Molecular dynamical calculations of the transport properties of a square-well fluid: V. The coefficient of self-diffusion

The diffusivity of a system, consisting of square-well molecules, has been determined by means of the dynamical computer simulation technique. The calculations offer a systematic investigation of the diffusivity covering almost the whole fluid region. It is found that at low and intermediate densities, the addition of a square negative potential on hard spheres lowers the diffusion coefficient but that at high densities, an increase can also be found, depending on the choice of the potential parameters. A negative first-order and a positive second-order density dependence of the product of density and diffusion coefficient has been found, which is in qualitative correspondence with results of real experiments.     

Combined effects of headgroup charge and tail unsaturation of lipids on lateral organization and diffusion of lipids in model biomembranes

Lateral organization and dynamics of lipids in plasma membranes are crucial for several cellular processes such as signal transduction across the membrane and still remain elusive.In this paper,using coarse-grained molecular dynamics simulation,we theoretically study the combined effects of headgroup charge and tail unsaturation of lipids on the lateral organization and diffusion of lipids in ternary lipid bilayers.In neutral ternary lipid bilayers composed of saturated lipids,unsaturated lipids,and cholesterols,under the conditions of given temperature and components,the main factor for the phase separation is the unsaturation of unsaturated lipids and the bilayers can be separated into liquid-ordered domains enriched in saturated lipids and cholesterols and liquid-disordered domains enriched in unsaturated lipids.Once the headgroup charge is introduced,the electrostatic repulsion between the negatively charged lipid headgroups will increase the distance between the charged lipids.We find that the lateral organization and diffusion of the lipids in the(partially) charged ternary lipid bilayers are determined by the competition between the headgroup charge and the unsaturation of the unsaturated lipids.In the bilayers containing unsaturated lipids with lower unsaturation,the headgroup charge plays a crucial role in the lateral organization and diffusion of lipids.The headgroup charge may make the lipid domains unstable and even can suppress phase separation of the lipids in some systems.However,in the bilayers containing highly unsaturated lipids,the lateral organization and diffusion of lipids are mainly dominated by the unsaturation of the unsaturated lipids.This work may provide some theoretical insights into understanding the formation of nanosized domains and lateral diffusion of lipids in plasma membranes.     

Computational investigation of lipid hydration water of Lα 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine at three hydration levels

Atomistic-scale simulations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayer systems were performed at three hydration levels from 12 water per lipid to 41 water per lipid. The structural and dynamical properties of water at each hydration level are the focus of the current study. The water properties are reported as a function of slab position relative to the lipid central plane using a floating slab model. Water properties depend more strongly on the relative slab position to the lipid than on the hydration level of the lipid. Water hydrogen bonds strengthen as water enters into the lipid, reaching a maximum at the phosphorus density maximum. The ratio of lateral and normal diffusion coefficients of water varies as water enters into the lipid and demonstrates an interesting crossover phenomenon at the phosphorus density maximum regardless of the overall hydration level. While most properties do not change beyond the commonly established excess water point, the water diffusion coefficient still increases upon further hydration. We propose a scheme that classifies lipid hydration water into buried water, interface water, and bulk-like water.     

Lateral diffusion in equimolar mixtures of natural sphingomyelins with dioleoylphosphatidylcholine

Cellular membranes of mammals are composed of a complex assembly of diverse phospholipids. Sphingomyelin (SM) and phosphatidylcholine (PC) are important lipids of eukaryotic cellular membranes and neuronal tissues, and presumably participate in the formation of membrane domains, known as "rafts," through intermolecular interaction and lateral microphase decomposition. In these two-dimensional membrane systems, lateral diffusion of lipids is an essential dynamic factor, which might even be indicative of lipid phase separation process. Here, we used pulsed field gradient nuclear magnetic resonance to study lateral diffusion of lipid components in macroscopically oriented bilayers composed of equimolar mixtures of natural SMs of egg yolk, bovine brain, bovine milk and dipalmitoylphosphatidylcholine (DPPC) with dioleoylphosphatidylcholine (DOPC). In addition, differential scanning calorimetry was used as a complementary technique to characterize the phase state of the lipid bilayers. In fully liquid bilayers, the lateral diffusion coefficients in both DOPC/DPPC and DOPC/SM systems exhibit mean values of the pure bilayers. For DOPC/SM bilayer system, this behavior can be explained by a model where most SM molecules form short-lived lateral domains with preferential SM-SM interactions occurring within them. However, for bilayers in the presence of their low-temperature gel phase, lateral diffusion becomes complicated and cannot simply be understood solely by a simple change in the liquid phase decomposition.     

疏水纳米颗粒对仿生膜流动性的影响: 尺寸效应

金属纳米颗粒与仿生膜的组装材料具有广阔的应用前景, 但纳米颗粒载入对仿生膜的物理性质的影响却存在争议. 在本文中, 我们合成了大于与小于膜厚度的疏水金纳米颗粒 (LPNs与SNPs), 系统地研究了粒子载入对膜流动性的影响. 通过荧光漂白恢复实验发现, LPNs与SNPs分别降低与提高磷脂的侧向扩散率. 基于荧光光谱实验与理论分析, 得到LPNs与SNPs分别通过空间位阻、改变磷脂有序度两种机制影响磷脂的侧向运动. 研究结果对构建功能性纳米颗粒磷脂组装体, 通过纳米材料调控仿生膜功能方面提供了参考.     

Simulation study of lateral diffusion in lipid-sterol bilayer mixtures

We employ off-lattice Monte Carlo simulations to study lateral diffusion in lipid-sterol bilayers using a two-dimensional model system which has been designed to simulate the experimental phase diagrams of both lipid-cholesterol and lipid-lanosterol systems. We focus on the effects of varying sterol concentration and temperature on the tracer diffusion coefficient, D, which characterizes the lateral motion of single tagged lipids in a bilayer. Generally, we find that increasing the cholesterol concentration suppresses D due to an increased conformational ordering of lipid chains. We argue that this effect competes with an increase in the average free area per lipid, which favours an increase in D. At temperatures close to the main transition temperature, the competition between the two effects leads to intriguing behavior of D. Overall, the model results are in excellent qualitative agreement with available experimental results for lipid-cholesterol mixtures. Additional studies of a model lipid-lanosterol system, for which experimental diffusion results are not available, predict that the presence of lanosterol has a smaller effect than cholesterol on reducing D relative to the pure lipid system. We conclude that the molecular model employed contains the essential features required to describe many of the qualitative features of the lateral diffusion behavior in lipid-sterol systems. Received 24 November 2000 and Received in final form 30 April 2001     

Flexoelectricity of lyotropics and biomembranes

Summary Flexoelectric properties of the following lyotropic systems are described: monolayers, bilayers, lamellar lipid-water phases, black lipid membranes and biomembranes. Lipid layers (one-component and mixed) and lipid-protein layers are considered. Different molecular mechanisms (dipolar and quadrupolar) at free and blocked flip-flop and at free and blocked lateral diffusion are discussed in details. Surface potential measurements in monolayers and diamagnetic anisotropy of bilayers are used to evaluate the contribution of the different mechanisms. The area flexoelectric coefficient is typically −5·10⁻¹¹ statC. Biomembranes with free lateral diffusion of the integral proteins (conical and dipolar ones) would exhibit dipolar flexoelectricity, while those with blocked lateral diffusion (high protein content) would exhibit quadrupolar flexoelectricity. The flexoelectric coefficient of biomembranes seems to be one order of magnitude higher than that of the protein-free bilayers and both positive and negative signs are possible. Some mechano-electrical phenomena in membrane systems are discussed in connection to the flexoelectricity. Paper presented at the ?Meeting on Lyotropics and Related Fields?, held in Rende, Cosenza, September 13–18, 1982.     

Effect of Polyacrylic Acid on Phase State of Lipids and Diffusion in Lipid-Water System

Lipid vesicles interacting with polyanions are promising for controlled drug delivery. However, different aspects of the interaction of these polymers with lipids are far from complete understanding. In this work we studied the influence of polyacrylic acid (PAA) with small concentrations (1–4 mol%) on the change of the phase state, lateral diffusion of these lipids in lamellar phase and transmembrane water diffusion in macroscopically oriented bilayers of lipid-water systems formed by dimiristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine. Measurements were performed by ³¹P nuclear magnetic resonance (NMR) spectroscopy and the ¹H NMR technique with a pulsed field gradient. It was found that the presence of PAA does not change the lamellar structure of the system. However, a part of bilayers changes their originally flat geometry and forms vesicles with a higher surface curvature. Macroscopic orientation of bilayers disappeared. For DMPC the presence of PAA leads to a shift of the gel-to-liquid crystalline phase-transition temperature to higher temperatures. An increase of PAA concentration leads to a monotonous decrease in the lateral diffusion coefficient of lipids that is caused, probably, by the ordering of lipids in bilayers. The transbilayer diffusion coefficient of water increases in the presence of PAA, but it depends slightly on the PAA concentration. An increase of pH leads to a change of the lipid lateral and transbilayer diffusion coefficients to the values typical for a pure bilayer. Authors' address: Andrey Filippov, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Microscopic observation of lateral diffusion at Si-SiO2 interface by photoelectron emission microscopy using synchrotron radiation

The lateral surface diffusion at Si-SiO₂ interface has been observed at nanometer scale using photoelectron emission microscopy (PEEM) combined with synchrotron soft X-ray excitation. The samples investigated were Si-SiO_x micro-patterns prepared by O₂⁺ ion implantation in Si (0 0 1) wafer using a mask. The lateral spacial resolution of the PEEM system was about 41 nm. The brightness of each spot in the PEEM images changed depending on the photon energy around the Si K-edge, in proportion to the X-ray absorption intensity of the corresponding valence states. It was found that the lateral diffusion occurs by 400-450 °C lower temperature than that reported for the longitudinal diffusion at the Si-SiO₂ interface. It was also found that no intermediate valence states such as SiO (Si²⁺) exist at the Si-SiO₂ interface during the diffusion. The observed differences between lateral and longitudinal diffusion are interpreted by the sublimated property of silicon monoxide (SiO).     

Dielectric and transport properties of a supercooled symmetrical molten salt

The liquid-glass transition of the restricted primitive model for a symmetrical molten salt is studied using mode-coupling theory. The transition at high densities is predicted to obey the Lindemann criterion for melting, and the charge-density peak found in neutron-scattering experiments on ionic glass formers is qualitatively reproduced. Frequency-dependent dielectric functions, shear viscosities, and dynamical conductivities of the supercooled liquid are presented. Comparing the latter to the diffusion constant, we find that mode-coupling theory reproduces the Nernst-Einstein relation. The Stokes-Einstein radius is found to be approximately equal to the particle radius only near the high-density glass transition.     

Dynamics of a Spinor Exciton–Polariton System in Laterally Strained GaAs Microcavities under Resonant Photoexcitation

The evolution of the spatial coherence and the polarization has been studied in a freely decaying polariton condensate that is resonantly excited by linearly polarized picosecond laser pulses at the lower and upper sublevels of the lower polariton branch in a high-Q GaAs-based microcavity with a reduced lateral symmetry without excitation of the exciton reservoir. It is found that the condensate inherits the coherence of the exciting laser pulse at both sublevels in a wide range of excitation densities and retains it for several dozen picoseconds. The linear polarization of the photoexcited condensate is retained only in the condensate at the lower sublevel. The linearly polarized condensate excited at the upper sublevel loses its stability at the excitation densities higher a threshold value: it enters a regime of internal Josephson oscillations with strongly oscillating circular and diagonal linear degrees of polarization. The polariton–polariton interaction leads to the nonlinear Josephson effects at high condensate densities. All the effects are well described in terms of the spinor Gross–Pitaevskii equations. The cause of the polarization instability of the condensate is shown to be the spin anisotropy of the polariton–polariton interaction.     

Anisotropic water diffusion in macroscopically oriented lipid bilayers studied by pulsed magnetic field gradient NMR

The anisotropy, D(parallel)/D( perpendicular ), of water diffusion in fully hydrated bilayers of dimyristoylphosphatidylcholine at 29 degrees C has been measured by pulsed magnetic field gradient (pfg) NMR. By using NMR imaging hardware to produce magnetic field gradients in an arbitrary direction with respect to a stack of macroscopically aligned lipid bilayers, translational diffusion of water was measured as a function of the angle between the direction of the magnetic field gradient and the normal of the lipid membrane. The observed diffusion coefficient is found to depend strongly on this angle. The anisotropy cannot be accurately determined due to the very small value of D( perpendicular ), but a lower limit of about 70 can be estimated from the observed diffusion coefficients. The results are discussed in terms of the relatively low permeability of water across the lipid bilayer, instrumental limitations, and/or possible defects in the lamellae.     

A theoretical study of the influence of ambipolar diffusion on the population distribution of excited states in a plasma

The effect of charged particle diffusion on excitation temperature was investigated theoretically, assuming that the diffusion processes are ambipolar and that the charged particles return to the plasma in the form of ground state atoms, after recombination at the plasma boundary. An approximate expression was obtained by solving a set of balance equations for the excited state atom densities. It shows that, with increase of the plasma pressure, the excitation temperature rises and approaches the electron temperature. It was found also that, as the electron density increases, the diffusion effect on the excitation temperature diminishes. A criterion was obtained which should be satisfied so that the excitation temperature may be in agreement with the electron temperature. The theoretical results of this paper are in good agreement with experimental results.     

Nonlinear shift of spatial solitons at a graded dielectric interface

We investigate total internal reflection of optical spatial solitons at the interface between two regions of nematic liquid crystals with different optical densities. Due to nonlinear molecular reorientation, the solitons experience a penetration depth, hence, a lateral shift that depends on the excitation, with lateral shifts from 0.7 to 1.2 mm as input powers increased from 1.6 to 9.3 mW.     

Studies of porosity and diffusion coefficient in porous matrices by computer simulations

We present a series of molecular dynamics simulations to study the porosity on different matrix configurations. The matrices were prepared using two different processes. In the fist method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at different densities and at different matrix particle interactions. The results showed that the matrix structure and the matrix porosity were affected by the way the porous matrices were prepared. Finally, we also investigated the diffusion of a fluid inside the matrices. The diffusion coefficient was measured by mean square displacements of the particles in the fluid. It was observed that this quantity was also affected by the kind of porous matrix employed. The calculations were performed for several fluids at different densities in the different porous matrices. From these studies we observed that the highest porosity and diffusion coefficient were found in matrices prepared with attractive particle interactions and without any template.     

Simulation of phase and component segregation in biological membranes

Phase and component segregation in lipid membranes was studied by means of mathematical modeling. The time dependence of phase segregation on the lateral diffusion coefficient was calculated. The phase and component segregations were also simulated by multiparticle Monte-Carlo methods, and the phase diagrams of the system were obtained.     

Self-diffusion of particles interacting through a square-well or square-shoulder potential

The diffusion coefficient and velocity autocorrelation function for a fluid of particles interacting through a square-well or square-shoulder potential are calculated from a kinetic theory similar to the Davis-Rice-Sengers theory and the results are compared to those of computer simulations. At low densities the theory yields too low estimates due to the neglect of correlations between subsequent partial collisions of identical pairs; in particular, the neglect of boundstate effects appears important. At intermediate densities the theory makes reasonable predictions and at high densities it produces too high values, due to the neglect of ring terms and other correlated collision events. The results for the square-shoulder potential generally exhibit better agreement between theory and simulations than do those for the square-well potential.     

Diffusion slowdown in the nanostructured liquid Ga‐Sn alloy

The diffusion of gallium in liquid Ga‐Sn alloy embedded into different porous silica matrices was studied by NMR. Spin relaxation was measured for two gallium isotopes, ⁷¹Ga and ⁶⁹Ga, at two magnetic fields. Pronounced rise of quadrupole contribution to relaxation was observed for the nanostructured alloy which increased with decreasing the pore size. The correlation time of atomic mobility was evaluated and found to be much larger than in the relevant bulk melt which evidenced a pronounced diffusion slowdown in the Ga‐Sn alloy under nanoconfinement. It is shown that the diffusion was slower by a factor of 30 for the alloy within 7 nm pores. The spectral densities of electric field gradients at zero frequency were found to double for the finest pores. The Knight shift was found to decrease but slightly for the nanostructured alloy.     

Diffusion of ions in supercritical water: Dependence on ion size and solvent density and roles of voids and necks

We have performed molecular dynamics simulations of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and halide (F⁻, Cl⁻, Br⁻, I⁻) ions in supercritical water at 673 K. The calculations were done for water at three different densities of 1.0, 0.7 and 0.35 g cm⁻³ to investigate the effects of solute size on the diffusion of ions in supercritical water. On increase of ion size, we observe a maximum for diffusion of ions in supercritical water of higher densities (1.0 and 0.7 g cm⁻³). However, no such maximum is found for ion diffusion in the supercritical water of low density (0.35 g cm⁻³) or for diffusion of neutral solutes at all densities. These results are analyzed in terms of passage through voids and necks present in supercritical water. Correlations of the observed diffusion behavior with the sizes of ions and voids present in the systems are discussed.     

Space charge solution of Rittner photoconductor equation for a HgCdTe detector

The steady-state charge continuity equations are linearized to derive a space charge field that accompanies the ambipolar diffusion and drift described by the Rittner equation. The space charge field is evaluated for a typical 14.2 μm cutoff wavelength HgCdTe detector operating at 85 K. It is found that the space charge density is ∼10⁻⁵ times the hole and electron population density generated by photon flux. This corroborates that Rittner's equation gives an accurate solution for the hole and electron densities. But, at relatively high photon flux levels that are found in some Geostationary Operational Environment Satellite instrument channels, the small average space charge field can have a noticable effect on the linearity of detector response. Divergence of electric field terms in the continuity equations, which are absent from the Rittner equation, can also contribute a non-linearity to detector response.