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Biomedical subjects

Mary Elizabeth Williams

Publications and source records attributed to Mary Elizabeth Williams.

15 recordsLinked to original sources
PubMed2006

TEM-induced structural evolution in amorphous Fe oxide nanoparticles.

Exposure to the high energy electron beam of a TEM changes the morphology of amorphous Fe oxide nanoparticles from solid spheres to hollow shells. Amorphous Fe oxide nanoparticles prepared via high-temperature methods using hexadecylamine and trioctylphosphine oxide surfactants were compared to crystalline gamma-Fe2O3 particles of similar size. Both sets of particles are fully characterized via SQUID magnetometry, X-ray powder diffraction, BET surface analysis, EPR spectroscopy, high-resolution transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). Time-resolved TEM images reveal that the amorphous Fe oxide particles evolve from solid spheres into hollow shells in <2 min, whereas crystalline gamma-Fe2O3 are unaffected by the electron beam. The resulting nanocrystalline Fe oxide shells bear striking resemblance to core-shell nanocrystals, but are a result of a morphology change attributed to restructuring of particle voids and defects induced by quasi-melting in the TEM. These results thus imply that caution is necessary when using TEM to analyze nanoparticle core-shell and heterostructured nanoparticles.

Journal Article↗
PubMed2006

Versatile routes toward functional, water-soluble nanoparticles via trifluoroethylester-PEG-thiol ligands.

This paper reports the synthesis of a trifluoroethylester-PEG-thiol ligand (TFEE-PEG-SH) and its use to create water-soluble, chemically functional Au metal and FePt magnetic nanoparticles. The trifluoroethylester terminus facilitates attachment of any primary-amine-containing molecule via amide bond formation at room temperature without the use of coupling agents. Three possible routes of nanoparticle functionalization are demonstrated: synthesis of Au nanoparticles in the presence of functionalized R-PEG-SH; ligand-exchange of R-PEG-SH onto both Au and FePt nanoparticles; and exchange of TFEE-PEG-SH onto Au nanoparticles followed by subsequent amide condensation. A series of primary-amine-containing molecules, including biotin and fluorescamine, are easily attached to the water-soluble particles, and the resulting materials are characterized by NMR, UV-visible absorption, and emission spectroscopies.

Journal Article↗
PubMed2005

Tetraplatinated artificial oligopeptides afford high affinity intercalation into dsDNA.

This paper reports the binding of an artificial tetrapeptide to which are tethered four Pt(II) complexes (i.e., [Pt(tpy)(py)]48+) with a 12 base pair duplex DNA oligonucleotide. Isothermal Titration Calorimetry reveals that two tetrametallic peptides stoichiometrically bind to each DNA duplex with a binding constant, KB, of 1.7 x 106 M-1, with a change in free energy of -8.5 kcal/mol. This KB represents an affinity 2 orders of magnitude greater than that of the monometallic analogue [Pt(tpy)(pic)]2+ for the same dsDNA sequence. The metalated peptides bind by intercalation into the DNA, partially unwinding the helix while stabilizing the structure, causing an increase in the dsDNA melting temperature of 25 degrees C.

Circular Dichroism↗
PubMed2005

Millimeter scale alignment of magnetic nanoparticle functionalized microtubules in magnetic fields.

The conjugation of 14 nm diameter CoFe2O4 nanoparticles to the surface of biotinylated microtubules enables their manipulation with externally applied magnetic fields of small, permanent NdFeB magnets. Microtubules are selectively patterned on kinesin motor-modified glass surfaces in coparallel arrays that mimic the orientation of the magnetic field lines over millimeter distances. The magnetic field is simultaneously used to increase surface loading of microtubules. We demonstrate that motility across the kinesin motor surface is retained following magnetic functionalization of the microtubules, while gliding speed is dependent on loading level of the neutravidin linker as well as magnetic nanoparticles.

Animals↗
PubMed2005

Pyridine-substituted oligopeptides as scaffolds for the assembly of multimetallic complexes: variation of chain length.

This paper presents the synthesis and characterization of pyridine-substituted artificial oligopeptides with an aminoethylglycine backbone of varying length, which are designed to act as scaffolds for the self-assembly of multimetallic structures. The identities and purities of the oligopeptides are confirmed with mass spectrometry, (1)H NMR, HPLC, and pH titrations. The acid dissociation constants for the oligopeptides were determined and were found to decrease with increasing pyridine units. Titrations of the oligopeptides with Cu(II) and Pt(II) complexes containing the tridentate ligands 2,2':6',2''-terpyridine and pyridine 2,6-dicarboxylic acid were monitored using UV-visible absorption spectroscopy and showed stoichiometric binding based on the number of pyridines on the peptide strand. Metal titrations performed using an analogous oligopeptide with methyl substituents (in place of the pyridine ligands) showed very weak or no binding. In the case of the oligopeptides containing bound Pt(terpyridine)(2+) complexes, cyclic voltammetry reveals two sequential one-electron reductions at formal potentials that do not vary as a function of oligopeptide length. The measured diffusion coefficients were measured with chronoamperometry and were found to decrease with increasing oliopeptide length.

Copper↗
PubMed2005

Ion atmosphere relaxation controlled electron transfers in cobaltocenium polyether molten salts.

A room-temperature redox molten salt for the study of electron transfers in semisolid media, based on combining bis(cyclopentadienyl)cobalt with oligomeric polyether counterions, [Cp2Co](MePEG350SO3), is reported. The transport properties of the new molten salt can be varied (plasticized) by varying the polyether content. The charge transport rate during voltammetric reduction of the ionically conductive [Cp2Co](MePEG350SO3) molten salt exceeds the actual physical diffusivity of [Cp2Co]+ because of rapid [Cp2Co](+/0) electron self-exchanges. The measured [Cp2Co](+/0) electron self-exchange rate constants (k(EX)) are proportional to the diffusion coefficients (D(CION)) of the counterions in the melt. The electron-transfer activation barrier energies are also close to those of ionic diffusion but are larger than those derived from optical intervalent charge-transfer results. Additionally, the [Cp2Co](+/0) rate constant results are close to those of dissimilar redox moieties in molten salts where D(CION) values are similar. All of these characteristics are consistent with the rates of electron transfers of [Cp2Co](+/0) (and the other donor-acceptor pairs) being controlled not by the intrinsic electron-transfer rates but by the rate of relaxation of the ion atmosphere around the reacting pair. In the low driving force regime of mixed-valent concentration gradients, the ion atmosphere relaxation is competitive with electron transfer. The results support the generality of the recently proposed model of ionic atmosphere relaxation control of electron transfers in ionically conductive, semisolid materials.

Journal Article↗
PubMed2005

Capillary magnetic field flow fractionation and analysis of magnetic nanoparticles.

This paper reports the purification and analysis of magnetic nanoparticles using capillary magnetic field flow fractionation, which utilizes an applied magnetic field oriented orthogonal to the capillary flow. To validate this approach as a separation method for nanometer-scale particles, samples of magnetic nanoparticles composed of either gamma-Fe2O3 (maghemite) or CoFe2O4 with average diameters ranging from 4 to 13 nm were prepared and characterized by transmission electron microscopy and SQUID magnetometry. Retention of the samples on the capillary was investigated as a function of solvent flow rate and the nanoparticle size and composition; the elution times of the nanoparticles are strongly dependent on their magnetic moments. We demonstrate the use of this method to separate a mixture of nanoparticles into size-monodisperse fractions. The magnetic moments of the particles are calculated based on analysis of the retention parameters and correlate with values obtained in separate SQUID magnetometry measurements.

Journal Article↗
PubMed2005

Artificial oligopeptide scaffolds for stoichiometric metal binding.

Two artificial peptides with pendant pyridine or bipyridine ligands have been synthesized and incorporated into oligomeric strands that are analogous to peptide nucleic acid. Spectrophotometric titrations with Cu(2+) and Fe(2+) show that the oligomers bind stoichiometric quantities of transition metals based on the number of pendant ligands. The identities of the titration products are confirmed by high resolution mass spectrometry. In the case of the bipyridine tripeptides, the titration stoichiometry and mass spectra indicate that the metal ions form interstrand cross-links between two oligopeptides, creating duplex structures linked exclusively by metal ions. Calculated molecular structures of the metalated oligopeptides and duplexes indicate that the peptide backbone acts as a scaffold for the directed assembly of metal ions. Electron paramagnetic resonance spectroscopy of the Cu-containing molecules have varying degrees of electronic interaction based on their charge and supramolecular structure. Cyclic voltammetry of the Fe(2+)- and Cu(2+)-linked bpy oligopeptide duplexes shows that they possess unique electrochemical signatures based on the redox reactivity of the metal complex.

Cations, Divalent↗
PubMed2005

Mutations in the Arabidopsis phosphoinositide phosphatase gene SAC9 lead to overaccumulation of PtdIns(4,5)P2 and constitutive expression of the stress-response pathway.

Phosphoinositides (PIs) are signaling molecules that regulate cellular events including vesicle targeting and interactions between membrane and cytoskeleton. Phosphatidylinositol (PtdIns)(4,5)P(2) is one of the best characterized PIs; studies in which PtdIns(4,5)P(2) localization or concentration is altered lead to defects in the actin cytoskeleton and exocytosis. PtdIns(4,5)P(2) and its derivative Ins(1,4,5)P(3) accumulate in salt, cold, and osmotically stressed plants. PtdIns(4,5)P(2) signaling is terminated through the action of inositol polyphosphate phosphatases and PI phosphatases including supressor of actin mutation (SAC) domain phosphatases. In some cases, these phosphatases also act on Ins(1,4,5)P(3). We have characterized the Arabidopsis (Arabidopsis thaliana) sac9 mutants. The SAC9 protein is different from other SAC domain proteins in several ways including the presence of a WW protein interaction domain within the SAC domain. The rice (Oryza sativa) and Arabidopsis SAC9 protein sequences are similar, but no apparent homologs are found in nonplant genomes. High-performance liquid chromatography studies show that unstressed sac9 mutants accumulate elevated levels of PtdIns(4,5)P(2) and Ins(1,4,5)P(3) as compared to wild-type plants. The sac9 mutants have characteristics of a constitutive stress response, including dwarfism, closed stomata, and anthocyanin accumulation, and they overexpress stress-induced genes and overaccumulate reactive-oxygen species. These results suggest that the SAC9 phosphatase is involved in modulating phosphoinsitide signals during the stress response.

Amino Acid Sequence↗
PubMed2004

Rhenium-based molecular rectangles as frameworks for ligand-centered mixed valency and optical electron transfer.

A series of six neutral, tetrametallic, molecular rectangles has been synthesized that have the form ([Re(CO)(3)](2)BiBzIm)(2)-mu,mu'-(LL)(2), where BiBzIm is 2,2'-bisbenzimidazolate and LL is a reducible, dipyridyl or diazine ligand. X-ray crystallographic studies of the six show that the rectangle frameworks, as defined by the metal atoms, range in size from 5.7 A x 7.2 A to 5.7 A x 19.8 A. The singly reduced rectangles are members of an unusual category of mixed-valence compounds in which the ligands themselves are the redox centers and interligand electronic communication is controlled by direct ligand orbital overlap rather than by superexchange through the metal ions. Despite nominally identical coordination-defined ligand positioning, the spectrally determined electronic strengths, H(ab)2, vary by roughly 100-fold. As shown by X-ray crystallography and computational modeling, the observed differences largely reflect detailed geometric configurational differences that can either facilitate or frustrate productive direct orbital overlap.

Journal Article↗
PubMed2002

Shape-selective transport through rectangle-based molecular materials: thin-film scanning electrochemical microscopy studies.

Microporous thin films (approximately equal to 50 to 400 nm) composed of discrete, cavity-containing molecular rectangles have been prepared. The films, which contain both amorphous and microcrystalline domains, display shape-selective transport behavior. They are permeable to small molecules and to molecules that are short or narrow in at least one dimension--for example, elongated planar molecules--but are impermeable to molecules lacking a narrow dimension. However, the shape selectivity is based on transport through intramolecular rather than intermolecular cavities. By using redox-active probe molecules, rates of transport through the rectangle-based material have been extracted from electrochemical measurements. Spatially resolved measurements obtained via scanning electrochemical microscopy have permitted transport through individual microcrystals to be evaluated semiquantitatively. The measurements reveal that transport is roughly two orders of magnitude slower than observed with thin microcrystalline films of molecular squares featuring similar-sized cavities. The differences likely reflect the fact that cavities within the square-based materials, but not the rectangle-based material, align to form simple one-dimensional channels.

Carbon↗
PubMed1997

Redox Polyether Hybrid Copper Bipyridine Complex Molten Salts.

The covalent attachment of poly(ethylene oxide) chains to metal bipyridine complexes produces viscous molecular melts with sufficient ionic conductivity to permit microelectrode voltammetry. This paper describes the use of this chemistry to prepare a new copper complex with "tailed" bipyridine ligands, [Cu(bpy(CO(2)MePEG-350)(2))(2)](ClO(4))(2), where MePEG-350 is methyl poly(ethylene glycol) with molecular weight 350 g mol(-)(1). The coupling of physical diffusion with electron hopping during voltammetry allows measurement of the electron self-exchange rate constant, k(EX), for the Cu(II/I) couple. Activation studies indicate the reaction to be nearly adiabatic, with an activation barrier larger than expected for purely inner-sphere rearrangements. This study also examines the depression of both electron transport and ionic conductivity caused by the addition of 1.4 M LiClO(4) to the undiluted room-temperature melt.

Journal Article↗