Stacks Of Bilayers Research Articles

Multi-Stacked Supported Lipid Bilayer Micropatterning through Polymer Stencil Lift-Off.

Complex multi-lamellar structures play a critical role in biological systems, where they are present as lamellar bodies, and as part of biological assemblies that control energy transduction processes. Multi-lamellar lipid layers not only provide interesting systems for fundamental research on membrane structure and bilayer-associated polypeptides, but can also serve as components in bioinspired materials or devices. Although the ability to pattern stacked lipid bilayers at the micron scale is of importance for these purposes, limited work has been done in developing such patterning techniques. Here, we present a simple and direct approach to pattern stacked supported lipid bilayers (SLBs) using polymer stencil lift-off and the electrostatic interactions between cationic and anionic lipids. Both homogeneous and phase-segregated stacked SLB patterns were produced, demonstrating that the stacked lipid bilayers retain lateral diffusivity. We demonstrate patterned SLB stacks of up to four bilayers, where fluorescence resonance energy transfer (FRET) and quenching was used to probe the interactions between lipid bilayers. Furthermore, the study of lipid phase behaviour showed that gel phase domains align between adjacent layers. The proposed stacked SLB pattern platform provides a robust model for studying lipid behaviour with a controlled number of bilayers, and an attractive means towards building functional bioinspired materials or devices.

Stabilising lamellar stacks of lipid bilayers with soft confinement and steric effects.

Structure and interactions stabilising the lamellar stack of mixed lipid bilayers in their fluid state are investigated by means of small-angle X-ray scattering. The (electrically neutral) bilayers are composed of a mixtures of lecithin, a zwitterionic phospholipid, and Simulsol, a non-ionic cosurfactant with an ethoxylated polar head. The soft confinement of the bilayer hydrophilic components is varied by changing hydration and bilayer composition, as well as the length of the cosurfactant polar head. Structural transitions are observed at low hydration, in the stacking order for the longer cosurfactant, and in the mixed bilayers for the shorter one. At higher hydration, the swelling of the lamellar stacks occurs with a significant, but continuous evolution in the mixed bilayer structure. The bilayer structural changes are discussed in analogy with the so-called "brush-to-mushroom" transition induced by lateral confinement, relevant for long linear polymers grafted onto rigid surfaces, taking also into account the role of vertical confinement.

Structure and dynamics of POPC bilayers in water solutions of room temperature ionic liquids.

Molecular dynamics simulations in the NPT ensemble have been carried out to investigate the effect of two room temperature ionic liquids (RTILs), on stacks of phospholipid bilayers in water. We consider RTIL compounds consisting of chloride ([bmim][Cl]) and hexafluorophosphate ([bmim][PF6]) salts of the 1-buthyl-3-methylimidazolium ([bmim](+)) cation, while the phospholipid bilayer is made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Our investigations focus on structural and dynamical properties of phospholipid and water molecules that could be probed by inelastic and quasi-elastic neutron scattering measurements. The results confirm the fast incorporation of [bmim](+) into the lipid phase already observed in previous simulations, driven by the Coulomb attraction of the cation for the most electronegative oxygens in the POPC head group and by sizeable dispersion forces binding the neutral hydrocarbon tails of [bmim](+) and of POPC. The [bmim](+) absorption into the bilayer favours the penetration of water into POPC, causes a slight but systematic thinning of the bilayer, and further stabilises hydrogen bonds at the lipid/water interface that already in pure samples (no RTIL) display a lifetime much longer than in bulk water. On the other hand, the effect of RTILs on the diffusion constant of POPC (DPOPC) does not reveal a clearly identifiable trend, since DPOPC increases upon addition of [bmim][Cl] and decreases in the [bmim][PF6] case. Moreover, because of screening, the electrostatic signature of each bilayer is only moderately affected by the addition of RTIL ions in solution. The analysis of long wavelength fluctuations of the bilayers shows that RTIL sorption causes a general decrease of the lipid/water interfacial tension and bending rigidity, pointing to the destabilizing effect of RTILs on lipid bilayers.

Self-induced uniaxial strain in MoS2 monolayers with local van der Waals-stacked interlayer interactions.

Strain engineering is an effective method to tune the properties of electrons and phonons in semiconductor materials, including two-dimensional (2D) layered materials (e.g., MoS2 or graphene). External artificial stress (ExAS) or heterostructure stacking is generally required to induce strains for modulating semiconductor bandgaps and optoelectronic functions. For layered materials, the van der Waals-stacked interlayer interaction (vdW-SI) has been considered to dominate the interlayer stacking and intralayer bonding. Here, we demonstrate self-induced uniaxial strain in the MoS2 monolayer without the assistance of ExAS or heterostructure stacking processes. The uniaxial strain occurring in local monolayer regions is manifested by the Raman split of the in-plane vibration modes E2g(1) and is essentially caused by local vdW-SI within the single layer MoS2 due to a unique symmetric bilayer stacking. The local stacked configuration and the self-induced uniaxial strain may provide improved understanding of the fundamental interlayer interactions and alternative routes for strain engineering of layered structures.

Probing Cellular Mechano-Sensitivity using Biomembrane-Mimicking Cell Substrates of Adjustable Stiffness

Mechanical cues are increasingly recognized to play a crucial role in the regulation of cellular fate and function. However, the underlying mechanisms of cellular mechano-sensing still remain a topic of open debate. Traditionally, advancements in this field have been made using polymeric substrates of adjustable stiffness with immobilized linkers. Recently, we introduced an alternative strategy, in which cells are plated on a laminin-functionalized, polymer-tethered multi-bilayer stack of adjustable substrate stiffness.1-3 Here, we apply the biomembrane-mimicking cell substrate design to explore the mechano-sensitivity of C2C12 myoblasts in the presence of N-cadherin linkers. Experiments are presented, which illustrate a relationship between the degree of bilayer stacking and properties of plated cells, such as morphology, cytoskeletal organization, cellular traction forces, and migration speed. Furthermore, we demonstrate the dynamic assembly of bilayer-bound N-cadherin linkers underneath cellular adherens junctions. In addition, properties of individual and clustered N-cadherins are examined in the polymer-tethered bilayer system in the absence of plated cells.On the basis of the biomembrane-mimicking cell substrate concept, we also discuss recent results on a linker-functionalized single polymer-tethered bilayer substrate with a lateral gradient in lipopolymer concentration (substrate viscoelasticity). Specifically, we show that the lipopolymer gradient has a notable impact on spreading, cytoskeletal organization, and motility of 3T3 fibroblasts. Two cases are discussed: (1) polymer-tethered bilayers with a sharp boundary between low and high lipopolymer concentration regions and (2) polymer-tethered bilayers with a gradual gradient in lipopolymer concentration. Another set of experiments demonstrates that substrates with lipopolymer gradient can also be built using a polymer-tethered double bilayer system.1. D. E. Minner et al. (2013) Soft Matter 9, 9643.2. D. E. Minner et al. (2014) Soft Matter 10, 1189.3. L. A. Lautscham et al. (2014) Biomaterials 35, 3198.

Fluorination of isotopically labeled turbostratic and Bernal stacked bilayer graphene.

Fluorination of graphene opens up a bandgap, which creates opportunities for optoelectronics, and also paves the way for the creation of extremely thin insulating layers, which can be important for applications in devices. However, in spite of many interesting features offered by, for example, unequally doped layers in multilayered systems, most of the work has concerned the fluorination of graphene monolayers. Here, the fluorination process of graphene bilayers is investigated through high-resolution Raman mapping followed by analysis of more than 10,000 spectra of bilayer graphene. Isotopically labeled bilayers are used, allowing each individual layer in bilayer graphene to be addressed unambiguously. The fluorinated graphene is prepared through exposure to XeF2. Monolayer graphene is found to be significantly more sensitive to fluorination than bilayer graphene. Through comparison of the D/G area ratio and the position of the G band for turbostratic and Bernal stacked (AB) bilayers, it is found that the fluorination process is more effective for turbostratic than for AB-stacked bilayer graphene. The fluorination changes the electronic structure similarly for the top and bottom layers in turbostratic bilayers. However, the top layer is more sensitive than the bottom layer in AB-stacked bilayers.

Penetration of Cell Membranes and Synthetic Lipid Bilayers by Nanoprobes

Nanoscale devices have been proposed as tools for measuring and controlling intracellular activity by providing electrical and/or chemical access to the cytosol. Unfortunately, nanostructures with diameters of 50–500 nm do not readily penetrate the cell membrane, and rationally optimizing nanoprobes for cell penetration requires real-time characterization methods that are capable of following the process of membrane penetration with nanometer resolution. Although extensive work has examined the rupture of supported synthetic lipid bilayers, little is known about the applicability of these model systems to living cell membranes with complex lipid compositions, cytoskeletal attachment, and membrane proteins. Here, we describe atomic force microscopy (AFM) membrane penetration experiments in two parallel systems: live HEK293 cells and stacks of synthetic lipid bilayers. By using the same probes in both systems, we were able to clearly identify membrane penetration in synthetic bilayers and compare these events with putative membrane penetration events in cells. We examined membrane penetration forces for three tip geometries and 18 chemical modifications of the probe surface, and in all cases the median forces required to penetrate cellular and synthetic lipid bilayers with nanoprobes were greater than 1 nN. The penetration force was sensitive to the probe's sharpness, but not its surface chemistry, and the force did not depend on cell surface or cytoskeletal properties, with cells and lipid stacks yielding similar forces. This systematic assessment of penetration under various mechanical and chemical conditions provides insights into nanoprobe-cell interactions and informs the design of future intracellular nanoprobes.

Role of Synovial Fluid Constituents on Tribological Behavior of Synovial Joints - Application for Developing Novel Therapeutic Substitutes

Joint diseases represent a major health problem because they evolve towards the wear of cartilage for which no treatment is really effective. The difficulties in identifying the causes of these diseases are related to the biochemical and structural complexity of synovial fluid that allow cartilage lubrication. Thereby, recent studies show that synovial fluid contains micro-vesicles filled with a glycoprotein gel (hyaluronic acid and protein) surrounded by stacks of lipid bilayers which gives it excellent lubricating properties [1]. On the other side, this stable structure in vivo, becomes unstable in the ex vivo conditions. In this context, the study aims to tests the lubricating properties of synovial fluid constituents in order to identify their role on the tribological behavior of synovial joints and to develop a biomimetic synovial fluid using self-assembly of polyelectrolytes in order to stabilize the ex vivo structure for the potential therapeutic synovial fluid substitutes.Our results show that the new synthetized polysaccharides influence the friction coefficient, the stability and the wear of lipids bilayers, providing lubricating properties superior to synovial fluid biological constituents. In addition, their capacity to be structured in micro-spheres and included in lipid micro-vesicles indicates them as good biomimetic lubricants. This will allow a longer stability of synovial fluid substitutes in ex-vivo conditions in order to achieve the best lubricating properties and to improve the joint diseases treatment.

Ca(2+)-mediated anionic lipid-plasmid DNA lipoplexes. Electrochemical, structural, and biochemical studies.

Several experimental methods, such as zeta potential, gel electrophoresis, small-angle X-ray scattering, gene transfection, fluorescence microscopy, flow cytometry, and cell viability/cytotoxicity assays, have been used to analyze the potential of anionic lipids (AL) as effective nontoxic and nonviral DNA vectors, assisted by divalent cations. The lipoplexes studied are those comprised of the green fluorescent protein-encoding plasmid DNA pEGFP-C3, an anionic lipid as 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) or 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS), and a zwitterionic lipid, the 1,2-dioleoyl-sn -glycero-3-phosphatidylethanolamine (DOPE, not charged at physiological pH). The studies have been carried on at different liposome and lipoplex compositions and in the presence of a variety of [Ca2+]. Electrochemical experiments reveal that DOPG/DOPE and DOPS/DOPE anionic liposomes may compact more effectively pDNA at low molar fractions (with an excess of DOPE) and at AL/pDNA ratios ≈20. Calcium concentrations around 15-20 mM are needed to yield lipoplexes neutral or slightly positive. From a structural standpoint, DOPG/DOPE-Ca2+-pDNA lipoplexes are self-assembled into a HIIc phase (inverted cylindrical micelles in hexagonal ordering with plasmid supercoils inside the cylinders), while DOPS/DOPE-Ca2+-pDNA lipoplexes show two phases in coexistence: one classical HIIc phase which contains pDNA supercoils and one Lα phase without pDNA among the lamellae, i.e., a lamellar stack of lipidic bilayers held together by Ca2+ bridges. Transfection and cell viability studies were done with HEK293T and HeLa cells in the presence of serum. Lipoplexes herein studied show moderate-to-low transfection levels combined with moderate-to-high cell viability, comparable to those yield by Lipofectamine2000*, which is a cationic lipid (CL) standard formulation, but none of them improve the output of typical CL gen vectors, mostly if they are gemini or dendritic. This fact would be indicating that, nowadays, lipofection via anionic lipids and divalent cations as mediators still needs to enhance transfection levels in order to be considered as a real and plausible alternative to lipofection through improved CLs-based lipoplexes.

Probing inhomogeneous doping in overlapped graphene grain boundaries by Raman spectroscopy

The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB–stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, as well as peak intensities depend on the width of the overlap. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped regions via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4eV below the charge neutrality point. Our results suggest an enhancement of electrical conductivity across overlapped grain boundaries, similar to previously observed measurements (Tsen et al., 2012). The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity.

Contribution of galactoglycerolipids to the 3‐dimensional architecture of thylakoids

Thylakoid membranes, the universal structure where photosynthesis takes place in all oxygenic photosynthetic organisms from cyanobacteria to higher plants, have a unique lipid composition. They contain a high fraction of 2 uncharged glycolipids, the galactoglycerolipids mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), and an anionic sulfolipid, sulfoquinovosediacylglycerol (SQDG). A remarkable feature of the evolution from cyanobacteria to higher plants is the conservation of MGDG, DGDG, SQDG, and phosphatidylglycerol (PG), the major phospholipid of thylakoids. Using neutron diffraction on reconstituted thylakoid lipid extracts, we observed that the thylakoid lipid mixture self-organizes as a regular stack of bilayers. This natural lipid mixture was shown to switch from hexagonal II toward lamellar phase on hydration. This transition and the observed phase coexistence are modulated by the fine-tuning of the lipid profile, in particular the MGDG/DGDG ratio, and by the hydration. Our analysis highlights the critical role of DGDG as a contributing component to the membrane stacking via hydrogen bonds between polar heads of adjacent bilayers. DGDG interactions balance the repulsive electrostatic contribution of the charged lipids PG and SQDG and allow the persistence of regularly stacked membranes at high hydration. In developmental contexts or in response to environmental variations, these properties can contribute to the highly dynamic flexibility of plastid structure.

Molecular Origins of the Ripple Phase

The ripple phase of lipid bilayers is characterized by periodic ripples in one dimension. We explore the molecular origins of this peculiar phase. Since the ripple wavelength is of the order of the bilayer thickness, molecular simulation can provide a detailed microscopic picture of lipid behavior in the ripple phase. In the first simulations of their kind, we study stacks of rippled bilayers to provide a more realistic molecular perspective that can be directly compared to X-ray diffraction and AFM experiments. We find that saddle-splay and tilt are key order parameters that characterize the long-range order observed in ripple phases. In addition, we elucidate the effects of lipid length, head group interaction, and bilayer hydration on ripple structure. The impact of lipid orientations in the ripple phase to biological processes such as membrane fusion is discussed.

Field Theoretic Approach for the Energetics of Stalk Formation

A numerical procedure was developed to calculate the minimum energy pathway for stalk formation between two flat and parallel lipid bilayer membranes. Bilayers were modeled as axially symmetric, freely deformable surfaces with lipid director fields. Energies were obtained by calculating the elastic energy--splay, tilt, and stretching--of each monolayer, along with undulation and hydration forces that repel membranes from each other. The topological transition between the planar and stalk phase was explicitly treated by allowing the bilayer to form a circular fissure along the axis of symmetry; the energy of fissure was approximated by the water-hydrocarbon surface and the area of exposed acyl chains. Generally, stalk formation is not spontaneous; it requires that external energy, such as provided by fusion proteins, be supplied to the bilayers. To quantify the energy required to surmount the transition barrier that separates a stalk and the original separate membranes, least energy paths were calculated by the string method. The string method finds a morphological transition between two energy basins_in this case the planar and stalk phases_that minimizes the height of the energy barrier. Sequences of images of bilayer shapes calculated by the string method show that opposing bilayers deform into cusped shapes that then form hydrophobic fissures in each membrane, which then promotes merger into a stalk shape. The energetics of stalk formation and morphologies of stalks were analyzed as a function of lipid composition and bilayer dimensions. The shapes of stalks were compared with shapes of stalks that form between stacks of planar bilayers, as obtained by x-ray diffraction: the theoretically calculated shapes are in agreement with those measured experimentally. Supported by NIH R01 GM101539.

Atomic resolution view into the structure-function relationships of the human myelin peripheral membrane protein P2.

P2 is a fatty acid-binding protein expressed in vertebrate peripheral nerve myelin, where it may function in bilayer stacking and lipid transport. P2 binds to phospholipid membranes through its positively charged surface and a hydrophobic tip, and accommodates fatty acids inside its barrel structure. The structure of human P2 refined at the ultrahigh resolution of 0.93 Å allows detailed structural analyses, including the full organization of an internal hydrogen-bonding network. The orientation of the bound fatty-acid carboxyl group is linked to the protonation states of two coordinating arginine residues. An anion-binding site in the portal region is suggested to be relevant for membrane interactions and conformational changes. When bound to membrane multilayers, P2 has a preferred orientation and is stabilized, and the repeat distance indicates a single layer of P2 between membranes. Simulations show the formation of a double bilayer in the presence of P2, and in cultured cells wild-type P2 induces membrane-domain formation. Here, the most accurate structural and functional view to date on P2, a major component of peripheral nerve myelin, is presented, showing how it can interact with two membranes simultaneously while going through conformational changes at its portal region enabling ligand transfer.

Polymer-tethered lipid multi-bilayers: a biomembrane-mimicking cell substrate to probe cellular mechano-sensing

Cells tiptoe through their environment forming highly localized and dynamic focal contacts. Experiments on polymeric gels of adjustable elasticity have shown that cells probe the viscoelasticity of their environment through an adaptive process of focal contact assembly/disassembly that critically affects cell adhesion, morphology, and motility. However, the specific mechanisms of this process have not yet been fully revealed. Here we report, for the first time, that fibroblast adhesion, morphology, and migration can also be controlled by altering the number of bilayers in a stack of multiple polymer-tethered lipid bilayers stabilized via maleimide-sulfhydral coupling chemistry. The observed changes in cell morphology, migration, and cytoskeletal organization in response to bilayer stacking correspond well with those previously observed on polymeric substrates of different polymer crosslinking density suggesting that variations in bilayer stacking are associated with changes in substrate viscoelasticity. This is in conceptual agreement with the existing knowledge about the structural, dynamic, and mechanical properties of polymer-lipid composite materials. Several distinct features, such as the lateral mobility of individual cell linkers and the immobilization of linker clusters, make the described substrates highly attractive tools for the study of dynamic, mechano-regulated cell linkages and cellular mechano-sensing.

Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures

Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing non-trivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three-dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with interlayer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a non-trivial topological phase transition occurs above a critical number of Rashba bilayers. In the topological phase, we find the formation of a single spin-polarized Dirac cone at the -point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry.

Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy

The growth of large-area bilayer graphene has been of technological importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which determines both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). In this work, we developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature associated with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, we were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chemical Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Furthermore, by combining surface sensitive fluorination with such hybrid (12)C/(13)C bilayer samples, we are able to identify that the second layer grows underneath the first-grown layer, which is similar to a recently reported observation.

Interlamellar Organization of Phase Separated Domains in Multi-Component Lipid Multilayers: Energetic Considerations

A recent experimental study [1] has demonstrated the alignment of phase separated domains across hundreds of bilayer units in multicomponent stacked lipid bilayers. The origin of this alignment is the interlamellar coupling of laterally phase separated domains. Here, we develop a theoretical model that presents the energetics description of this phenomenon based on the minimization of the free energy of the system. Specifically, we use solution theory to estimate the competition between energy and entropy in different stacking configurations. The model furnishes an elemental phase diagram, which maps the domain distributions in terms of the strength of the intra- and inter-layer interactions and estimates the value of inter-layer coupling for complete alignment of domains in the stacks of five and ten bilayers. The area fraction occupied by co-existing phases was calculated for the system of the minimum free energy, which showed a good agreement with experimental observations.

Imaging Ellipsometry of Spin-Coated Membranes: Mapping of Multilamellar Films, Hydrated Membranes, and Fluid Domains

Imaging ellipsometry (IE) has been applied to generate laterally resolved thickness maps of spin-coated membranes in both the dry and fully hydrated state. Spin-coating offers a convenient preparation method for stacked supported membranes, and in-depth thickness maps for such films can be measured by IE, thereby going beyond topography measurements of the top surface. We find that dry lipid films of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) have a highly ordered multilamellar structure which allows counting of the number of individual bilayers in a thick film from the progression in a concentration series. The average film thickness is approximately proportional to the coating concentration with a constant of proportionality of 5.2 nm/mM and 6.2 nm/mM for POPC and DSPC (1,2-distearoyl-sn- glycero-3-phosphocholine), respectively. The root-mean-square roughness of the dry films is also approximately proportional to concentration with constants of 3.7 nm/mM (DSPC) and 0.87 nm/mM (POPC). Fully hydrated POPC membranes with several stacked bilayers show decreasing thickness for increasing temperature. An apparent excess in thickness by 1.2 nm for the proximal membrane can possibly be linked to the presence of a structured water film next to the solid support. This is supported by modeling of spectroscopic data. Thickness maps of double supported ternary membranes show resolvable liquid-ordered domains in the second membrane while domains are below the resolution limit in the proximal membrane. A thickness difference of 1.69 and 1.89 nm between the liquid-ordered (lo) and liquid-disordered (ld) phases is found for two different ternary membrane compositions. This is approximately twice the height difference measured by AFM on domains, thus indicating that the relative excess thickness of the lo phase is symmetrically distributed.

Ordered stacking of F-actin layers and mixed lipid bilayers: a columnar liquid crystal.

In this Letter, we show how the grooved helical structure of actin microfilaments (F-actin) interacting with mixed fluid lipid bilayers leads to handedness-independent 1D lipid bilayer undulations coupled to longitudinal in-plane ordering of the microfilaments. This longitudinal ordering is forced by the emerging in-plane compression and curvature energy terms of the straight 1D bilayer undulation wave fronts. Thereby, adjacent helices are set into registry along their long axis in their monolayer and π shifted between adjacent monolayers. An ordered composite multilamellar structure emerges by alternate stacking of these lipid bilayers and monolayers of F-actin. This two-dimensionally ordered system has the symmetries of a centered rectangular columnar liquid crystal, the straight 1D wave fronts playing the role of the classical molecular columns.