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PubMed2004

Organic spin clusters: annelated macrocyclic polyarylmethyl polyradicals and a polymer with very high spin S=6-18.

Synthesis and magnetic studies of annelated macrocyclic polyradicals and a related high-spin polymer with macrocyclic repeat units are described. Polyarylmethyl polyether precursors to the polyradicals and the related polymer are prepared by using Negishi cross-coupling of difunctionalized calix[4]arene-based macrocycles. The three lowest homologues, with high degree of monodispersity, are tetradecaether (14-ether) 3-(OCH(3))(14), octacosaether (28-ether) 4-(OCH(3))(28), and dotetracontaether (42-ether) 5-(OCH(3))(42), in which 2, 4, and 6 calix[4]arene-based macrocycles are annelated to the center macrocycle, respectively. The evidence for their annelated structures (ladder connectivities) is based upon FAB-MS and the (1)H NMR based end-group analysis. The absolute masses (4-12 kDa) were determined by FAB-MS and GPC/MALS. Small angle neutron scattering (SANS) provides the radii of gyration of 1.7, 2.0, and 3.2 nm for 4-(OCH(3))(28), 5-(OCH(3))(42), and polymer 6-(OCH(3))(n), respectively. The corresponding polyarylmethyl polyradicals 3 and 4, and polymer 6 possess average values of S approximately 6-7, S approximately 10, and S approximately 18, respectively, as determined by SQUID magnetometry and numerical fits to linear combinations of Brillouin functions. The quantitative values of magnetization at saturation and of magnetic susceptibilities indicate that about 40-60 % of unpaired electrons are present at low temperatures (T=1.8-5 K). For polyradical 3, the variable temperature magnetic data are fit to the Heisenberg Hamiltonian based model. The variable magnetic field data at low temperatures are also fit to a percolation-based model for organic spin cluster, with random distribution of chemical defects, and ferromagnetic versus antiferromagnetic couplings, providing quantitative agreement between the experiment and the theory. For polyradical 3 (with S approximately 6-7), annealing at room temperature for 0.5 h leads to a polyradical with S approximately 5.

Free Radicals↗

PubMed1987

Multiple wavelength illumination in flow cytometry using a single arc lamp and a dispersing element.

The principle of a multiple wavelength illumination method for flow cytometers, based upon a combination of a helium-neon laser and an arc lamp as illumination sources is described. By using a prism, the light from the arc lamp is dispersed and the different colors are imaged at different places on the sample stream. The small angle light scattering from the helium-neon laser light is measured as a relevant parameter and serves as a trigger signal for subsequent measurements of fluorescence or scattering of light from the arc lamp. Two experimental systems are described utilizing this principle: a system where the emission is detected orthogonally with respect to the direction of the illumination beams, and an epi-illumination system. With the orthogonal set-up multiple wave-length right angle scattering measurements are possible. This is illustrated by showing that the orthogonal scattering from erythrocytes is strongly dependent on the illumination wavelength. It is further shown that the apparatus is suitable for the measurement of intracellular pH using the pH dependence of the excitation spectrum of fluorescein. The epi-illumination system allows excitation of two (or more) fluorescent dyes with different excitation spectra. In this case the emission spectra of the fluorescent dyes may overlap substantially. This is shown by simultaneous measurement of DNA and protein of Chinese hamster lung cells using mitramycin and tetramethyl rhodamin isothiocyanate (TRITC).

DNA↗

PubMed1996

Polyacrylamide solutions for DNA sequencing by capillary electrophoresis: mesh sizes, separation and dispersion.

Two preparations of linear polyacrylamide with average molecular weights of 0.37 million and 1.14 million Da, and a deuterated preparation with an average molecular weight of 1.71 million Da, were used to study the effects of molecular weight, polydispersity, and concentration on the mesh size of entangled polymers in a DNA sequencing buffer solution and their ability to resolve DNA sequencing reactions by capillary electrophoresis. The polyacrylamide concentrations were above the overlap threshold C*, the concentration above which an entangled polymer network is expected to form. Small angle neutron scattering experiments showed that between 1% and 8% polyacrylamide, the mesh size ( xi ) can be expressed by the relation xi = 2.09C-0.76, where xi is in A and C is the polymer concentration in g/mL. The mesh size depended only on the concentration and was independent of the average molecular weight of the polyacrylamide. Consistent with this result, electrophoretic mobilities of DNA moving through the polymer network depended almost entirely on the polyacrylamide concentration and not on its molecular weight or polydispersity. Although separation was little affected, band sharpness persisted to longer DNAs when the polymer network contained a higher fraction of larger polyacrylamide molecules. We postulate a dispersive effect that depends on the size of the DNA and the resiliency of the polymer network. This interpretation provides a rationale for optimizing the design of polymer solutions to sieve DNA for sequencing by capillary electrophoresis.

Acrylic Resins↗

PubMed2002

Collagen fiber disruption occurs independent of calcification in clinically explanted bioprosthetic heart valves.

The durability of bioprosthetic heart valves (BHV) is severely limited by tissue deterioration, manifested as calcification and mechanical damage to the extracellular matrix. Extensive research on mineralization mechanisms has led to prevention strategies, but little work has been done on understanding the mechanisms of noncalcific matrix damage. The present study tested the hypothesis that calcification-independent damage to the valvular structural matrix mediated by mechanical factors occurs in clinical implants and could contribute to porcine aortic BHV structural failure. We correlated quantitative assessment of collagen fiber orientation and structural integrity by small angle light scattering (SALS) with morphologic analysis in 14 porcine aortic valve bioprostheses removed from patients for structural deterioration following 5-20 years of function. Calcification of the explants varied from 0 (none) to 1+ (minimal) to 4+ (extensive), as assessed radiographically. SALS tests were performed over entire excised cusps using a 0.254-mm spaced grid, and the resultant structural information used to generate maps of the local collagen fiber damage that were compared with sites of calcific deposits. All 42 cusps showed clear evidence of substantial noncalcific structural damage. In 29 cusps that were calcified, structural damage was consistently spatially distinct from the calcification deposits, generally in a distribution similar to that noted in porcine BHV subjected to in vitro durability testing. Our results suggest a mechanism of noncalcific degradation dependent on cuspal mechanics that could contribute to porcine aortic BHV failure.

Animals↗

PubMed1998

Excluded volume in the configurational distribution of a strongly-denatured protein.

The configurational distribution of phosphoglycerate kinase (PGK) strongly-denatured in 4 M guanidine hydrochloride solution is investigated using small-angle neutron scattering (SANS) and Monte Carlo computer simulation. It is shown that the experimental scattering profile can be represented by a random flexible chain of spheres of excess scattering density with excluded volume interactions, the best agreement being achieved when partial sphere intersection is allowed. The radius of gyration of the chain increases by a factor of 4 on denaturation, whereas the average length of segments approximately 5 residues long increases by only approximately 10%, consistent with a picture in which the large expansion on denaturation originates primarily from increased long-range flexibility of the polypeptide chain. The results provide a description of the chain statistics from which the construction of starting points for simulation studies of folding of the protein can be envisaged.

Computer Simulation↗

PubMed2006

Conformational flexibility of RecA protein filament: transitions between compressed and stretched states.

RecA protein is a central enzyme in homologous DNA recombination, repair and other forms of DNA metabolism in bacteria. It functions as a flexible helix-shaped filament bound on stretched single-stranded or double-stranded DNA in the presence of ATP. In this work, we present an atomic level model for conformational transitions of the RecA filament. The model describes small movements of the RecA N-terminal domain due to coordinated rotation of main chain dihedral angles of two amino acid residues (Psi/Lys23 and Phi/Gly24), while maintaining unchanged the RecA intersubunit interface. The model is able to reproduce a wide range of observed helix pitches in transitions between compressed and stretched conformations of the RecA filament. Predictions of the model are in agreement with Small Angle Neutron Scattering (SANS) measurements of the filament helix pitch in RecA::ADP-AlF(4) complex at various salt concentrations.

Amino Acids↗

PubMed2002

Tri-alpha-naphthylbenzene as a crystalline or glassy matrix for matrix-assisted laser desorption/ionization: a model system for the study of effects of dispersion of polymer samples at a molecular level.

Tri-alpha-naphthylbenzene (TalphaNB) can exist as either a crystalline or glassy solid at ambient temperatures, making it a unique matrix in matrix-assisted laser desorption/ionization (MALDI) spectroscopy. Electrosprayed TalphaNB is crystalline and has a melting point of 180 +/- 2 degrees C, as measured by differential scanning calorimetry (DSC). A glass of TalphaNB is obtained upon heating above the crystalline melting point with a glass transition temperature of 68 +/- 2 degrees C having no remaining crystallinity. MALDI samples containing mass fraction 1% polystyrene (PS) are run in both the crystalline and amorphous states. In the crystalline state, there is a strong spectrum typical of PS, but upon melting and quenching to the glassy state, the MALDI signal disappears. If the transparent, amorphous sample is treated with 1-butanol, it becomes white, and the MALDI signal returns. DSC shows that the 1-butanol treatment leads to the return of some of the crystallinity. Small angle neutron scattering (SANS) shows that the crystalline state has large aggregations of PS while the amorphous state has molecularly dispersed PS molecules. MALDI gives strong signals only when there are large aggregations of polymer molecules, with individually dispersed molecules producing no signal.

Journal Article↗

PubMed2006

Near-edge X-ray absorption fine structure spectroscopy as a tool for investigating nanomaterials.

We have demonstrated near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a particularly useful and effective technique for simultaneously probing the surface chemistry, surface molecular orientation, degree of order, and electronic structure of carbon nanotubes and related nanomaterials. Specifically, we employ NEXAFS in the study of single-walled carbon nanotube and multi-walled carbon nanotube powders, films, and arrays, as well as of boron nitride nanotubes. We have focused on the advantages of NEXAFS as an exciting, complementary tool to conventional microscopy and spectroscopy for providing chemical and structural information about nanoscale samples.

Crystallography↗

PubMed1997

The Structure of Nonionic Micelles in Less Polar Solvents

Small angle neutron scattering, SANS, has been used to investigate the structure and geometry of nonionic micelles of monododecyl octaethylene glycol C12EO8, monododecyl hexaethylene glycol C12EO6, and monohexadecyl octaethylene glycol C16EO8 in less polar solvents: mixed solvents of water and glycerol, sorbitol, and ethylene glycol. For C12EO8 in glycerol/water solvent mixtures the addition of glycerol results in an increase in the micelle aggregation number and a decrease in the ethylene oxide (EO) headgroup hydration. The increase in micelle size is associated with the dehydration of the ethylene oxide headgroup and the consequent closer proximity of the lower consolute boundary. This follows from the observation that the same trends are seen at fixed glycerol content and increasing temperature where the intermicellar interactions become attractive. For C12EO6 micelles in water/ethylene glycol mixtures similar trends are observed, and the same conclusions may be drawn despite an increase in the clouding temperature. This can be explained by a greater dehydration of the solvent ethylene glycols such that the rate of change of solvent quality for the micelles with temperature is suppressed. A combination of SANS and Couette shear flow alignment has been used to investigate the structure of C16EO8 micelles in water/sorbitol and water/glycerol mixtures. The addition of sorbitol or glycerol results in less anisotropically shaped micelles. The resulting micelles are more labile, and show evidence of flexibility and shear induced transformations or extreme polydispersity. The temperature dependence of the micelle rod length of C16EO8/water/glycerol is similar to that previously observed for C16EO6 and C16EO8 micelles in water and for the mixed micelles of C16EO6/C16TAB.

Journal Article↗

PubMed1997

Microemulsions with Didodecyldimethylammonium Bromide Studied by Neutron Contrast Variation

The structure of water-in-oil microemulsion droplets, stabilized by didodecyldimethylammonium bromide (DDAB), has been investigated by small-angle neutron scattering (SANS). Detailed information about the curved surfactant film has been obtained by selectively deuterating the water, DDAB, and cyclohexane components. For each surfactant h -DDAB and d -DDAB and concentration, three sets of complementary neutron contrast data were analyzed together in terms of a Schultz distribution of core-shell particles. The modeling was consistent with a simple liquid-like surfactant layer, of density 0.80 g cm-3 , with no evidence for any solvent penetration. This film thickness was found to be 11-12 A, about 70% of an all-trans C12 chain length. At the water interface the area per head was 56-61 A2 , while for the alkyl chains at the outer surface it was 90-125 A2 (15-30% lower than that for a truncated cone molecular configuration). The cyclohexane-water interfacial tensions gammao/w , measured by surface light scattering, were used along with the droplet polydispersities to find that the rigidity of the DDAB film, 2K + K ; is close to 1.0k B T . This means that rather than acting as an effective parameter in the SANS analysis, the polydispersity is a natural consequence of the film rigidity. These results show that the film bending energy model accounts well for the behavior of such DDAB microemulsions.

Journal Article↗

PubMed1997

Repeptization Determined by Turbidity and Photon Correlation Spectroscopy Measurements: Particle Size Effects

The aggregation of four polystyrene latexes with similar surface charge and different diameter has been examined by small angle light scattering. The DLVO model including aggregation at the secondary minimum qualitatively agrees with the experimental dependence of colloidal stability on particle size. Repeptization experiments, where the flocs were redispersed by washing away the coagulating salt, were carried out using two different methods: dialysis-turbidity and dilution-photon correlation spectroscopy. It was found that for the larger latexes the aggregates fragment after dilution, while for the smaller latexes the aggregates do not fragment. These results correlate closely with the hypothesis that particles may be captured within the secondary minimum of the theoretical interaction energy curves. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗

PubMed1998

Influence of Alcohol on the Behavior of Sodium Dodecylsulfate Micelles.

The effect of medium chain alcohol molecules on the size and shape of sodium dodecylsulfate micelles, and on the self-diffusion coefficient of the surfactant and alcohol, has been investigated by means of small angle neutron scattering (SANS), and Fourier transform pulsed field gradient spin echo (FT-PGSE) nuclear magnetic resonance measurements. All measurements were done in D2O containing a sodium chloride concentration of 0.4 mol/kg, and a surfactant concentration of 0.04 mol/kg. The alcohols used were 1-propanol, 1-butanol, and 1-pentanol. The data obtained from the different techniques agrees qualitatively. The results show that propanol successively breaks down the micelles while pentanol brings about a structural change toward large wormlike aggregates. Butanol shows a highly complex behavior on the structure of the micelles and can decrease and increase the size of the aggregates, depending on the added alcohol concentration range. All analyzed solutions show a distribution of the alcohol between the aqueous bulk solution and the palisade layer of the micelles, resulting in an increased "hydration" of D2O in the micellar surface. Moreover, the structural changes of the micelles indicate that butanol and pentanol solubilize in the micellar core in the high molality range, near the end of the solubility limit. Propanol is apparently too hydrophilic to do so. Copyright 1998 Academic Press.

Journal Article↗

PubMed1998

Interactions in Colloidal Mixtures: Partial Structure Factors in Mixtures of Colloidal Silica and an Anionic Oil-in-Water Microemulsion.

We have investigated the colloidal stability and interactions in a mixture of colloidal silica (LudoxTM) and an oil-in-water microemulsion (AOT/decane/brine). Stable silica-microemulsion mixtures containing large fractions of dispersed phase (phioil + phiLudox > 0.36) can be prepared. For fixed silica and AOT concentrations, the addition of decane increases the size of the microemulsion droplets. Since the surfactant concentration remains constant, this droplet growth is balanced by a decrease in the number density of droplets. Partial structure factors for the mixture are extracted from sets of small-angle neutron scattering (SANS) data using a solvent contrast variation technique. The extracted partial structure factors are compared qualitatively to model calculations for a multicomponent sticky hard sphere (SHS) fluid. A shallow contact potential (approximately 2.6 kT) between the Ludox particles is necessary to obtain agreement between model and experiment in the low-q region of the partial structure factors. The magnitude of this contact potential does not change with decane concentration, in agreement with the macroscopic observation that there is no observable change in the stability of these silica-microemulsion mixtures. Copyright 1998 Academic Press.

Journal Article↗

PubMed1999

Large-Scale Bundle Ordering in Sterically Stabilized Latices.

Time-resolved small-angle light scattering and linear conservative dichroism measurements are presented for concentrated, sterically stabilized, aqueous latices under simple shear flow. At low stress levels, flow causes a mild distortion of the liquid-like structure in colloidally stable dispersions, which is quite well understood. In this paper flow-induced structures are investigated in concentrated dispersions when the system is brought far from equilibrium by means of hydrodynamic forces. At high stress levels various structural changes have been predicted by numerical simulation, among others string phases oriented in the flow direction. Here, experimental results are reported on a bundle-like ordering in very dense systems, which involves a length scale much larger than that of a single string of particles. Two latices, with different particle sizes and different thicknesses of the stabilizing layer, are compared. The occurrence of the bundle-like ordering is related to the rheological behavior: it causes a significant decrease in viscosity. It is shown that the presence of this phase results in a structural hysteresis, which explains a thixotropic behavior that is encountered in some stable colloidal suspensions. Also the relaxation behavior of the bundle-like phases has been studied. Interparticle forces are found to have a very strong effect on the relaxation time scales. Copyright 1999 Academic Press.

Journal Article↗

PubMed2000

Structure of Hydrous Ferric Oxide Aggregates.

The small angle light scattering behavior of hydrous ferric oxide flocs is examined here and found to provide useful insights into the nature of the aggregates formed despite the large size of these aggregates at later times. The flocs appear to exhibit fractal properties over a significant size range though the aggregates appear to be easily disrupted through mixing effects resulting in breakup and/or restructuring to denser assemblages. Background electrolyte concentrations also have some impact on floc structure but mixing effects and apparent destabilization by ferric ions limit the effect of added electrolytes on the stability and structure of ferric oxyhydroxides. Similar estimates of fractal dimensions of these hydrous ferric oxide flocs are obtained both by static light scattering analysis and by a cluster mass scaling approach. The choice of density distribution cutoff function has some impact on derived size and structure parameters and further refinement in this area is needed. Copyright 2000 Academic Press.

Journal Article↗

PubMed2000

Flow-Induced Anisotropy in Mixtures of Associative Polymers and Latex Particles.

The effect of associative polymers on the structure and rheological behavior of colloidal suspensions is discussed. Adding associative polymer is known to increase the viscosity of the suspensions. At high shear rates the increase is close to what could be expected on the basis of the hydrodynamic effects of the added polymer. At low shear rates the viscosity increases much more. Small-angle light scattering (SALS) during flow is used here to investigate the underlying structural mechanisms. The SALS patterns indicate that the associative polymer changes the particulate structure: characteristic butterfly patterns appear even at relatively low particle volume fractions. They are not present in the suspensions without associative polymer. The patterns indicate that fluctuations in particle concentration are more pronounced in the flow direction than in the vorticity direction and that anisotropic particulate structures with an orientation along the vorticity direction develop. The evolution of their characteristic length scale during flow has been followed over time. Changing the hydrophilic part of the polymer from polyacrylamide to polyacrylic acid induces stronger associative interactions. In the suspensions this results in a reduction of the relative viscosity rather than an increase. The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspensions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competition between adsorption of the associative polymer on the particles with the intermolecular associations between the polymer chains seems to be responsible for the observed differences. Copyright 2000 Academic Press.

Journal Article↗

PubMed2000

Osmotic Compressibility of Poly(propylene imine) Dendrimers in Deuterated Methanol.

The inverse osmotic compressibility of the poly(propylene imine) dendrimers in deuterated methanol has been measured as a function of concentration with small-angle neutron scattering. The experimental results reveal three different regimes. First, there is a dilute regime going up to a maximum in the inverse osmotic compressibility. This region can be subdivided into a very dilute region, where the behavior is hard-sphere-like, and a denser region, where the solvation layers overlap. The maximum, occurring around volume fraction 0.30 for each generation, is found to be the concentration where the distance between the centers of two dendrimers is twice their radius of gyration. It designates the crossover to the second regime of a semidilute phase with shrinking dendrimers. Interpenetration of the dendrimers does not seem to take place. Finally, for the higher generations, at high concentrations, the dendrimers are collapsed and the inverse osmotic compressibility starts to increase again. As dendrimers are regularly and very highly branched molecules, they can be considered as ultimately hyperbranched polymers. For both types, the experimental inverse osmotic compressibility shows similar features. The dendrimers seem to be more compact, however. Copyright 2000 Academic Press.

Journal Article↗

PubMed2001

Aggregation and Polymerization of PEG-Based Macromonomers with Methacryloyl Group as the Only Hydrophobic Segment.

Aggregation behavior in aqueous solution of a series of poly (ethylene glycol) (PEG)-based macromonomers with methacryloyl group as the only hydrophobic segment has been investigated using surface tension, steady-state and time-resolved fluorescence spectroscopy using pyrene as a probe, and small-angle neutron scattering techniques. The general formula of these macromonomers is CH(2)&dbond;C(CH(3))-CO-O-E(m)-CH(3), where E is the ethylene glycol unit and m=8 (ME(8)), 18 (ME(18)), 49 (ME(49)), and 120 (ME(120)). The results indicate that a macromonomer with 8 ethylene glycol units forms as an aggregate above a certain critical concentration, which can be defined as critical aggregation concentration. The observed high value of I(1)/I(3) in pyrene emission spectra at the interface of these aggregates and the inability to scatter a neutron beam by these aggregates indicate that the hydrophobic cluster formed by this macromonomer is remarkably solvated. ME(18) has a tendency to aggregate but others do not form any hydrophobic cluster. The homopolymerization behaviors of these macromonomers in an aqueous medium at 70 degrees C are consistent with these possibi- lities. Copyright 2001 Academic Press.

Journal Article↗