Confinement Zone Research Articles

Effects of characteristics of Mo dispersions on the plasticity of Mg-based bulk metallic glass composites

The Mg 58Cu 28.5Gd 11Ag 2.5 bulk metallic glass composites (BMGCs) dispersion strengthened by porous Mo particles with different volume fractions and particle sizes were synthesized by casting and characterized. The presence of porous Mo particles could restrict the shear band propagation. It was found for a given volume fraction of Mo particles, smaller particles would lead to more interfacial areas, shorter inter-particle free spacings, smaller confinement zone sizes than the larger particles, and consequently the improvement of compression plasticity from 10% up to 27%. Also, for a given Mo particle size, higher volume fraction would lead to larger compression plasticity. The inter-particle free spacing, as well as the confinement zone size (mean free path of shear bands), appears to be the controlling factor in limiting the propagation of shear bands which in turn affecting the plasticity of BMGCs.

Membrane Cholesterol Strongly Influences Hop-Diffusion of Prestin

Prestin is the membrane motor protein that drives outer hair cell (OHC) electromotility via electromechanical coupling of membrane potential to whole cell length changes. Various groups have demonstrated the importance of the interaction of prestin with the membrane environment. Biochemical evidence and optical microscopy strongly suggest that prestin localizes in cholesterol-rich microdomains, and electrophysiology experiments demonstrate that prestin-associated charge movement depends on membrane cholesterol concentration. We have previously measured prestin diffusion using fluorescence recovery after photobleaching (FRAP). However FRAP is an ensemble measurement making it difficult to discriminate between free and confined diffusion resulting from interactions with the cytoskeleton or confinement in membrane compartments. We have thus measured diffusion of prestin molecules in a model system using single particle tracking and total internal reflection microscopy. Utilizing a combination of single molecule fluorescence microscopy and site-directed labeling with a highly photostable fluorophore, we have robustly determined that prestin diffuses in and hops between confinement zones on the order of 1 micron in average size. We observe that depletion of membrane cholesterol increases average confinement size and decreases confinement strength. Statistical analysis of squared displacements reveals that depletion of cholesterol removes domains of intermediate size between 142 and 500 nm. From measurements of the initial increase of the mean squared deviation with time, the microscopic diffusion constant was determined to be 0.05 X 10−12 m2/s, and was unchanged by cholesterol depletion. Our results suggest that membrane cholesterol affects prestin function by changing prestin crowding in confinement zones, consistent with the hypothesis that the microscale organization of prestin in the membrane influences prestin function.

Nano-Scale, Microsecond Diffusion Imaging of Membrane Protein - lipid Raft Interaction in the Plasma Membrane

Spatial membrane domains, such as created by lipid rafts and the membrane cytoskeleton, influence membrane protein mobility and hence membrane bound processes, i.e. cell signaling. However, visualization of these lipid domains in intact cells is challenging because of their small dimension and dynamics. We have visualized lipid raft and cytoskeleton domains by tracking the diffusion of membrane proteins with thermal noise imaging (TNI). TNI tracks the diffusion of a colloid labeled membrane protein with microsecond and nanometer precision. In addition, an optical trap confines the diffusion to a small area of the membrane providing sufficient statistics for high resolutions maps of the diffusion. We observe millisecond transient confinement zones consistent with 10nm lipid rafts which redistribute after depolymerization of the actin cytoskeleton. Also, we observed larger (∼80nm) cytoskeleton caused confinement zones.

A geophysical analysis of hydro-geomorphic controls within a headwater wetland in a granitic landscape, through ERI and IP

Abstract. Wetlands are undergoing considerable degradation in South Africa. As interventions are often technical and costly, there is a requirement to develop conceptual process models for these wetland systems so that rehabilitation attempts will be successful. This paper presents an approach using the geophysical methods of Electrical Resistivity Imaging (ERI) and Induced Polarization (IP) to delineate sub-surface hydro-geomorphic controls that maintain equilibrium disconnectivity of wetland-catchment processes, which through gully erosion are increasing the catchments connectivity through loss of water and sediment. The findings presented here give insight into the geomorphic processes that maintain the wetland in an un-degraded state, this allows for the development of a conceptual model outlining the wetland forming processes. The analysis suggests that sub-surface clay-plugs, within an otherwise sandy substrate are created by illuviation of clays from the surrounding hillslopes particularly at zones of valley confinement.

Confined within: National territories as zones of confinement

The securitization of immigration has led to increased reliance on border enforcement, detention, and deportation to control unauthorized movements. Based on a case study of the ways that Salvadoran immigrants to the United States have experienced these tactics, this paper analyzes the spatial implications of current enforcement strategies. As movement across borders becomes more difficult for the unauthorized, national territories become zones of confinement. This carceral quality is a dimension of national territory in that undocumented and temporarily authorized migrants cannot exit their countries of residence without losing territorially-conferred rights, while if they are deported, their countries of origin become extensions of the detention centers they occupied before exit. This transformation of national spaces is accompanied by internal differentiation, as interior enforcement confines migrants to subnational spaces where they must remain to avoid detection or harassment. Securitization thus entails both extraterritoriality, that is the extension of U.S. legal regimes into foreign territories, and intraterritoriality, or the operation of different legal regimes within national territories. The paper also highlights the ways that securitization contributes to multidimensionality, such that spatial locations are rendered ambiguous, both inside and outside at the same time. Finally, the paper considers how these spatial transformations redefine citizenship and belonging.

Nanoscale Arrangement of Apoptotic Ligands Reveals a Demand for a Minimal Lateral Distance for Efficient Death Receptor Activation

Cellular apoptosis, the prototype of programmed cell death, can be induced by activation of so-called death receptors. Interestingly, soluble and membrane-bound members of death receptor ligands can differentially activate their receptors. Using the death receptor ligand tumor necrosis factor (TNF) presented on a surface in a nanoscaled pattern with spacings between 58 and 290 nm, we investigated its requirements for spatial arrangement and motility to efficiently activate TNF receptor (TNFR)1 and TNFR2 as well as its chimeras TNFR1-Fas and TNFR2-Fas. We show that the mere mechanical fixation of TNF is insufficient to efficiently activate TNFR2 that is responsive to only the membrane bound form of TNF but not its soluble form. Rather, an additional stabilization of TNFR2(-Fas) by cluster formation seems to be mandatory for efficient activation. In contrast, TNFR1(-Fas) is strongly activated by TNF spaced within up to 200 nm distances, whereas larger spacings of 290 nm fails completely. Furthermore, unlike for TNFR2(-Fas) no dose-response relationship to increasing distances of nanostructured ligands could be observed for TNFR1-(Fas), suggesting that compartmentalization of the cell membrane in confinement zones of approximately 200 nm regulates TNFR1 activation.

Time Series Analysis of Particle Tracking Data for Molecular Motion on the Cell Membrane

Biophysicists use single particle tracking (SPT) methods to probe the dynamic behavior of individual proteins and lipids in cell membranes. The mean squared displacement (MSD) has proven to be a powerful tool for analyzing the data and drawing conclusions about membrane organization, including features like lipid rafts, protein islands, and confinement zones defined by cytoskeletal barriers. Here, we implement time series analysis as a new analytic tool to analyze further the motion of membrane proteins. The experimental data track the motion of 40 nm gold particles bound to Class I major histocompatibility complex (MHCI) molecules on the membranes of mouse hepatoma cells. Our first novel result is that the tracks are significantly autocorrelated. Because of this, we developed linear autoregressive models to elucidate the autocorrelations. Estimates of the signal to noise ratio for the models show that the autocorrelated part of the motion is significant. Next, we fit the probability distributions of jump sizes with four different models. The first model is a general Weibull distribution that shows that the motion is characterized by an excess of short jumps as compared to a normal random walk. We also fit the data with a chi distribution which provides a natural estimate of the dimension d of the space in which a random walk is occurring. For the biological data, the estimates satisfy 1 < d < 2, implying that particle motion is not confined to a line, but also does not occur freely in the plane. The dimension gives a quantitative estimate of the amount of nanometer scale obstruction met by a diffusing molecule. We introduce a new distribution and use the generalized extreme value distribution to show that the biological data also have an excess of long jumps as compared to normal diffusion. These fits provide novel estimates of the microscopic diffusion constant. Previous MSD analyses of SPT data have provided evidence for nanometer-scale confinement zones that restrict lateral diffusion, supporting the notion that plasma membrane organization is highly structured. Our demonstration that membrane protein motion is autocorrelated and is characterized by an excess of both short and long jumps reinforces the concept that the membrane environment is heterogeneous and dynamic. Autocorrelation analysis and modeling of the jump distributions are powerful new techniques for the analysis of SPT data and the development of more refined models of membrane organization. The time series analysis also provides several methods of estimating the diffusion constant in addition to the constant provided by the mean squared displacement. The mean squared displacement for most of the biological data shows a power law behavior rather the linear behavior of Brownian motion. In this case, we introduce the notion of an instantaneous diffusion constant. All of the diffusion constants show a strong consistency for most of the biological data.

The impact of negative selection on thymocyte migration in the medulla

Developing thymocytes are screened for self-reactivity before exiting the thymus, but how thymocytes scan the medulla for self-antigens is unclear. Using two-photon microscopy, we observed that medullary thymocytes migrated rapidly and made frequent, transient contacts with dendritic cells. In the presence of a negative selecting ligand, thymocytes slowed, became confined to areas of approximately 30 microns in diameter, and had increased contact with dendritic cells surrounding confinement zones. One third of polyclonal medullary thymocytes also exhibited confined, slower migration, and may correspond to auto-reactive thymocytes. Our data suggest that many auto-reactive thymocytes do not undergo immediate arrest and death upon encounter with a negative selecting ligand, but rather adopt an altered migration program while remaining within the medullary microenvironment.

High Resolution Imaging Methods to Probe FcεRI Membrane Organization &amp; Behavior (139.6)

Abstract We have applied a combination of high resolution microscopy approaches, together with lipid mass spec analysis, to study the membrane organization of high affinity IgE receptors (FcεRI), their signaling partners and the local lipid environment. Using immunogold labeling and transmission electron microscopy (TEM) imaging of fixed membrane sheets, we find resting receptors in singlets and small clusters. After crosslinking, clusters increase dramatically in size and recruit specific downstream signaling molecules to form primary signaling domains. Atomic Force Microscopy (AFM) studies provide maps of topographical features of the cytoplasmic face of the membrane and demonstrate the importance of cholesterol to the height of these features. Consistent with this, lipid mass spec analysis of isolated FcεRI membrane patches show that 50% of the lipid surrounding receptors is cholesterol. Recently, we have generated novel quantum dot (QD) based probes to study the dynamics of early events in FcεRI signaling. Monovalent IgE-QD complexes are used to track the lateral motion of individual FcεRI in resting and activated states. These studies document transient co-confinement of resting IgE-bound receptors, as well as the immobilization of receptors after extensive crosslinking. Mathematical models show that clusters form when receptors rapidly exchange between domains or confinement zones, consistent with experimental results. The motion of QD-IgE has also been tracked in live cells transfected with GFP-actin using total internal reflection microscopy (TIRF), providing direct evidence that actin corrals provide one mechanism for defining confinement zones. Supported by NIH RO1 AI051575 (BW), NIH RO1 GM49814 (JM0) and Sandia Labs (LDRD).

Heat confinement in tunnels between two double-stream twin-jet air curtains

Heat confining in tunnels by means of double-stream twin-jet (DS-TJ) air curtains was studied numerically to simulate the behavior of an experimental facility built for research and design purposes. The results of 2D and 3D simulations were compared. The axial distributions of mean velocity, turbulent kinetic energy and Reynolds stresses were investigated over the entire jet flow. The temperature field obtained by numerical simulations was compared with the results of experiments. The simulations show that the flow structure within the domain is essentially two dimensional. Radiation effects from the lateral walls in the 3D case induce higher temperatures within the confinement zone. Both the 2D and 3D approaches overestimate the temperature in the domain with respect to experiments.

Measurement of Turbulence Propagation Velocity Using Doppler Reflectometer on HL-2A Tokamak

O-mode Doppler reflectometer has been successfully developed as an important diagnostic system on HL-2A. It can be used to measure the turbulence propagation in both plasma edge and confinement zone. The Doppler reflectometer system consists of two fixed frequency homodyne receivers: 15 GHz (corresponding to cutoff density of 0.3×1019 m−3) and 33 GHz (corresponding to cutoff density of 1.35×1019 m−3). The Doppler reflectometry principle and the experimental arrangements on HL-2A are presented. Meanwhile, the experimental Doppler reflectometric spectra under different discharge conditions, with and without ECRH, were obtained. Furthermore, the turbulence propagation velocity change and the profile were also observed in different discharge conditions.

Fiber microaxicons fabricated by a polishing technique for the generation of Bessel-like beams

We report a simple method for generating microaxicons at the extremity of commercial optical fibers. The proposed solution, based on a polishing technique, can readily produce any desired microaxicon cone angle and is independent of the nature of the fiber. An optical study of microaxicon performance, in terms of confinement ability and length of the generated Bessel-like beams, is presented as a function of the microaxicon angle. This study, made possible by the experimental acquisition of the 3D light distribution of the Bessel-like beams, reveals the relationship between the Bessel-like beam confinement zone and the beam length. Finally, the effect of diffraction of the Bessel-like beams, induced by the limited lateral extent of the incident fiber mode, is studied and discussed.

Anomalous particle pinch and scaling of vin/D based on transport analysis and multiple regression

Predictions of density profiles in current tokamaks and ITER require a validated scaling relation for vin/D where vin is the anomalous inward drift velocity and D is the anomalous diffusion coefficient. Transport analysis is necessary for determining the anomalous particle pinch from measured density profiles and for separating the impact of particle sources. A set of discharges in ASDEX Upgrade, DIII-D, JET and ASDEX is analysed using a special version of the 1.5-D BALDUR transport code. Profiles of ρsvin/D with ρs the effective separatrix radius, five other dimensionless parameters and many further quantities in the confinement zone are compiled, resulting in the dataset VIND1.dat, which covers a wide parameter range. Weighted multiple regression is applied to the ASDEX Upgrade subset which leads to a two-term scaling with x′ = ρ/ρs, effective radius ρ and average value . The rmse value of the scaling equals 15.2%. The electron temperature gradient length is the key parameter of the anomalous particle pinch which yields the main contribution. A further parameter is the loop voltage UL which introduces the electron collisionality parameter νe*. All exponents are statistically significant. The parameters UL and νe* suggest a new anomalous particle pinch term driven by the Ohmic inductive electric field. The nonlinearities in the two-term scaling show that quasilinear theory is disproved by experiment. Regression analysis of the whole dataset VIND1.dat from four tokamaks shows that the scaling covers the dependence of ρsvin/D on the effective plasma radius. It is further found that the ρsvin/D values from transport analysis do not respond to a change in collisionality regime and are not clearly related to the prevailing turbulence type. The new scaling law predicts for ITER high values of ρsvin/D and peaked density profiles, caused by the term and central heating due to alpha particles. The density peaking improves the energy confinement by some 20%.

Observation of high energy ion tail in the SINP tokamak plasma

The present study examines the possibility of acceleration of ions by large scale magnetohydrodynamic activities in the SINP tokamak. The analysis is done with a retarding field ion energy analyzer looking into the plasma in the tangential direction. The experiment shows the presence of high energy ions escaping the confinement zone along with the bulk low energy ions. This process seems to have a close connection with electron acceleration due to the electric field of the m∕n=2∕1 tearing modes.

Phospholipid surfaces regulate the delivery of substrate to tissue factor:VIIa and the removal of product

For many years, the essential role of tissue factor (TF) in coagulation and thrombogenesis has been recognized. The catalytic complex of TF and VIIa (TF:VIIa) is membrane-bound whereas its substrate, factor X (FX), is distributed between a phospholipid-bound fraction and one that is in true solution in 3-dimensional space. This complicates analytical solutions for the kinetic mechanisms describing this reaction because dimensionality must be preserved. We believe that, at the time of activation, FX is simultaneously bound to TF:VIIa and the phospholipid surface. The hydrolysis of a peptide bond activates FX and the product, Xa, is yet bound to the catalytic complex in a manner such that it must leave before a new molecule of X encounters the complex. This means that, in principle, the classically defined Vmax does not apply because on a surface, infinite substrate and its attendant infinite collision frequency do not apply. We show that increasing the lipid surface area available to each TF:VIIa increases the apparent k cat and that it approaches a maximum asymptotically, exhibiting a K ½ at a 40 nm lipid radius. Notably, this is of the same order as transient confinement zones that have been identified on the surface of living cells. We believe the increased lipid surface area allows the Xa to easily diffuse away from the enzyme complex along the 2D lipid surface, thereby allowing new substrate to approach the enzyme and minimizing collisions between the product and the enzyme complex (product inhibition). Thus, after Xa leaves the vicinity of the enzyme, a new FX molecule is able to bind TF:VIIa and the rate at which this complex forms cannot exceed the leaving rate of Xa from the TF:VIIa and phospholipid sites. Thus, this parameter is of critical interest. Starting with the off-rate of Xa from appropriate phospholipid surfaces, we note that the literature values differ by a factor of ∼500. Using energy transfer techniques between 30% phosphatidylserine/70% phosphatidylcholine vesicles and human F.Xa, we measured this off rate and found it agrees closely with the Biacore generated data. We have determined the binding parameters of Xa to vesicles and a continuous supported bilayer. Our data are in excellent agreement with the data derived using a lipid coated Biacore chip.

Detection of confinement and jumps in single-molecule membrane trajectories

We propose a variant of the algorithm by [R. Simson, E. D. Sheets, and K. Jacobson, Biophys. 69, 989 (1995)]. Their algorithm was developed to detect transient confinement zones in experimental single-particle tracking trajectories of diffusing membrane proteins or lipids. We show that our algorithm is able to detect confinement in a wider class of confining potential shapes than that of Simson et al. Furthermore, it enables to detect not only temporary confinement but also jumps between confinement zones. Jumps are predicted by membrane skeleton fence and picket models. In the case of experimental trajectories of mu-opioid receptors, which belong to the family of G-protein-coupled receptors involved in a signal transduction pathway, this algorithm confirms that confinement cannot be explained solely by rigid fences.

Cusp confinement zones on quiet and disturbed dayside magnetosphere

The possibility of the charged particle confinement in the dayside regions of magnetosphere cusps is investigated. The analysis, on the basis of Tsyganenko geomagnetic field model, consisted of a numerical simulation of the single charged particle trajectories passing through the regions of the high latitude magnetic field minima. It is shown that the magnetic field topology with off-equatorial field minima itself does not guarantee a particle trap in cusps. In determined conditions relatively stable particle traps exist at different levels of magnetic disturbance, and the topology strongly depends on the seasonal influence. The particles captured in the zones form a kind of cusp radiation belt where particles drift with periods of several minutes conserving the 1st and 2nd adiabatic invariants. This capture and other features of the cusp trapped radiation are discussed.

Segregation of micron‐scale membrane sub‐domains in live murine sperm

Lipid rafts, membrane sub-domains enriched in sterols and sphingolipids, are controversial because demonstrations of rafts have often utilized fixed cells. We showed in living sperm that the ganglioside G(M1) localized to a micron-scale membrane sub-domain in the plasma membrane overlying the acrosome. We investigated four models proposed for membrane sub-domain maintenance. G(M1) segregation was maintained in live sperm incubated under non-capacitating conditions, and after sterol efflux, a membrane alteration necessary for capacitation. The complete lack of G(M1) diffusion to the post-acrosomal plasma membrane (PAPM) in live cells argued against the transient confinement zone model. However, within seconds after cessation of sperm motility, G(M1) dramatically redistributed several microns from the acrosomal sub-domain to the post-acrosomal, non-raft sub-domain. This redistribution was not accompanied by movement of sterols, and was induced by the pentameric cholera toxin subunit B (CTB). These data argued against a lipid-lipid interaction model for sub-domain maintenance. Although impossible to rule out a lipid shell model definitively, mice lacking caveolin-1 maintained segregation of both sterols and G(M1), arguing against a role for lipid shells surrounding caveolin-1 in sub-domain maintenance. Scanning electron microscopy of sperm freeze-dried without fixation identified cytoskeletal structures at the sub-domain boundary. Although drugs used to disrupt actin and intermediate filaments had no effect on the segregation of G(M1), we found that disulfide-bonded proteins played a significant role in sub-domain segregation. Together, these data provide an example of membrane sub-domains extreme in terms of size and stability of lipid segregation, and implicate a protein-based membrane compartmentation mechanism.

Cusp region radiation belts in the dayside magnetosphere

The possibility of quasi-stable trapping of charged particles of hundreds keV–MeV energy on the front side Earth magnetosphere is explored in this article by numerical modeling of the single particle orbits in the geomagnetic field utilizing empirical Tsyganenko model. On the front side magnetosphere the remote equatorial magnetic field lines are compressed to balance the solar wind pressure and exhibit two minima in the geomagnetic field strength magnitude along the field line in high latitudes on both sides of the equator which results in stable confinement structures in the north or/and the south hemispheres, providing energetic particle trapping for times from several minutes to durations of seasonal scale. Simulation of the energetic proton orbits in magnetosphere passing through the regions of magnetic field minima with different disturbance level and the Earth's tilt shows when and where these trapped radiation zones could exist. It is noted that the existence of this adiabatic confinement zone depends on the tilt of the geomagnetic dipole axis to the Sun–Earth direction. As a result the northern polar cap (cusp) confinement zone appears only during a summer solstice and similarly the southern hemisphere capture zone appears during a winter solstice. During equinox the confinement zones exist in both hemispheres in the disturbed magnetospheric conditions, however, they are less pronounced. The zones are essentially restricted to the sunlit magnetosphere. They form a kind of cusp radiation belt, where a proton drifts with a period of several minutes, conserving its 1st and the 2nd adiabatic invariants. The latitudinal width of the belt is very thin, about 2–5 latitudinal degrees. The particle orbits passing through opposite off-equatorial field minimum reveal another effect: a bound of the geomagnetic equatorial plane on the day sector. These and other features of the confinement zones in the two minima off-equatorial magnetic field regions are discussed.

Seasonal cusp radiation belt on dayside magnetosphere

The possibility of quasi-stable trapping of charged particles of hundreds keV to MeV energy on the frontside Earth magnetosphere is explored in by numerical modeling of the single particle orbits in the geomagnetic field utilizing empirical Tsyganenko magnetic field model. Due to solar wind pressure the remote magnetic field lines on the frontside of the magnetosphere exhibit two minima in the geomagnetic field strength along the field line in high latitudes on the both sides of the equator. These minima may result in stable confinement structures, a kind of radiation belts, in the northern or/and the southern hemispheres, providing energetic particle trapping for times from several minutes to duration of seasonal scale. Simulation of energetic proton orbits passing through the regions of the magnetic field minima with different disturbance level and the Earth’s tilt reveals conditions in which these trapped radiation zones could result. It is shown that the existence of the adiabatic confinement zones strongly depends on the seasonal inclination of the Earth’s rotation axis. As a result the northern cusp confinement zone appears only in a summer solstice and similarly the southern cusp capture zone appears only in a winter solstice. In equinox time the confinement zones exist in both hemispheres in the disturbed magnetospheric conditions, however, they are less pronounced. The zones are essentially restricted to the sunlit magnetosphere. They form a kind of cusp radiation ring/belt, where a proton drifts with a period of several minutes, conserving its 1 st and the 2 nd adiabatic invariants. The latitudinal width of the ring is very thin, about 2-5 latitudinal degrees. The proton orbits passing through the off-equatorial field minimum opposite to those cusp belts reveal another interesting effect: a bound of the geomagnetic equatorial plane on the day sector. These and other features of the confinement zones in the two minima off-equatorial magnetic field regions are discussed.