Parametric decay instability of an inhomogeneous plasma in a frequency-modulated pump wave

It is found experimentally that the broadening of the pump-wave spectrum affects the parametric instability in an inhomogeneous plasma more weakly than is predicted by theory. The suppression of the absolute instability is only observed for a pump-wave spectrum width of 2πΔf>100γ, which is much greater than the instability growth rate γ.     

Translation driven by an eIF4G core domain in vivo.

Most eukaryotic mRNAs possess a 5' cap structure (m(7)GpppN) and a 3' poly(A) tail which promote translation initiation by binding the eukaryotic translation initiation factor (eIF)4E and the poly(A) binding protein (PABP), respectively. eIF4G can bridge between eIF4E and PABP, and-through eIF3-is thought to establish a link to the small ribosomal subunit. We fused the C-terminal region of human eIF4GI lacking both the eIF4E- and PABP-binding sites, to the IRE binding protein IRP-1. This chimeric protein suffices to direct the translation of the downstream cistron of bicistronic mRNAs bearing IREs in their intercistronic space in vivo. This function is preserved even when translation via the 5' end is inhibited. Deletion analysis defined the conserved central domain (amino acids 642-1091) of eIF4G as an autonomous 'ribosome recruitment core' and implicated eIF4A as a critical binding partner. Our data reveal the sufficiency of the conserved eIF4G ribosome recruitment core to drive productive mRNA translation in living cells. The C-terminal third of eIF4G is dispensable, and may serve as a regulatory domain.     

A hall dynamo effect driven by two-fluid tearing instability

A quasi-linear prediction of the two-fluid dynamo effect is analyzed with the use of tearing eigenfunctions obtained for force-free equilibrium. In the range of parameters of practical interest, the basic shear Alfvén mode is decoupled from fast compressional Alfvén and slow magneto-acoustic modes. Kinetic Alfvén modification of the shear Alfvén wave drives an instability with a growth rate ∝δ^1/3ρ _s ^2/3 , where δ is the electron skin depth and ρ_s is the ion-sound gyroradius. A net dynamo effect parallel to the magnetic field is calculated at ρ _s ?δ for large values of the stability factor \(\Delta '\rho _s^{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}} \delta ^{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}} \gg 1\). The dynamo effect caused by the j×B Hall term dominates the contribution from the v×B term (the alpha effect) by a factor ∝(ρ_s/δ)² in the narrow electron layer, while in the broader ion layer these contributions are comparable. The results are compared with the case of a strong guiding field where ρ _s ?δ and the tearing instability is described by resistive MHD.     

A diffusion driven instability in systems that separate particles by velocity sedimentation.

Velocity sedimentation has been used extensively to separate particles according to the magnitude of their sedimentation velocity in suitable media. This technique has been used over a wide range of particle size from protein molecules, viruses, subcellular particles to whole cells. Successful separation demands that collective particle motion should not occur. In practice it is observed that such systems may, under certain circumstances, suffer from a particular type of instability which destroys the normal dependence of sedimentation velocity on particle size and density. The aim of this paper is to identify the critical parameters that determine the development of this instability. Stability criteria are deduced and predictions of the theory compared with published observations. Satisfactory agreement between theory and observation is obtained. It is concluded that the simple stability criterion, namely that stable sedimentation will occur if the total density gradient is in the direction of the sedimenting force, grossly overestimates the particle load that can be separated in practice. Some specific recommendations for optimum particle loading are included. Earlier theoretical and experimental works are briefly reviewed.     

Sphingomyelinase-induced domain shape relaxation driven by out-of-equilibrium changes of composition

Sphingomyelinase (SMase)-induced ceramide (Cer)-enriched domains in a lipid monolayer are shown to result from an out-of-equilibrium situation. This is induced by a change of composition caused by the enzymatic production of Cer in a sphingomyelin (SM) monolayer that leads to a fast SM/Cer demixing into a liquid-condensed (LC), Cer-enriched and a liquid-expanded, SM-enriched phases. The morphological evolution and kinetic dependence of Cer-enriched domains is studied under continuous observation by epifluorescence microscopy. Domain shape annealing is observed from branched to rounded shapes after SMase activity quenching by EDTA, with a decay halftime of ∼10 min. An out-of-equilibrium fast domain growth is not the determinant factor for domain morphology. Domain shape rearrangement in nearly equilibrium conditions result from the counteraction of intradomain dipolar repulsion and line tension, according to McConnell's shape transition theory. Phase separation causes a transient compositional overshoot within the LC phase that implies an increased out-of-equilibrium enrichment of Cer into the LC domains. As a consequence, higher intradomain repulsion leads to transient branched structures that relax to rounded shapes by lowering the proportion of Cer in the domain to equilibrium values. The fast action of SMase can be taken as a compositional perturbation that brings about important consequences for the surface organization.     

Diffusion as a probe of peptide-induced membrane domain formation

Recently, we have shown that association with an antimicrobial peptide (AMP) can drastically alter the diffusion behavior of the constituent lipids in model membranes (Biochemistry 49, 4672-4678). In particular, we found that the diffusion time of a tracer fluorescent lipid through a confocal volume measured via fluorescence correlation spectroscopy (FCS) is distributed over a wide range of time scales, indicating the formation of stable and/or transient membrane species that have different mobilities. A simple estimate, however, suggested that the slow diffusing species are too large to be attributed to AMP oligomers or pores that are tightly bound to a small number of lipids. Thus, we tentatively ascribed them to membrane domains and/or clusters that possess distinctively different diffusion properties. In order to further substantiate our previous conjecture, herein we study the diffusion behavior of the membrane-bound peptide molecules using the same AMPs and model membranes. Our results show, in contrast to our previous findings, that the diffusion times of the membrane-bound peptides exhibit a much narrower distribution that is more similar to that of the lipids in peptide-free membranes. Thus, taken together, these results indicate that while AMP molecules prompt domain formation in membranes, they are not tightly associated with the lipid domains thus formed. Instead, they are likely located at the boundary regions separating various domains and acting as mobile fences.     

Diacylglycerol-rich domain formation in giant stearoyl-oleoyl phosphatidylcholine vesicles driven by phospholipase C activity

We have studied the effect of phospholipase C from Bacillus cereus and Clostridium perfringens (alpha-toxin) on giant stearoyl-oleoyl phosphatidylcholine (SOPC) vesicles. Enzyme activity leads to a binary mixture of SOPC and the diacylglycerol SOG, which phase separates into a SOPC-rich bilayer phase and a SOG-rich isotropic bulk-like domain embedded within the membrane, as seen directly by phase contrast microscopy. After prolonged enzymatic attack, all bilayer membranes are transformed into an isotropic pure SOG phase as characterized by fluorescence microscopy, differential scanning calorimetry, fluorescence anisotropy measurements, and small angle x-ray scattering. These domains may have biological relevance, serving as storage compartments for hydrophobic molecules and/or catalyzing cellular signaling events at their boundaries. Furthermore, in the early stages of asymmetric enzymatic attack to the external monolayer of giant vesicles, we observe a transient coupling of the second-messenger diacylglycerol to membrane spontaneous curvature, which decreases due to enzyme activity, before domain formation and final vesicle collapse occurs.     

Transient transport across an inhomogeneous blood-retina barrier

A mathematical model for transport across the blood-retina barrier and diffusion into the vitreous body of the human eye is formulated. The eye is modeled as a sphere, the surface of which represents the blood-retina barrier. The equations of the model are solved analytically, using an expansion in spherical harmonics and inversion of the Laplace transform in the time variable. The numerical properties of the solution are investigated and the applicability of the model to the analysis of data from three-dimensional vitreous fluorometry is discussed.     

Structural instability in an autophosphorylating kinase switch

We analyse a simple kinase model that exhibits bistability when there is no protein turnover, and show analytically that the property of being bistable is not necessarily conserved when degradation and synthesis of the kinase are taken into account.     

Diffusion of KCl in an amylose film

A method has been developed measuring the diffusion coefficient of KCl in amylose films. The films were soaked in potassium chloride solutions, then immersed in pure water and conductivity measured as a function of time. Different concentrations of the soaking solution were used and the measurements were made at several temperatures. The diffusion coefficient of KCl was found to be independent of the soaking solution KCl concentration, but found to increase with increasing temperature. The diffusion coefficient values were about one quarter of those found in water and varied from 4.8×10⁻¹⁰ to 11×10⁻¹⁰ m² s⁻¹. The activation energy of diffusion was close to that found in water. Two values for the activation energy were obtained, 20.1 and 14 kJ mol⁻¹, indicating a change in the film structure at 45 °C. Amylose films swelled equally in KCl-solutions and water. The thickness of amylose films doubled and the increase in mass was 100–200% corresponding the decrease of amylose content from about 87 to 37%, when the conditions changed from normal humidity conditions to water.     

A phenomenological model of the current filamentation instability driven by cathode processes in the Livermore spheromak

The current density on the open field lines of the Livermore spheromak (SSPX) typically exceeds the saturation current density of the bulk plasma. We assume that the mechanism that provides conditions for that is associated with the formation of a thin layer near the cathode surface, where both the plasma and the neutral density are higher than in the bulk plasma and where intense ionization occurs. The ions formed in this layer fall back onto the cathode, whereas electrons contribute to the high current density in the bulk plasma. The particle balance in the ionizing layer is determined by the recycling coefficient, which, in turn, depends on the cathode temperature and the sheath voltage. As it turns out, these dependences give rise to an instability that leads to the current filamentation and the formation of hot spots on the cathode surface. The instability can be characterized in a phenomenological manner without going into the details of the structure of the ionizing layer, whose effect on the instability shows up in the form of a couple of numerical coefficients of the order of one. We predict the characteristic size and the shape of the filaments (and the hot spots), which are in a general agreement with discoloration patterns on the surface of the cathode in the SSPX. If the magnetic field is tilted to the surface, the footpoints of the filaments move with a significant velocity, whose direction depends on the ratio of the ion gyroradius and the thickness of the ionizing layer. This instability, although primarily considered in conjunction with the SSPX experiment, may play a role in spherical tokamaks and other systems with coaxial helicity injection.     

Hydromagnetic modes in an inhomogeneous collisionless plasma of finite pressure

The spatial structure and growth rate of hydromagnetic waves with frequencies are considered in a one-dimensional model. It is shown that the wave under consideration is an Alfvén mode modified by the inhomogeneity and anisotropy of the plasma and its finite pressure. Because of these factors, the magnetic field lines oscillate differently in the radial and azimuthal directions and the wave frequency depends on the radial wavenumber. There may be two types of mode structure in the direction across the magnetic shells. When the magnetospheric parameters vary monotonically along the radial coordinate, the mode propagates across the magnetic field lines; because of its resonance with high-energy particles, the radial wavenumber acquires a nonzero imaginary part, which vanishes at the Alfvén resonance surface. In the magnetospheric regions where the main plasma parameters (density or pressure) reach their extreme values, the mode is a standing wave in a direction transverse to the magnetic field lines. In this case, because of the instability, the eigen-frequency of the cavity has a nonzero imaginary part. Under certain, very specific conditions, there can exist drift-mirror waves in the magnetosphere. Such conditions, however, are unlikely to occur in reality. In terms of the modes to be described, it is possible to explain some types of oscillations of the geomagnetic field.     

Reflectionless passage of an electromagnetic wave through an inhomogeneous plasma layer

An exactly solvable model is used as a basis to study the reflectionless passage of a transverse electromagnetic wave through an inhomogeneous plasma containing large-amplitude, small-scale (subwave-length) structures (in particular, opaque regions) that cannot be correctly described by approximate methods. It is shown that, during the reflectionless passage of an electromagnetic wave, strong wave field splashes can occur in certain plasma sublayers. The nonuniform spatial plasma density profile is characterized by a number of free parameters describing the modulation depth of the dielectric function, the characteristic sizes of the structures and their number, the thickness of the inhomogeneous plasma region, and so on. Such plasma density structures are shown to be very diverse when, e.g., a wave that is incident from vacuum propagates without reflection through a plasma layer (wave barrier transillumination). With the cubic nonlinearity taken into account, a one-dimensional problem of the nonlinear transillumination of an inhomogeneous plasma can be solved exactly.     

Structural instability, multiple stable states, and hysteresis in periphyton driven by phosphorus enrichment in the Everglades

Periphyton is a key component of the Everglades ecosystems. It is a major primary producer, providing food and habitat for a variety of organisms, contributing material to the surface soil, and regulating water chemistry. Periphyton is sensitive to the phosphorus (P) supply and P enrichment has caused dramatic changes in the native Everglades periphyton assemblages. Periphyton also affects P availability by removing P from the water column and depositing a refractory portion into sediment. A quantitative understanding of the response of periphyton assemblages to P supply and its effects on P cycling could provide critical supports to decision making in the conservation and restoration of the Everglades. We constructed a model to examine the interaction between periphyton and P dynamics. The model contains two differential equations: P uptake and periphyton growth are assumed to follow the Monod equation and are limited by a modified logistic equation. Equilibrium and stability analyses suggest that P loading is the driving force and determines the system behavior. The position and number of steady states and the stability also depend upon the rate of sloughing, through which periphyton deposits refractory P into sediment. Multiple equilibria may exist, with two stable equilibria separated by an unstable equilibrium. Due to nonlinear interplay of periphyton and P in this model, catastrophe and hysteresis are likely to occur.     

Evolution of the electron Langmuir oscillations in an inhomogeneous magnetized plasma

The conditions under which the energy of the electron Langmuir oscillations can escape from the plasma into vacuum are determined in the simplest model of a plane slab of an inhomogeneous cold magnetized plasma in a uniform magnetic field.     

Lateral Diffusion in an Archipelago: Effects of Impermeable Patches on Diffusion in a Cell Membrane

Lateral diffusion of molecules in lipid bilayer membranes can be hindered by the presence of impermeable domains of gel-phase lipid or of proteins. Effective-medium theory and percolation theory are used to evaluate the effective lateral diffusion constant as a function of the area fraction of fluid-phase lipid and the permeability of the obstructions to the diffusing species. Applications include the estimation of the minimum fraction of fluid lipid needed for bacterial growth, and the enhancement of diffusion-controlled reactions by the channeling effect of solid patches of lipid.     

Effects of an inhomogeneous magnetic field on flowing erythrocytes

Effects of an inhomogeneous magnetic field on narrow erythrocyte streams in a wide and transparent laminar buffer flow were studied. The stream line of erythrocytes containing paramagnetic hemoglobin showed distinct displacement toward the stronger magnetic field. The displacement increased in the order, oxygenated erythrocytes (no displacement), erythrocytes containing cyanomethemoglobin, deoxygenated erythrocytes, erythrocytes containing methemoglobin in the high spin state; more precisely the displacement was proportional to the square of the paramagnetic moment of hemoglobin contained in the erythrocytes. In addition, the displacement was proportional to the product of the magnetic flux density and its gradient, and approximately proportional to the hematocrit of the flowing-erythrocyte suspension, and was much larger than that calculated for a single erythrocyte. These phenomena could be successfully interpreted by the interaction of paramagnetic erythrocytes with the inhomogeneous magnetic field, the resistance force (Stokes Law) from the bulk water, and the hydrodynamic interaction between erythrocytes.