Browsing by Subject "chemistry, multidisciplinary"
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Item Open Access Adsorption of a Nonionic Symmetric Triblock Copolymer on Surfaces with Different Hydrophobicity(2010) Liu, Xiaomeng; Wu, Dong; Turgman-Cohen, Salomon; Genzer, Jan; Theyson, Thomas W; Rojas, Orlando JThis study investigates the adsorption of a symmetric triblock nonionic polymer comprising ethylene oxide (EO) and propylene oxide (PO) blocks (Pluronic P-105, EO37PO56EO37) on a range of substrates including hydrophobic, i.e., polypropylene (PP), poly(ethylene terephthalate) (PET), nylon, and graphite, and hydrophilic, i.e., cellulose and silica. The adsorption process and the structure of the hydrated adsorbed layers are followed by quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR), and atomic force microscopy. The unhydrated surfaces are characterized by ellipsometry and contact angle techniques. The adsorption kinetics and the extent of adsorption are determined by monitoring the changes in resonance frequency and refractive index of sensors coated with ultrathin films of the various substrates. Langmuirian-type adsorption kinetics is observed in all cases studied. The amount of adsorbed Pluronic on hydrophobic polymer surfaces (PP, PET, and nylon) exceeds that on the hydrophilic cellulose. The hydrophobic (graphite) mineral surface adsorbs relatively low polymer mass, typical of a monolayer, while micellar structures are observed on the hydrophilic silica surface. The amount of water coupled to the adsorbed polymer layers is quantified by combining data from QCM, and SPR are found to increase with increasing polarity of the substrate. On the basis of contact angle data, the nonhydrated adsorbed structures produce modest increases in hydrophilicity of all the substrates investigated. Overall, insights are provided into the structure and stability of both hydrated and nonhydrated adsorbed triblock copolymer.Item Open Access Calcium Dependence of Fibrin Nanomechanics: The gamma 1 Calcium Mediates the Unfolding of Fibrinogen Induced by Force Applied to the "A-a" Bond(2010) Akhremitchev, Boris BThe interactions between the constituent monomers of fibrin, the polymerized protein network that provides the structural stability of blood clots, ale frequently under stress because of the dynamic nature of blood flow Herein, the calcium dependence of the structural unfolding linked to the forced dissociation of the "A-a" knob-hole bond between fibrin monomers is reported The presence of calcium was shown to influence the incidence of the last event in the unfolding pattern characteristic of "A-a" rupture This effect, attributed to the function of the gamma 1 calcium-binding site, was found to be reversible and specific Our results indicate that binding of calcium at the gamma 1 site has no effect on the strength of the knob-hole bond prior to unfolding of the hole-containing gamma module Rather, calcium bound at the gamma 1 site makes the structure of the hole mole resilient to such forced unfolding, leading to survival of the "A-a" knob-hole bond during large: extensions of the fibrinogen molecule but at the cost of rupture of the bond at lower forcesItem Restricted Direct Carbon-Carbon Bond Formation via Reductive Soft Enolization: A Kinetically Controlled syn-Aldol Addition of alpha-Halo Thioesters and Enolizable Aldehydes(2010) Sauer, Scott J; Garnsey, Michelle R; Coltart, Don MThe direct addition of enolizable aldehydes and a-halo thioesters to produce beta-hydroxy thioesters enabled by reductive soft enolization is reported. The transformation is operationally simple and efficient and has the unusual feature of giving high syn-selectivity, which is the opposite of that produced for (thio)esters under conventional conditions. Moreover, excellent diastereoselectivity results when a chiral nonracemic alpha-hydroxy aldehyde derivative is used.Item Open Access Direct carbon-carbon bond formation via soft enolization: aldol addition of alpha-halogenated thioesters(2011) Yost, Julianne M; Alfie, Rachel J; Tarsis, Emily M; Chong, Insun; Coltart, Don Malpha-Halo thioesters undergo soft enolization and syn-selective direct aldol addition to aldehydes in the presence of MgBr2 center dot OEt2 and i-Pr2NEt to produce alpha-halo-beta-hydroxy thioesters.Item Open Access Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires(2010) Schmidt, V; Wittemann, JV; Gösele, UItem Open Access Kinetic Parameters from Detection Probability in Single Molecule Force Spectroscopy(2010) Ray, Chad; Guo, Senli; Brown, Jason; Li, Nan; Akhremitchev, Boris BThe detection probability of rupture events in A FM force spectroscopy measurements presents a viable alternative to standard methods for extracting kinetic parameters of dissociation. The detection probability has a maximum as a function of the probe velocity where (1) the probability to form a molecular bond is independent of the probe velocity and (2) the detection of rupture events is limited by noise and performed with a constant density of data points per distance of the probe displacement. This newly developed model indicates that the optimal detection velocity is independent of dissociation rate and depends on the distance to the barrier kinetic parameter. Therefore, the kinetic parameters of bond dissociation can be extracted from the dependence of detection probability on probe velocity and the detection threshold. This approach is sensitive to low rupture forces and therefore is complementary to the common most probable force data analysis approach. The developed approach is tested using rupture forces measured with specific bonds between biotin and streptavidin and with nonspecific bonds between linear alkalies in water. Results for the analysis of specific bonds rupture are consistent with the previous measurements, suggesting that rupture forces spanning a wide range of values originate from the same binding potential. Kinetic parameters obtained for linear alkalies are significantly different from previous measurements suggesting possible heterogeneity of the bound state.Item Open Access Mechanical Distortion of Protein Receptor Decreases the Lifetime of a Receptor-Ligand Bond(2010) Guo, Senli; Li, Nan; Lad, Nimit; Ray, Chad; Akhremitchev, Boris BSubstantial experimental evidence indicates that the mechanical force applied to pull apart non-covalent molecular bonds (such as receptor ligand pairs) can significantly decrease the bond lifetime. This evidence is often generated in single-molecule experiments that are designed to specifically test effects of pulling forces. However, the effect of compressive forces on the lifetime of receptor ligand bonds remains largely unexplored. Here we extend the common usage of the atomic force microscopy technique to study whether compressive forces applied to bound streptavidin-biotin species can significantly accelerate the rate of dissociation. Presented experimental data indicate that compressive forces can substantially decrease the lifetime of the molecular bond. Surprisingly, the efficiency of accelerating dissociation by compressive forces sometimes exceeds the enhancement of the dissociation rate measured in pulling experiments, indicating that compressive forces applied to the bound species might be efficiently used to control the lifetime of adhesion bonds.Item Open Access The Red and the Black(2010) Simon, John D; Peles, Dana NPigmentation, which is primarily determined by the amount, the type, and the distribution of melanin, shows a remarkable diversity in human populations, and in this sense, it is an atypical trait.-E.J. Parra. Melanin is found throughout the human body, skin, eye, brain, hair, and inner ear, yet its molecular structure remains elusive. Researchers have characterized the molecular building blocks of melanin but have not been able to describe how those components fit together in the overall architecture of the pigment. Melanin is categorized into two distinct classes, pheomelanin (red) and eumelanin (black). Although these classes share a common biosynthetic origin, specific molecular reactions occurring early in pigment production differentiate these two types. Pure eumelanin is found throughout nature, which has allowed researchers to characterize and quantify its chemical properties. However, pure pheomelanin is not observed in nature and rarely makes up more than similar to 25% of the total melanin present. In this Account, we explore our current understanding of the structure and reactivity of the red and black pigments. Epidemiological studies of skin and ocular cancers suggest that increasing relative proportions of pheomelanin correlate with increased risk factors for these diseases. Therefore, understanding the factors that control the relative abundance of the two pigments has become increasingly important. Consequently, researchers have worked to elucidate the chemistry of pheomelanin to determine whether the pigment could cause these cancers and, if so, by what mechanisms. The photoactivation of oxygen by pheomelanin in the UV-A range could contribute to the development of UV-induced cancers: recent measurement of the surface photoionization threshold of intact melanosomes reveals a lower photoionization potential for pheomelanin than eumelanin. A complementary study of intact human melanosomes isolated from different colored irides reveals that the absorption coefficient of the melanosome decreases with increasing pheomelanin content. These results suggest that the epidemiological data may simply result from an increased exposure of the underlying tissues to UV light.Item Open Access Theoretical Framework for Nanoparticle Reactivity as a Function of Aggregation State(2010) Hotze, Ernest M; Bottero, Jean-Yves; Wiesner, MarkTheory is developed that relates the reactivity of nanoparticles to the structure of aggregates they may form in suspensions. This theory is applied to consider the case of reactive oxygen species (ROS) generation by photosensitization of C-60 fullerenes. Variations in aggregate structure and size appear to account for an apparent paradox in ROS generation as calculated using values for the photochemical kinetics of fullerene (C-60) and its hydroxylated derivative, fullerol (C-60(OH)(22-24)) and assuming that structure varies between compact and fractal objects. A region of aggregation-suppressed ROS production is identified where interactions between the particles in compact aggregates dominate the singlet oxygen production. Intrinsic kinetic properties dominate when aggregates are small and/or are characterized by low fractal dimensions. Pseudoglobal sensitivity analysis of model input variables verifies that fractal dimension, and by extension aggregation state, is the most sensitive model parameter when kinetics are well-known. This theoretical framework qualitatively predicts ROS production by fullerol suspensions 2 orders of magnitude higher compared with aggregates of largely undifferentiated C-60 despite nearly an order of magnitude higher quantum yield for the undifferentiated C-60 based on measurements for single molecules. Similar to C-60, other primary nanoparticles will exist as aggregates in many environmental and laboratory suspensions. This work provides a theoretical basis for understanding how the structure of nanoparticle aggregates may affect their reactivity.Item Open Access Tunable Leuko-polymersomes That Adhere Specifically to Inflammatory Markers(2010) Rawson, Jeff; Therien, Michael JThe polymersome, a fully synthetic cell mimetic, is a tunable platform for drug delivery vehicles to detect and treat disease (theranostics). Here, we design a leuko-polymersome, a polymersome with the adhesive properties of leukocytes, which can effectively bind to inflammatory sites under flow. We hypothesize that optimal leukocyte adhesion can be recreated with ligands that mimic receptors of the two major leukocyte molecular adhesion pathways, the selectins and the integrins. Polymersomes functionalized with sialyl Lewis X and an antibody against ICAM-1 adhere avidly and selectively to surfaces coated with inflammatory adhesion molecules P-selectin and ICAM- I under flow. We find that maximal adhesion occurs at intermediate densities of both sialyl Lewis X and anti-ICAM- I, owing to synergistic binding effects between the two ligands. Leuko-polymersomes bearing these two receptor mimetics adhere under physiological shear rates to inflamed endothelium in an in vitro flow chamber at a rate 7.5 times higher than those to uninflamed endothelium. This work clearly demonstrates that polymersomes bearing only a single ligand bind less avidly and with lower selectivity, thus suggesting proper mimicry of leukocyte adhesion requires contributions from both pathways. This work establishes a basis for the design of polymersomes for targeted drug delivery in inflammation.Item Open Access Weave Tile Architecture Construction Strategy for DNA Nanotechnology(2010) Hansen, Majken N; Zhang, Alex M; Rangnekar, Abhijit; Bompiani, Kristin M; Carter, Joshua D; Gothelf, Kurt V; LaBean, Thomas HArchitectural designs for DNA nanostructures typically fall within one of two broad categories: tile-based designs (assembled from chemically synthesized oligonucleotides) and origami designs (woven structures employing a biological scaffold strand and synthetic staple strands). Both previous designs typically contain many Holliday-type multi-arm junctions. Here we describe the design, implementation, and testing of a unique architectural strategy incorporating some aspects of each of the two previous design categories but without multi-arm junction motifs. Goals for the new design were to use only chemically synthesized DNA, to minimize the number of component strands, and to mimic the back-and-forth, woven strand routing of the origami architectures. The resulting architectural strategy employs "weave tiles" formed from only two oligonucleotides as basic building blocks, thus decreasing the burden of matching multiple strand stoichiometries compared to previous tile-based architectures and resulting in a structurally flexible tile. As an example application, we have shown that the four-helix weave tile can be used to increase the anticoagulant activity of thrombin-binding aptamers in vitro.