Session V63: Membranes, Micelles and Vesicles

Electrostatic shape control of a charged molecular membrane from ribbon to scroll

Bilayers of amphiphiles can organize into a number of distinct mesoscopic shapes, which interconvert under suitable conditions. The pathway for such transformations is often elusive. We use a charged amphiphile (palmitoyl-lysine, C₁₆-K₁) to elucidate the planar nanoribbon to cochleate transition induced by salt (NaCl) concentration (c). In-situ small- and wide-angle X-ray scattering (SAXS/WAXS), atomic force and cryogenic transmission electron microscopies (AFM and cryo-TEM) tracked transformations over Å to µm length scales. AFM reveals that the large length (L) to width (W) ratio nanoribbons (L/W > 10) convert to sheets (L/W ~ 1) before rolling into cochleates. A theoretical model based on electrostatic and surface energies shows that the ribbon to sheet conversion is a first order transition, occurring at a critical Debye length. SAXS shows that the interbilayer spacing (D) in the cochleates scales linearly with Debye length (or c ^-1/2). Theoretical arguments that include electrostatic, bending and van der Waals energies explain the membrane rolling and the linearity between D and Debye length. These models suggest that the salt-induced ribbon to cochleate transition should be common to all charged bilayers possessing an intrinsic curvature.
  

A gel based on vesicle-vesicle adhesion induced by polyelectrolyte: from morphology to rheology

Phospholipid membranes are of great interest for encapsulation due to its outstanding barrier properties. Here we show that one can build an elastic solid with giant vesicles (1-30 mm diameter) that adhere to one another. A slight negative potential on the lipid membrane enables electrostatic attraction with polycations, which induces vesicle-vesicle adhesion. The vesicle adhesion leads to a macroscopic vesicle based gel that is >99% water but has a closed-cell structure. Shear rheology measurements show a low-frequency modulus of a few Pa and linear response extending to very large strains of approximately 0.7. We find that the vesicle adhesion strength and shear modulus can be tuned with polyelectrolyte concentration. We also find that the shear modulus does not scale with vesicle size as long as the adhesion strength is held constant. The vesicle gel system shows great potential as a soft-solid platform for a stimulus-responsive material that can deliver reagents on demand.   

Controlling the elasticity of lipid membranes with pluronic block copolymers

Pluronic triblock copolymers consisting of a hydrophobic poly(propylene-oxide) (PPO) block, flanked by two hydrophilic poly(ethylene-oxide) (PEO) blocks have been widely investigated in drug delivery applications as membrane sealants and plasticizers. However, despite a single pluronic (F68) being FDA approved, there is minimal insight into how block copolymer architecture can be engineered to dictate membrane response. Here, we investigate the influence of the structure of the pluronic on the interaction with artificial biological membranes using the large-area model biomembrane (LAMB) technique. We systematically measured the change in membrane tension and elasticity of a planar, freestanding DOPC membrane upon interaction with several pluronics with with varying PEO and PPO chain lengths and HLB values. Based on the membrane tension - area relationship, the apparent elasticity modulus was calculated. Interestingly, the main factor determining the membrane elasticity is the length of the hydrophilic PEO chain, rather than the hydrophobic PPO block. We also discuss the effect of concentration above and below the cmc on the membrane elasticity. These findings may prove beneficial for future medical applications of pluronics as treatment for membrane-tension mediated maladies.   

Spontaneous division of model protocell membranes

Prior to the existence of phospholipid membranes, the ancestors of modern day cells likely had membranes consisting of much simpler molecules. On early Earth, these membranes would have provided the function that modern cell rely on proteins for - nutrient transport, growth and division - prompting the question of whether such functions can arise from a lipid-only system.

We show that fatty acids can self-assemble into giant unilamellar vesicles (GUVs) without the need for scaffolding droplets, substrates, or microfluidics. Remarkably, such GUVs are capable of spontaneously dividing upon an increase in surface area to volume ratio, instead of simply forming tubular vesicles. When nucleic acids are encapsulated, the protocells - membranous compartments containing nucleic acids - are able to divide into daughter protocells without apparent loss of content.

We find that the bilayer properties of fatty acid membranes depend on the pH and ionic strength of the surrounding solution, and explore how the differences between fatty acid membranes and the better-studied phospholipid systems enable the unusual self-assembly and morphology changes we see.   

Direct imaging of micelle dynamics with an electron microscope

Direct imaging of single micelle dynamics and the kinetics of their assembly is not possible with conventionally used imaging techniques such as transmission electron microscopy (TEM) or cryo-TEM. In this talk, I will discuss our efforts to image the dynamics of simple block copolymer micelles using liquid-phase TEM. By directly visualizing small micellar assemblies in solution, we demonstrate the structure-dependent dynamics of these assemblies, with intriguing surprises.   

The role of particle shape in the deformation and disruption of lipid membranes : Experiments with tunable particle shape and adhesion

We seek to understand the effects of DNA origami nano-rods on membrane structure and morphology. We combine giant unilamellar lipid vesicles (GUVs) with a sufficiently high concentration of oppositely charged nano-rods and observe the interactions. The adhesion of the nano-rods to the membrane is a tunable parameter controlled by the lipid composition, and results in three primary behaviors. At weak adhesion strengths, vesicles adhere to one another and form a stable gel, with the nano-rods acting as a glue that holds the gel together. At intermediate adhesion strengths, gel forms but is subsequently destroyed by avid binding of the nano-rods. At higher adhesion strengths, the vesicles are ruptured by the nano-rods without ever forming a gel. These behaviors can be explained respectively by shallow, deep, or complete wrapping of the nano-rods onto the lipid membrane. These results are a robust example of tuning response in a synthetic membrane system and provide a physical understanding of the design principles toward controlled membrane morphologies. These results will lead to a bio-inspired membrane material that is stimuli-responsive, has high surface area and is reconfigurable.   

Controlling Cargo Trafficking in Multicomponent Membranes

Biological membranes typically contain a large number of different components dispersed in small concen-trations in the main membrane phase, including proteins, sugars, and lipids of varying geometrical properties. Most of these components do not bind the cargo. Here, we show that such “inert” components can be crucial for the precise control of cross-membrane trafficking. Using a statistical mechanics model and molecular dynamics simulations, we demonstrate that the presence of inert membrane components of small isotropic curvatures dramatically influences cargo endocytosis, even if the total spontaneous curvature of such a membrane remains unchanged. Our results suggest a robust and general method of controlling cargo trafficking by adjusting the membrane composition without needing to alter the concentration of receptors or the average membrane curvature. This study indicates that cells can prepare for any trafficking event by incorporating curved inert components in either of the membrane leaflets.

Refs:
T. Curk, P. Wirnsberger, J. Dobnikar, D. Frenkel, and A. Šarić, Nano Lett., 18, 5350–5356 (2018)
T. Curk, J. Dobnikar, and D. Frenkel, PNAS. 114, 7210–7215 (2017)   

Precision measurements of lipid membrane hydrodynamic drag

The hydrodynamic drag at a lipid bilayer determines, among other things, the flow properties of suspensions of cells and liposomes. For spherical liposomes or lipid vesicles, the Stokes equation relates the drag coefficient ζ to the radius r and the fluid viscosity η by ζ = Cπηr, where the dimensionless C depends on the hydrodynamic boundary between the sphere and the fluid. For solid spheres C=6 and for liquid spheres C=4. This constant has been assumed to be 6 for lipid membranes, but we have found no direct measurements of it, and given the fluidity of lipid membranes it is not obvious that the solid-sphere value applies. The Stokes-Einstein relationship D = k_BT/(Cπηr), where D is the diffusion coefficient, k_B is Boltzmann's constant, and T is the temperature allows determination of C provided D can be determined precisely. We use light sheet fluorescence microscopy to image lipid vesicles far from surfaces that complicate application of the Stokes-Einstein relationship. Combined with high-accuracy image localization, this allows us to measure C, finding that it closely matches the C=6 value of a solid sphere. We comment on deviations from C=6 for non-spherical vesicles with thermally driven undulations.   

Shape Control of Deformable Nanocontainers by Modulating Surface Tension and Charge Patterning

Shape-reconfigurable polymeric vesicles, micelles and other soft-matter-based nanomembranes have important applications as adaptive drug-delivery carriers that change shape in response to external cues, and as dynamic building blocks for designing reconfigurable materials. The shape of these nanocontainers can be controlled by modulating their surface composition and environment. Molecular dynamics simulations are used to explore the role of surface tension and surface charge patterning in changing the shape of charged, elastic, volume-preserving nanocontainers. For homogeneously charged nanocontainers, sphere-to-rod-to-disk shape transitions are shown to be facilitated by decreasing surface tension; this could be realized by functionalizing the nanocontainer surface with surfactants. For inhomogeneous, hemispherically-charged Janus nanocontainers, transitions to hemispherical shapes are observed. Shape control of nanocontainers with inhomogeneous surface charge patterns characteristic of pH values near the pKa of the charged surface moieties is discussed. Robustness of shape transitions is investigated by including the effects of ions via mean-field models and explicit-ion simulations.   

Morphology of toroidal vesicles

The equilibrium shapes of toroidal fluid vesicles have been studied for many years. Recently, a new class of stable nonaxisymmetric circular shapes has been observed both from experiment and simulation. This shape contains a necklace-like main ring, usually accompanied by buds. Using Monte Carlo simulation and theoretical analysis, we investigate the morphology of toroidal vesicle. Based on Delaunay unduloids we give a perturbative estimation of the properties of the main ring.   

A

We report on a complex fingering pattern that forms in an approximately 25μm-diameter circular lipid membrane adhered by biotin-avidin bonds to a solid supported membrane. The experiment involves applying tension to form a tear in the bound circular membrane. The pattern in the torn membrane then evolves as 1μm-scale pores form intermittently on the scale of tens of seconds at the boundary of the tear. As the pores form, the biotin-avidin bonds are laterally compressed. As the density of bonds increases, the width of the pores decreases linearly and the rate of new pore formation decreases exponentially. Finally, when the biotin-avidin bonds are compressed into a solid-like state, the process arrests leaving a complex finger pattern in the membrane. We show that these findings are consistent with a thermodynamic description of tension induced pore formation.   

Arbitrary Lagrangian–Eulerian finite element method for biological lipid membranes

We present an arbitrary Lagrangian–Eulerian (ALE) finite element method for arbitrarily curved and deforming lipid membranes. We provide a formalism to determine the equations of motion governing lipid membrane behavior using an irreversible thermodynamic analysis of curved surfaces. We develop an ALE theory by endowing the surface with a mesh whose in-plane velocity is independent of the in-plane material velocity, and which can be specified arbitrarily. The general isoparametric finite element implementation of the theory, based on an arbitrary surface parametrization with curvilinear coordinates, is used to model lipid membranes in several biologically relevant situations. A new physical insight is obtained by applying the ALE developments to cylindrical lipid membrane tubes: though lipid membrane tubes are stable, in the limit of vanishing bending rigidity (the limiting case of a fluid film) we numerically and analytically find tubes to be unstable with respect to long-wavelength perturbations when their length exceeds their circumference.   

Stretching induced disk-to-ribbon transition of chiral colloidal membranes

Colloidal membranes are fluid monolayers consisting of rod-like virus particles held together by the depletion interaction. When optical tweezers are used to apply a diametric stretching force, these disks transform into twisted ribbons. We measure the force and pitch as functions of membrane extension and identify three distinct regimes of twisting. Assuming that the bending stiffness of the membrane is large, we also calculate the shape of the membrane as it deforms using a purely geometric theory that effectively incorporates liquid crystal degrees of freedom.   

A Universal Law for Interaction of 2D Materials with Cellular Membranes

Understanding the interaction of 2D materials including graphene-based nano-sheets, boron ni- tride and MoS2 with biological systems is a growing topic of interest to many applications such as biosensors, drug delivery, gene therapy and nanotoxicity. In this paper, we show that the interaction of 2D materials with cellular membranes at its early stage of approaching is dominantly controlled by entropic factors. Recent experiments indicate that graphene sheets, depending on their size, can either undergo a near-orthogonal cutting or a parallel attachment mode of interaction with cell membranes. Here we perform a theoretical statistical mechanics analysis as well as coarse-grained molecular dynamics simulations to characterize the entropic energy barrier for these modes of inter- actions. Our results indicate that micro-sized graphene sheets strongly prefer to approach cellular membranes through their sharp corners. In contrast, nano-sized sheets are likely to adhere to the membranesurface,insteadofpiercingthroughtheirsharpcorners.