adhesiveness↗
PubMed2002

Siting of antimony dopants and gallium in Ba(8)Ga(16)Ge(30) clathrates grown from gallium flux.

A series of antimony-doped Ba(8)Ga(16)Ge(30) clathrates was grown as large crystals from gallium flux. These compounds form in the cubic space group Pm(-)3n, with the unit cell parameter varying from 10.784(5) to 10.9008(6) A as the amount of GaSb substituting for germanium atoms in the framework is increased. It was found that more antimony than extra gallium was incorporated into the material and that a specific site (the 24k Wyckoff site) was favored by this element. (71)Ga NMR was carried out to determine the siting of gallium; it fills the 6c site preferentially.

Journal Article↗
PubMed2001

Mesostructured materials for optical applications: from low-k dielectrics to sensors and lasers.

Recent advances on the use of mesoporous and mesostructured materials for electronic and optical applications are reported. The focus is on materials which are processed by block-copolymer templating of silica under weakly acidic conditions and by employing dip- and spin-coating as well as soft lithographic methods to bring them into a well-defined macroscopic shape. Several chemical strategies allow the mesostructure architecture to be used for electronic/optical applications: Removal of the block-copolymers results in highly porous, mechanically and thermally robust materials which are promising candidates for low dielectric constant materials. Since the pores are easily accessible, these structures are also ideal hosts for optical sensors, when suitable are incorporated during synthesis. For example, a fast response optical pH sensor was implemented on this principle. As-synthesized mesostructured silica/block-copolymer composites, on the other hand, are excellently suited as host systems for laser dyes and photochromic molecules. Laser dyes like rhodamine 6G can be incorporated during synthesis in high concentrations with reduced dimerization. This leads to very-low-threshold laser materials which also show a good photostability of the occluded dye. In the case of photochromic molecules, the inorganic-organic nanoseparation enables a fast switching between the colorless and colored form of a spirooxazine molecule, attributed to a partitioning of the dye between the block-copolymer chains. The spectroscopic properties of these dye-doped nanocomposite materials suggest a silica/block-copolymer/dye co-assembly process, whereby the block-copolymers help to highly disperse the organic dye molecules.

Electronics↗
PubMed2001

Molecularly ordered inorganic frameworks in layered silicate surfactant mesophases.

Self-assembled lamellar silica-surfactant mesophase composites have been prepared with crystal-like ordering in the silica frameworks using a variety of cationic surfactant species under hydrothermal conditions. These materials represent the first mesoscopically ordered composites that have been directly synthesized with structure-directing surfactants yielding highly ordered inorganic frameworks. One-dimensional solid-state 29Si NMR spectra, X-ray diffraction patterns, and infrared spectra show the progression of molecular organization in the self-assembled mesophases from structures with initially amorphous silica networks into sheets with very high degrees of molecular order. The silicate sheets appear to be two-dimensional crystals, whose structures and rates of formation depend strongly on the charge density of the cationic surfactant headgroups. Two-dimensional solid-state heteronuclear and homonuclear NMR measurements show the molecular proximities of the silica framework sites to the structure-directing surfactant molecules and establish local Si-O-Si bonding connectivities in these materials.

Journal Article↗
PubMed2001

A novel fluorescent silica tracer for biological silicification studies.

BACKGROUND: Biological silica production has drawn intense attention and several molecules involved in biosilicification have been identified. Cellular mechanisms, however, remain unknown mainly due to the lack of probes required for obtaining information on live specimens. RESULTS: The fluorescence spectra of the compound 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole (PDMPO) are affected by the presence of >3.2 mM silicic acid. Increase in intensity and shift in the fluorescence coincide with the polymerization of Si. The unique PDMPO-silica fluorescence is explored here to visualize Si deposition in living diatoms. The fluorophore is selectively incorporated and co-deposited with Si into the newly synthesized frustules (the outer silica shells) showing an intense green fluorescence. CONCLUSIONS: We suggest that a fluorescence shift is due to an interaction between PDMPO and polymeric silicic acid. PDMPO is an excellent probe for imaging newly deposited silica in living cells and has also a potential for a wide range of applications in various Si-related disciplines, including biology of living organisms as diatoms, sponges, and higher plants, clinical research (e.g. lung fibrosis and cancer, bone development, artificial bone implantation), and chemistry and physics of materials research.

Diatoms↗
PubMed2000

Biomimetic synthesis of ordered silica structures mediated by block copolypeptides.

In biological systems such as diatoms and sponges, the formation of solid silica structures with precisely controlled morphologies is directed by proteins and polysaccharides and occurs in water at neutral pH and ambient temperature. Laboratory methods, in contrast, have to rely on extreme pH conditions and/or surfactants to induce the condensation of silica precursors into specific morphologies or patterned structures. This contrast in processing conditions and the growing demand for benign synthesis methods that minimize adverse environmental effects have spurred much interest in biomimetic approaches in materials science. The recent demonstration that silicatein-a protein found in the silica spicules of the sponge Tethya aurantia--can hydrolyse and condense the precursor molecule tetraethoxysilane to form silica structures with controlled shapes at ambient conditions seems particularly promising in this context. Here we describe synthetic cysteine-lysine block copolypeptides that mimic the properties of silicatein: the copolypeptides self-assemble into structured aggregates that hydrolyse tetraethoxysilane while simultaneously directing the formation of ordered silica morphologies. We find that oxidation of the cysteine sulphydryl groups, which is known to affect the assembly of the block copolypeptide, allows us to produce different structures: hard silica spheres and well-defined columns of amorphous silica are produced using the fully reduced and the oxidized forms of the copolymer, respectively.

Cysteine↗
PubMed1999

Silicatein filaments and subunits from a marine sponge direct the polymerization of silica and silicones in vitro.

Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators, optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis experiments. The catalytic activity of the "silicatein" (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.

Actin Cytoskeleton↗
PubMed1998

Silicatein alpha: cathepsin L-like protein in sponge biosilica.

Earth's biota produces vast quantities of polymerized silica at ambient temperatures and pressures by mechanisms that are not understood. Silica spicules constitute 75% of the dry weight of the sponge Tethya aurantia, making this organism uniquely tractable for analyses of the proteins intimately associated with the biosilica. Each spicule contains a central protein filament, shown by x-ray diffraction to exhibit a highly regular, repeating structure. The protein filaments can be dissociated to yield three similar subunits, named silicatein alpha, beta, and gamma. The molecular weights and amino acid compositions of the three silicateins are similar, suggesting that they are members of a single protein family. The cDNA sequence of silicatein alpha, the most abundant of these subunits, reveals that this protein is highly similar to members of the cathepsin L and papain family of proteases. The cysteine at the active site in the proteases is replaced by serine in silicatein alpha, although the six cysteines that form disulfide bridges in the proteases are conserved. Silicatein alpha also contains unique tandem arrays of multiple hydroxyls. These structural features may help explain the mechanism of biosilicification and the recently discovered activity of the silicateins in promoting the condensation of silica and organically modified siloxane polymers (silicones) from the corresponding silicon alkoxides. They suggest the possibility of a dynamic role of the silicateins in silicification of the sponge spicule and offer the prospect of a new synthetic route to silica and siloxane polymers at low temperature and pressure and neutral pH.

Amino Acid Sequence↗
PubMed1997

Molecular cloning and characterization of lustrin A, a matrix protein from shell and pearl nacre of Haliotis rufescens.

A specialized extracellular matrix of proteins and polysaccharides controls the morphology and packing of calcium carbonate crystals and becomes occluded within the mineralized composite during formation of the molluscan shell and pearl. We have cloned and characterized the cDNA coding for Lustrin A, a newly described matrix protein from the nacreous layer of the shell and pearl produced by the abalone, Haliotis rufescens, a marine gastropod mollusc. The full-length cDNA is 4,439 base pairs (bp) long and contains an open reading frame coding for 1,428 amino acids. The deduced amino acid sequence reveals a highly modular structure with a high proportion of Ser (16%), Pro (14%), Gly (13%), and Cys (9%). The protein contains ten highly conserved cysteine-rich domains interspersed by eight proline-rich domains; a glycine- and serine-rich domain lies between the two cysteine-rich domains nearest the C terminus, and these are followed by a basic domain and a C-terminal domain that is highly similar to known protease inhibitors. The glycine- and serine-rich domain and at least one of the proline-rich domains show sequence similarity to proteins of two extracellular matrix superfamilies (one of which also is involved in the mineralized matrixes of bone, dentin, and avian eggshell). The arrangement of alternating cysteine-rich domains and proline-rich domains is strikingly similar to that found in frustulins, the proteins that are integral to the silicified cell wall of diatoms. Its modular structure suggests that Lustrin A is a multifunctional protein, whereas the occurrence of related sequences suggest it is a member of a multiprotein family.

Amino Acid Sequence↗
PubMed1997

Modification of calcite crystal growth by abalone shell proteins: an atomic force microscope study.

A family of soluble proteins from the shell of Haliotis rufescens was introduced over a growing calcite crystal being scanned in situ by an atomic force microscope (AFM). Atomic step edges on the crystal surface were altered in shape and speed of growth by the proteins. Proteins attached nonuniformly to the surface, indicating different interactions with crystallographically different step edges. The observed changes were consistent with the habit modification induced by this family of proteins, as previously observed by optical microscopy. To facilitate further studies in this area, AFM techniques and certain AFM imaging artifacts are discussed in detail.

Animals↗