Increase In Local Order Research Articles

Combinatorial Polarized TIRF-AFM Study of Membrane Reorganization by Alpha-Synuclein and Human Prion Protein

Many neurodegenerative diseases are caused by aggregation or fibrillation of membrane-associated proteins. Alpha-Synuclein (aS) is associated with Parkinson's Disease as a major component of Lewy bodies, and misfolded aggregates of human Prion Protein (PrP) are associated with spongiform encephalopathies. Here we study the influence of these proteins on the organization and topography of canonical raft-mimetic supported lipid bilayers (DSPC/DOPC/cholesterol at 35/35/30 mol% on mica) using a correlated polarized Total Internal Reflectance Fluorescence-Atomic Force Microscopy (pTIRF-AFM) platform. pTIRF allows for high-speed measurements of dynamic processes at diffraction-limited spatial resolution, while AFM gives time-averaged nm-scale spatial resolution. Using pTIRF and an oriented lipophilic dye (DiI-C18) we determine the local order in the membrane by measuring the ratio of fluorescence parallel and perpendicular to the membrane plane, while simultaneously measuring membrane topography using AFM. Upon addition of aS to our phase-segregated membrane system, liquid-ordered and liquid-disordered domains were mixed and the local order increased such that lipid headgroups were oriented more parallel to the membrane plane. Moreover, AFM topography revealed the formation of long, closed-loop tubules. Addition of the active segment PrP(106-126) to these raft-mimetic membranes resulted in a similar mixing of domains and an increase of local order, while AFM revealed the formation of peptide fibrils in and around cholesterol-rich domains. This study provides direct evidence of membrane reorganization and restructuring by aS and PrP, and the effect of membrane chemistry on aS and PrP self-association. These results portend future detailed investigations into the molecular-level factors responsible for these effects.

Spectroscopic study of plasma-facing graphite tiles modified by electron and proton irradiation

We have carried out Raman, electron spin resonance (ESR), and X-ray photoemission (XPS) spectroscopy measurements on the graphite tiles modified by electron and proton irradiation. While increase in local order was observed in the electron-irradiated sample, structural disorder and amorphization were revealed in the proton-irradiated sample, with much reduced electrical conductivity. Besides, a narrow ESR peak attributed to localized spins, closely associated with C–OH bonds as identified by the XPS measurements, was detected in the proton-irradiated sample.

Rubbing effects on the structural and optical properties of poly(3-hexylthiophene) films

Steady state photoconductivity and x-ray diffraction combined with optical and electronic spectroscopies were applied to study spin coated poly(3-hexylthiophene) (P3HT) films mechanically rubbed using a teflon bar. As expected, photoconductivity evidenced a strong increase in the photo-response along the rubbing direction. But, while rubbing during annealing at a temperature of 70 °C resulted in a sizeable photocurrent anisotropy along the direction parallel or perpendicular to the rubbing, the samples rubbed at room temperature (RT) showed an isotropic and comparatively stronger enhancement of absorption and photoconductivity spectral features. According to these observations, rubbing at RT resulted in a significant increase in local order and macroscopic alignment of the film, while a higher degree of global order but with a relaxed local structure was obtained after rubbing at 70 °C.

A spherical model with directional interactions. I. Static properties

We introduce a simple spherical model whose structural properties are similar to the ones generated by models with directional interactions, by employing a binary mixture of large and small hard spheres, with a square-well attraction acting only between particles of different sizes. The small particles provide the bonds between the large ones. With a proper choice of the interaction parameters, as well as of the relative concentration of the two species, it is possible to control the effective valence. Here we focus on a specific choice of the parameters which favors tetrahedral ordering and study the equilibrium static properties of the system in a large window of densities and temperatures. Upon lowering the temperature we observe a progressive increase in local order, accompanied by the formation of a four-coordinated network of bonds. Three different density regions are observed: At low density the system phase separates into a gas and a liquid phase; at intermediate densities a network of fully bonded particles develops; at high densities--due to the competition between excluded volume and attractive interactions--the system forms a defective network. The very same behavior has been previously observed in numerical studies of nonspherical models for molecular liquids, such as water, and in models of patchy colloidal particles. Different from these models, theoretical treatments devised for spherical potentials, e.g., integral equations and ideal mode coupling theory for the glass transition, can be applied in the present case, opening the way for a deeper understanding of the thermodynamic and dynamic behavior of low valence molecules and particles.

Temperature Dependence of the Relaxation Rates of α and δ Relaxation in Liquid-Crystalline Side-Group Polymethacrylates

Dielectric spectroscopy in the frequency range from 10-2 to 106 Hz and in the temperature range from 190 to 430 K is employed to analyze the temperature dependence of the relaxation rates of the α and δ relaxation of liquid-crystalline polymethacrylates having derivatives of (p-alkoxyphenyl)benzoate as mesogenic units in the side group. Different mesophases were achieved by variation of the spacer length. When a temperature derivative method was applied, it was found that the temperature dependence of the relaxation rate of the dielectric α relaxation in these systems displays two different regions: an Arrhenius-like behavior at high temperatures and a behavior according to the Vogel−Fulcher−Tammann law at low temperatures which is characteristic for the dynamic glass transition in general. The crossover between these two dependencies characterized by a temperature Tdc is shifted to lower temperatures with increasing spacer length, but the ratio of Tdc and the glass transtion temperature is nearly independent of the spacer length. The result is discussed in the framework of the cooperativity of the glass transtion. The ratio of the relaxation rates of the α and δ processes increases with decreasing temperature in the liquid-crystalline mesophase range. This is explained by an increase of local order with decreasing temperature. Close to the glass transiton temperature, the ratio of the relaxation rates of the α and δ processes decreases. This indicates that the dielectric α and δ processes freeze at least together and cannot be regarded further as independent relaxation processes in that temperature regime.

Investigation of the Behaviour of Structural Changes during Recovery of Crystal Defects in Plastically Deformed Fe–50% Ni Alloy by Magnetic Measurements

Isochronal and isothermal annealing experiments in the temperature range 25 to 900 °C revealed the existence of different annealing stages in the annealing spectrum of plastically deformed Fe-50% Ni alloy by observing the associated changes in maximum magnetic permeability (μ max ) and magnetic coercivity (H er) . On annealing, a first stage centered around 225 °C appears, activated by (1.16 ± 0.02) eV which is explained on the basis of an increase in local order in the alloy matrix. A second stage in the temperature range 550 to 750 °C arises because of the dissociation of defect clusters, activated by (1.8 ± 0.1) eV. The third recovery stage appears above 750 °C, activated by (3.06 ± 0.02) eV, and the process is related to the climb motion of dislocations during the recrystallization process. Changes in initial lattice defect concentration were found to have influence on the amplitude and the position of the different observed stages. The kinetics of FeNi 3 precipitation observed at high temperatures (T a ≥ 800 °C) are discussed.

Recovery studies of neutron-irradiated and deformed NiCr alloys

Ni–10 wt% Cr and Ni–20 wt% Cr are irradiated with fast neutrons to fluences of 6.5 × 1021 and 1 × 1022 n/m2, and their isochronal annealing behaviour is studied between 343 and 945 K. On annealing, the resistivity increases between 343 and 575 K and decreases again at higher temperatures. The increase in resistivity is explained on the basis of an increase in local order on annealing. The decrease in resistivity at higher temperature arises because of dissociation of defect clusters.

Low-Temperature Thermal Conductivity in Neutron Irradiated Vitreous Silica

Measurements of the thermal conductivity, K, of vitreous silica have been made at low temperatures before and after fast neutron bombardment. The observed increase of K upon bombardment is in agreement with the prediction of Klemens. Over the exposure range used, K increases approximately linearly with dose to the level used (6×1019 n/cm2) whereas the density appears to have nearly saturated, suggesting that the two properties are not directly related. A possible explanation for the K increase upon bombardment is that the neutron bombardment relieves macroscopic strains. Alternatively, it is possible that bombardment causes an increase in local order in the vitreous material.