Determination of interphase line tension in Langmuir films

Hallmarks of Reversible Separation of Living, Unperturbed Cell Membranes into Two Liquid Phases

Controversy has long surrounded the question of whether spontaneous lateral demixing of membranes into coexisting liquid phases can organize proteins and lipids on micron scales within unperturbed, living cells. A clear answer hinges on observation of hallmarks of a reversible phase transition. Here, by directly imaging micron-scale membrane domains of yeast vacuoles both in vivo and cell free, we demonstrate that the domains arise through a phase separation mechanism. The domains are large, have smooth boundaries, and can merge quickly, consistent with fluid phases. Moreover, the domains disappear above a distinct miscibility transition temperature (T_mix) and reappear below T_mix, over multiple heating and cooling cycles. Hence, large-scale membrane organization in living cells under physiologically relevant conditions can be controlled by tuning a single thermodynamic parameter.

Structure of DPPC-hyaluronan interfacial layers - effects of molecular weight and ion composition

Hyaluronan and phospholipids play an important role in lubrication in articular joints and provide in combination with glycoproteins exceptionally low friction coefficients. We have investigated the structural organization of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) Langmuir layers at the solution-air interface at different length scales with respect to the adsorption of hyaluronan (HA). This allows us to assemble a comprehensive picture of the adsorption and the resulting structures, and how they are affected by the molecular weight of HA and the presence of calcium ions. Brewster angle microscopy and grazing incident diffraction were used to determine the lateral structure at the micro- and macro scale. The data reveals an influence of HA on both the macro and micro structure of the DPPC Langmuir layer, and that the strength of this effect increases with decreasing molecular weight of HA and in presence of calcium ions. Furthermore, from X-ray reflectivity measurements we conclude that HA adsorbs to the hydrophilic part of DPPC, but data also suggest that two types of interfacial structures are formed at the interface. We argue that hydrophobic forces and electrostatic interactions play important rules for the association between DPPC and HA. Surface pressure area isotherms were used to determine the influence of HA on the phase behavior of DPPC while electrophoretic mobility measurements were used to gain insight into the binding of calcium ions to DPPC vesicles and hyaluronan.

Accurate control of hyperbolic trajectories in any dimension

The unsteady (nonautonomous) analog of a hyperbolic fixed point is a hyperbolic trajectory, whose importance is underscored by its attached stable and unstable manifolds, which have relevance in fluid flow barriers, chaotic basin boundaries, and the long-term behavior of the system. We develop a method for obtaining the unsteady control velocity which forces a hyperbolic trajectory to follow a user-prescribed variation with time. Our method is applicable in any dimension, and accuracy to any order is achievable. We demonstrate and validate our method by (1) controlling the fixed point at the origin of the Lorenz system, for example, obtaining a user-defined nonautonomous attractor, and (2) the saddle points in a droplet flow, using localized control which generates global transport.

Coarsening dynamics of domains in lipid membranes

We investigate isothermal diffusion and growth of micron-scale liquid domains within membranes of free-floating giant unilamellar vesicles with diameters between 80 and 250 μm. Domains appear after a rapid temperature quench, when the membrane is cooled through a miscibility phase transition such that coexisting liquid phases form. In membranes quenched far from a miscibility critical point, circular domains nucleate and then progress within seconds to late stage coarsening in which domains grow via two mechanisms 1), collision and coalescence of liquid domains, and 2), Ostwald ripening. Both mechanisms are expected to yield the same growth exponent, α = 1/3, where domain radius grows as time(α). We measure α = 0.28 ± 0.05, in excellent agreement. In membranes close to a miscibility critical point, the two liquid phases in the membrane are bicontinuous. A quench near the critical composition results in rapid changes in morphology of elongated domains. In this case, we measure α = 0.50 ± 0.16, consistent with theory and simulation.

Molecular determinants for the line tension of coexisting liquid phases in monolayers

The line tension (λ) in biphasic membranes has been determined in monolayers and bilayers using a variety of techniques. In this work we present a novel approach to the determination of λ in monolayers with liquid/liquid phase coexistence, overcoming several of the drawbacks of current techniques. Using our method, we determined the line tension of liquid/liquid phases in binary mixtures of different lipids and a molecule similar to cholesterol but less oxidizable. We analyzed the effect of the hydrocarbon chain length and the polar head-group of the non-sterol lipid and found the latter to exert much more influence than the former. The presence of PE led to high λ values, PG to low values and PS and PC to intermediate values. The line tension showed a strong correlation with the critical packing parameter of the phospholipid. The spontaneous curvature displayed by the phases constituted by a particular lipid appears to be an important parameter for determining the line tension in mixed films.

Oxidized phosphatidylcholines promote phase separation of cholesterol-sphingomyelin domains

Lipid lateral segregation in the plasma membrane is believed to play an important role in cell physiology. Sphingomyelin (SM) and cholesterol (Chol)-enriched microdomains have been proposed as liquid-ordered phase platforms that serve to localize signaling complexes and modulate the intrinsic activities of the associated proteins. We modeled plasma membrane domain organization using Langmuir monolayers of ternary POPC/SM/Chol as well as DMPC/SM/Chol mixtures, which exhibit a surface-pressure-dependent miscibility transition of the coexisting liquid-ordered and -disordered phases. Using Brewster angle microscopy and Langmuir monolayer compression isotherms, we show that the presence of an oxidatively modified phosphatidylcholine, 1-palmitoyl-2-azelaoyl-sn-glydecero-3-phosphocholine, efficiently opposes the miscibility transition and stabilizes micron-sized domain separation at lipid lateral packing densities corresponding to the equilibrium lateral pressure of ∼32 mN/m that is suggested to prevail in bilayer membranes. This effect is ascribed to augmented hydrophobic mismatch induced by the oxidatively truncated phosphatidylcholine. To our knowledge, our results represent the first quantitative estimate of the relevant level of phospholipid oxidation that can potentially induce changes in cell membrane organization and its associated functions.

To mix or not to mix: 2D crystallization and mixing behavior of saturated and unsaturated aliphatic primary amides

Physisorbed monolayers based on relatively weak noncovalent interactions can serve as excellent model systems for understanding crystallization of materials in reduced dimensionality. Here we employ a powerful combination of scanning tunneling microscopy (STM), differential scanning calorimetry (DSC), and computational modeling to reveal two-dimensional (2D) crystallization and mixing behavior of saturated and unsaturated (cis as well as trans) aliphatic primary amides. The foundation of the present work is laid by DSC measurements, which reveal characteristic adsorption and mixing behavior of aliphatic amides. These results are further supported by STM visualization of the adlayers. STM reveals, at submolecular resolution, the adsorption as well as the two-component 2D phase behavior of these molecules at the liquid-solid interface. The saturated and trans-unsaturated amides exhibit random mixing in view of their size and shape complementarity. Binary mixtures of saturated and cis-unsaturated amides, on the other hand, display unprecedented mixing behavior. The linear saturated and bent cis-unsaturated amide molecules are found to mix surprisingly better at the liquid-solid interface than might have been expected on account of the dissimilarity in their shapes. Strong, directional intermolecular hydrogen-bonding interactions as well as the relative stabilization energies of the adlayers are responsible for such unusual mixing behavior. Computational modeling provides additional insight into all the possible interactions in 2D assemblies and their impact on stabilization energies of the supramolecular networks. This study provides a model for understanding the effect of nanoscale cocrystallization on the thin film structure at interfaces and demonstrates the importance of molecular geometry and hydrogen bonding in determining the coadsorption behavior.

Experimental study of hybrid nematic wetting films

Liquid crystal layers, with thickness less than 1 μm, are deposited on isotropic - solid or liquid - substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as "hybrid nematics". The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework.

Shear-induced domain deformation in a tilted lipid monolayer: from circle to ellipse and kinked stripe

The shear-induced domain deformation in a lipid monolayer comprised of tilted molecules is studied as a mechanical balance between surface pressure, line tension, electrostatic energy due to the dipole-dipole interaction, hexatic-elastic stress, and viscous stress. It is found that a simple shear can deform a circular domain into an elliptic shape with the long axis inclined 45 degrees from the shear direction. The "ellipse" is elongated in the long axis as shear rate increases, and evolves to a straight or kinked stripe, which was observed as a "shear band" by Fuller's group [Science 274, 233 (1996)] and "avalanche-like fronts" by Schwaltz's group [Langmuir 17, 3017 (2001)], at a threshold shear rate. The propagation of stripe-shaped domains is discussed in the context of electrostatic energy. The dependence of the threshold shear rate on surface pressure is predicted in good agreement with observation and can be used to estimate surface viscosity. The shear-induced domain deformation is maintained by the effect of the lattice elastic stress when shear ceases.

Line tension and structure of smectic liquid-crystal multilayers at the air-water interface

At the air-water interface, 4'-8-alkyl[1,1'-biphenyl]-4-carbonitrile (8CB) domains with different thicknesses coexist in the same Langmuir film, as multiple bilayers on a monolayer. The edge dislocation at the domain boundary leads to line tension, which determines the domain shape and dynamics. By observing the domain relaxation process starting from small distortions, we find that the line tension lambda is linearly dependent on the thickness difference DeltaL between the coexisting phases in the film, lambda=(3.3+/-0.2) mN/m DeltaL. Comparisons with theoretical treatments in the literature suggest that the edge dislocation at the boundary locates near the center of the film, which means that the 8CB multilayers are almost symmetric with respect to the air-water interface.