Hydrodynamic effects on concentration fluctuations in multicomponent membranes

A phenomenological model for the unbinding transitionof multicomponent fluid membranes is proposed,where the unbinding transition is described using atheory analogous to the Flory-Huggins theory for polymers.The coupling between the lateral phase separation of inclusionmolecules and the membrane-substrate potential provides arich phase behavior.Our model describes the first-order natureof the unbinding transition in multicomponent membranes as wasobserved in a recent experiment.In particular, we predictdifferent scenarios of phase coexistence between boundand unbound membrane states.

We consider how membrane fluctuations can modify the miscibility of lipid mixtures, that is to say how the phase diagram of a boundary-constrained membrane is modified when the membrane is allowed to fluctuate freely in the case of zero surface tension. In order for fluctuations to have an effect, the different lipid types must have differing Gaussian rigidities. We show, somewhat paradoxically, that fluctuation-induced interactions can be treated approximately in a mean-field type theory. Our calculations predict that, depending on the difference in bending and Gaussian rigidity of the lipids, membrane fluctuations can either favor or disfavor mixing.

Sub-ten-nanometer heterogeneity of solid supported lipid membranes determined by solution atomic force microscopy

Hydrodynamic Effects in Multicomponent Fluid Membranes

Biological membranes are involved in numerous intriguing biophysical and biological cellular phenomena of different length scales, ranging from nanoscale raft formation, vesiculation, to microscale shape transformations. With extended length and time scales as compared to atomistic simulations, solvent-free coarse-grained membrane models have been exploited in mesoscopic membrane simulations. In this study, we present a one-particle-thick fluid membrane model, where each particle represents a cluster of lipid molecules. The model features an anisotropic interparticle pair potential with the interaction strength weighed by the relative particle orientations. With the anisotropic pair potential, particles can robustly self-assemble into fluid membranes with experimentally relevant bending rigidity. Despite its simple mathematical form, the model is highly tunable. Three potential parameters separately and effectively control diffusivity, bending rigidity, and spontaneous curvature of the model membrane. As demonstrated by selected examples, our model can naturally simulate dynamics of phase separation in multicomponent membranes and the topological change of fluid vesicles.

The multi lipid-bilayer system immersed in the two-component solution is considered. By means of the Monte-Carlo method, the relationships between the fluctuation of the membrane sheets and the concentration fluctuation of the solution are investigated. It is found the concentration fluctuation gives rise to the two competitive effects on the membrane fluctuation.

We report a technique for simultaneous label-free quantification of cytoplasmic hemoglobin Hb concentration and dynamic membrane fluctuation in individual red blood cells (RBCs). Spectroscopic phase microscopy equipped with three different coherent laser sources and a color detector records three wavelength-dependent quantitative phase images in a single shot of a color-coded hologram. Using molecular specific dispersion, we demonstrate the extraction of Hb concentration and the dynamic membrane fluctuation from individual RBCs.

Use of chemigation for the application of fertilizers and chemicals through the irrigation system has increasedworldwide. The concentration is usually controlled and maintained constant over the irrigation duration. Dynamicfluctuation in the concentration may, however, result in inaccurate application of the fertilizers and chemicals. This may, inturn, cause a reduction in crop yield when the application amounts are lower than the planned amounts, or present anenvironmental hazard when they are higher. These dynamic fluctuations in the chemical concentration of the irrigation waterdecay with soil depth due to the convection-dispersion nature of the transport process of infiltrating water during irrigation.An analytical solution, which employs the Fourier Transform method, is used here to estimate the decay of these dynamicfluctuations as water infiltrates from the soil surface to the root zone. This solution assumed constant irrigation flux and soilmoisture conditions, typical to high frequency irrigation. It assumes periodic fluctuations in the chemical concentration ofthe irrigation water. A general analytical solution was derived for these asymmetric dynamic periodic fluctuations. The effectsof fluctuation period, asymmetric time factor, and high and low concentration values within the irrigation period on the decayprocess were evaluated and results are presented for clay loam and sandy loam soils. Damping depth was chosen tocharacterize this decay process. It is the depth at which the amplitude of the chemical concentration decays by a factor exp(-1). 37% of its value at the soil surface Two (high and low) damping depths characterize the decay process of asymmetric timefluctuations. Damping depths for symmetric fluidations in clay loam were 21, 58, 138 mm for periods of 2, 10, 30 days, and78, 239, 682 mm in sandy loam for the same periods, showing the significance of soil texture and fluctuation periods. Theeffect of asymmetry was assessed for constant average concentration. High damping depths values of 92, 156, 235, and308 mm were computed for asymmetric factors 0.1, 0.25, 0.50, and 0.80, while the low damping depths were 322, 296, 239,and 145 mm, respectively, in sandy loam and 10 days period, showing a significant effect of asymmetry. This analysis is usefulfor assessing the level of accuracy required for controlling chemical concentration during chemigation, considering the soilbuffer effect.

Extractant aggregation in liquid-liquid extraction organic phases impacts extraction energetics and is related to the deleterious efficiency-limiting liquid-liquid phase transition known as third phase formation. Using small angle X-ray scattering, we find that structural heterogeneities across a wide range of compositions in binary mixtures of malonamide extractants and alkane diluents are well described by Ornstein-Zernike scattering. This suggests that structure in these simplified organic phases originates from the critical point associated with the liquid-liquid phase transition. To confirm this, we measure the temperature dependence of the organic phase structure, finding critical exponents consistent with the 3D Ising model. Molecular dynamics simulations were also consistent with this mechanism for extractant aggregation. Due to the absence of water or any other polar solutes required to form reverse-micellar-like nanostructures, these fluctuations are inherent to the binary extractant/diluent mixture. We also show how the molecular structure of the extractant and diluent modulate these critical concentration fluctuations by shifting the critical temperature: critical fluctuations are suppressed by increasing extractant alkyl tail lengths or decreasing diluent alkyl chain lengths. This is consistent with how extractant and diluent molecular structure are known to impact metal and acid loading capacity in many-component LLE organic phases, suggesting phase behavior of practical systems may be effectively studied in simplified organic phases. Overall, the explicit connection between molecular structure, aggregation and phase behavior demonstrated here will enable the design of more efficient separations processes.

We study material transport and permeation through a lamellar stack of multi-component lipid membranes by performing Monte Carlo simulations of a stacked two-dimensional Ising model in the presence of permeants. In the model, permeants are transported through the stack via in-plane lipid clusters, which are inter-connected in the vertical direction. These clusters are formed transiently by concentration fluctuations of the lipid mixture, and the permeation process is affected especially close to the critical temperature of the binary mixture. We show that the permeation rate decays exponentially as a function of temperature and permeant lateral size, whereas the dependency on the characteristic waiting time obeys a stretched exponential function. The material transport through such lipid clusters can be significantly affected around physiological temperatures.

Diffusion of spheres in suspension: Three-body hydrodynamic interaction effects

Photon correlation spectroscopy (PCS) and dielectric spectroscopy (DS) are employed to study the dynamics of density fluctuations at temperatures above and below the T g and the dynamics of concentration fluctuations above T g in concentrated solutions of poly(methylmethacrylate)/bis(2-ethylhexyl phthalate) (PMMA/DOP). The PCS measurements covered the temperature range between 252 and 313 K, and the DS measurements were made between 200 and 398 K and in the frequency range between 10 -2 and 10 5 Hz. Above T g the polarized PCS correlation functions contain contributions from both density and concentration fluctuations for a range of temperatures

A thermodynamic model describing multicomponent fluid membranes under the influence of lateral forces due to a buried charge is developed to estimate the magnitude and dynamics of field-induced lateral reorganization in membranes of living cells. The calculations were performed for an electric field geometry such as might be produced by a charge in an ion channel but can apply to any situation where charge is buried in the low dielectric environment of a membrane, for example, during electron transfer. The model includes molecular size and charge differences as well as critical demixing effects. These calculations indicate that the local concentration of charged and uncharged membrane components can be substantially altered over the course of a few microseconds. Critical demixing effects enhance the propensity of a membrane toward lateral reorganization and provide an intriguing mechanism by which an electric field can induce separation between neutral molecules such as cholesterol and phosphatidylcholine.

In this paper, we present a method to quantify the extent of disorder in a system by using conditional entropies. Our approach is especially useful when other global, or mean field, measures of disorder fail. The method is equally suited for both continuum and lattice models, and it can be made rigorous for the latter. We apply it to mixing and demixing in multicomponent fluid membranes, and show that it has advantages over previous measures based on Shannon entropies, such as a much diminished dependence on binning and the ability to capture local correlations. Further potential applications are very diverse, and could include the study of local and global order in fluid mixtures, liquid crystals, magnetic materials, and particularly biomolecular systems.

Criticality in Plasma Membranes