Browsing by Subject "chemistry, physical"

Now showing 1 - 9 of 9

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 J
This 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.
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 B
The 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 forces
Open Access
Distributions of Parameters and Features of Multiple Bond Ruptures in Force Spectroscopy by Atomic Force Microscopy
(2010) Guo, S; Li, N; Lad, N; Desai, S; Akhremitchev, BB
Force spectroscopy measurement of rupture forces of bound molecules becomes an important physicochemical tool in characterizing intermolecular interactions, Atomic force microscopy (AFM) measurements are among the most common approaches in implementation of this technique. Kinetic information about the molecular bond under study is usually extracted assuming that the detected rupture force comes from rupturing of a single bond. However, multiple bond ruptures might occur in experiments. In this article, we consider how the presence of multiple bonds is manifested in the distribution of parameters that are typically extracted in force spectroscopy experiments. Of particular interest here are the distributions of rupture forces and Kuhn lengths of polymeric tethers. We show that multiple bond ruptures might contribute to the measured distributions even when these distributions have a well-defined single peak. Also, we consider how the probability to form multiple bonds depends on probe velocity. The developed analytical models are applied to experimental data of biotin streptavidin ruptures. The velocity dependence of the amplitude of high force tail supports the hypothesis of multiple bond nature of the measured high forces.
Open Access
Kinetic Parameters from Detection Probability in Single Molecule Force Spectroscopy
(2010) Ray, Chad; Guo, Senli; Brown, Jason; Li, Nan; Akhremitchev, Boris B
The 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.
Open Access
Quantification of the Binding Properties of Cu2+ to the Amyloid Beta Peptide: Coordination Spheres for Human and Rat Peptides and Implication on Cu2+-Induced Aggregation
(2010) Hong, Lian; Carducci, Tessa M; Bush, William D; Dudzik, Christopher G; Millhauser, Glenn L; Simon, John D
There is no consensus on the coordinating ligands for Cu2+ by A beta. However, the differences in peptide sequence between human and rat have been hypothesized to alter metal ion binding in a manner that alters Cu2+-induced aggregation of A beta. Herein, we employ isothermal titration calorimetry (ITC), circular dichroism (CD), and electron paramagnetic resonance (EPR) spectroscopy to examine the Cu2+ coordination spheres to human and rat A beta and an extensive set of A beta(16) mutants. EPR of the mutant peptides is consistent with a 3N1O binding geometry, like the native human peptide at pH 7.4. The thermodynamic data reveal an equilibrium between three coordination spheres, {NH2, O, NIm(His6), N-}, {NH2, O, N-Im(His6), N-Im(His13)}, and {NH2, O, N-Im(His6), N-Im(His14)}, for human A beta(16) but one dominant coordination for rat A beta(16), {NH2, O, N-Im(His6), N-}, at pH 7.4-6.5. ITC and CD data establish that the mutation R5G is sufficient for reproducing this difference in Cu2+ binding properties at pH 7.4. The substitution of bulky and positively charged Arg by Gly is proposed to stabilize the coordination {NH2, O-, NIm(His6), N-} that then results in one dominating coordination sphere for the case of the rat peptide. The differences in the coordination geometries for Cu2+ by the human and rat A beta are proposed to contribute to the variation in the ability of Cu2+ to induce aggregation of A beta peptides.
Open Access
The Ultraviolet Absorption Coefficient of Melanosomes Decreases with Increasing Pheomelanin Content
(2010) Peles, Dana N; Simon, John D
Uveal melanosomes from the iridal stroma contain both eumelanin and pheomelanin, the ratio of which varies with iris color. Herein, we report the absorption coefficient at lambda = 244 nm for individual human iridal stroma melanosomes from dark brown and blue-green irides. The melanosomes are nearly identical in size, but differ in the relative concentration composition, ranging from a eumelanin/pheomelanin ratio of 14.8:1 (dark brown) to 1.3:1 (blue-green or hazel). The absorption coefficient of the melanosome decreases as its pheomelanin content increases. The origin of this decrease is attributed to a corresponding decrease in the number of UV-absorbing chromophores, reflecting the different molecular volumes of the monomeric building blocks of the two pigments. In agreement with reported data on synthetic pigments, the absorption coefficient of pheomelanin is found to be slightly larger than that for eumelanin at lambda = 244 nm (by a factor of 1.2). On the basis of the reported optical properties of synthetic models, this result suggests that the absorption of pheomelanin is less than eumelanin at wavelengths of biological relevance (similar to 315-400 nm).
Open Access
Theoretical Framework for Nanoparticle Reactivity as a Function of Aggregation State
(2010) Hotze, Ernest M; Bottero, Jean-Yves; Wiesner, Mark
Theory 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.
Open Access
Tunable Leuko-polymersomes That Adhere Specifically to Inflammatory Markers
(2010) Rawson, Jeff; Therien, Michael J
The 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.
Open Access
Ultraviolet Absorption Coefficients of Melanosomes Containing Eumelanin As Related to the Relative Content of DHI and DHICA
(2010) Peles, Dana N; Lin, Erica; Wakamatsu, Kazumasa; Ito, Shosuke; Simon, John D
Central to understanding the photochemical function(s) of melanosomes is the determination of their absorption properties and an understanding of how the absorption varies with the molecular composition of melanin. Herein, the absorption coefficients for melanosomes containing predominantly eumelanin, a polymeric pigment derived from the molecular precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), are reported for) lambda = 244 nm. The absorption coefficient varies with the DHICA/DHI ratio, determined from chemical degradation analyses. With increasing DHICA content, the absorption coefficient of the melanosome increases. This observation is consistent with reported extinction coefficients, which reveal that at 244 nm, the extinction coefficient of DHICA is a factor of similar to 2.1 greater than that of DHI. The melanosome absorption coefficients are compared to predicted values based on a linear combination of the absorption of the constituent monomers.