Diffusion in supported lipid bilayers: influence of substrate and preparation technique on the internal dynamics

Electrophoretic mobility of a monotopic membrane protein inserted into the top of supported lipid bilayers

We have studied the translational migration of a monotopic membrane protein, the bacterial sulfide quinone reductase (SQR) in supported n-bilayers ([Formula: see text]) under the influence of an electric field parallel to the membrane plane. The direction of the migration changes when the charge of the protein changes its sign. Measuring mobilities at different pH enables us to gain experimental physico-chemical data on SQR as its isoelectric point and its estimated oligomeric state (at least trimeric) when inserted in a lipid membrane. Consequently, in addition to the migration study of membrane proteins in a lipid environment, this experimental system, previously used with a transmembrane protein, is thus suitable to define membrane protein properties in conditions approaching the native ones (in the absence of detergent).

Temperature-Controlled High-Speed AFM: Real-Time Observation of Ripple Phase Transitions

With nanometer lateral and Angstrom vertical resolution, atomic force microscopy (AFM) has contributed unique data improving the understanding of lipid bilayers. Lipid bilayers are found in several different temperature-dependent states, termed phases; the main phases are solid and fluid phases. The transition temperature between solid and fluid phases is lipid composition specific. Under certain conditions some lipid bilayers adopt a so-called ripple phase, a structure where solid and fluid phase domains alternate with constant periodicity. Because of its narrow regime of existence and heterogeneity ripple phase and its transition dynamics remain poorly understood. Here, a temperature control device to high-speed atomic force microscopy (HS-AFM) to observe dynamics of phase transition from ripple phase to fluid phase reversibly in real time is developed and integrated. Based on HS-AFM imaging, the phase transition processes from ripple phase to fluid phase and from ripple phase to metastable ripple phase to fluid phase could be reversibly, phenomenologically, and quantitatively studied. The results here show phase transition hysteresis in fast cooling and heating processes, while both melting and condensation occur at 24.15 °C in quasi-steady state situation. A second metastable ripple phase with larger periodicity is formed at the ripple phase to fluid phase transition when the buffer contains Ca²⁺ . The presented temperature-controlled HS-AFM is a new unique experimental system to observe dynamics of temperature-sensitive processes at the nanoscopic level.

Hydration lubrication and shear-induced self-healing of lipid bilayer boundary lubricants in phosphatidylcholine dispersions

Measurements of normal and shear (frictional) forces between mica surfaces across small unilamellar vesicle (SUV) dispersions of the phosphatidylcholine (PC) lipids DMPC (14:0), DPPC (16:0) and DSPC (18:0) and POPC (16:0, 18:1), at physiologically high pressures, are reported. We have previously studied the normal and shear forces between two opposing surfaces bearing PC vesicles across pure water and showed that liposome lubrication ability improved with increasing acyl chain length, and correlated strongly with the SUV structural integrity on the substrate surface (DSPC > DPPC > DMPC). In the current study, surprisingly, we discovered that this trend is reversed when the measurements are conducted in SUV dispersions, instead of pure water. In their corresponding SUV dispersion, DMPC SUVs ruptured and formed bilayers, which were able to provide reversible and reproducible lubrication with extremely low friction (μ < 10(-4)) up to pressures of 70-90 atm. Similarly, POPC SUVs also formed bilayers which exhibited low friction (μ < 10(-4)) up to pressures as high as 160 atm. DPPC and DSPC SUVs also provided good lubrication, but with slightly higher friction coefficients (μ = 10(-3)-10(-4)). We believe these differences originate from fast self-healing of the softer surface layers (which are in their liquid disordered phase, POPC, or close to it, DMPC), which renders the robustness of the DPPC or DSPC (both in their solid ordered phase) less important in these conditions. Under these circumstances, the enhanced hydration of the less densely packed POPC and DMPC surface layers is now believed to play an important role, and allows enhanced lubrication via the hydration lubrication mechanism. Our findings may have implications for the understanding of complex biological systems such us biolubrication of synovial joints.

Photopolymerization of Dienoyl Lipids Creates Planar Supported Poly(lipid) Membranes with Retained Fluidity

Polymerization of substrate-supported bilayers composed of dienoylphosphatidylcholine (PC) lipids is known to greatly enhance their chemical and mechanical stability; however, the effects of polymerization on membrane fluidity have not been investigated. Here planar supported lipid bilayers (PSLBs) composed of dienoyl PCs on glass substrates were examined to assess the degree to which UV-initiated polymerization affects lateral lipid mobility. Fluorescence recovery after photobleaching (FRAP) was used to measure the diffusion coefficients (D) and mobile fractions of rhodamine-DOPE in unpolymerized and polymerized PSLBs composed of bis-sorbyl phosphatidylcholine (bis-SorbPC), mono-sorbyl-phosphatidylcholine (mono-SorbPC), bis-dienoyl-phosphatidylcholine (bis-DenPC), and mono-dienoyl phosphatidylcholine (mono-DenPC). Polymerization was performed in both the Lα and Lβ phase for each lipid. In all cases, polymerization reduced membrane fluidity; however, measurable lateral diffusion was retained which is attributed to a low degree of polymerization. The D values for sorbyl lipids were less than those of the denoyl lipids; this may be a consequence of the distal location of polymerizable group in the sorbyl lipids which may facilitate interleaflet bonding. The D values measured after polymerization were 0.1-0.8 of those measured before polymerization, a range that corresponds to fluidity intermediate between that of a Lα phase and a Lβ phase. This D range is comparable to ratios of D values reported for liquid-disordered (Ld) and liquid-ordered (Lo) lipid phases and indicates that the effect of UV polymerization on lateral diffusion in a dienoyl PSLB is similar to the transition from a Ld phase to a Lo phase. The partial retention of fluidity in UV-polymerized PSLBs, their enhanced stability, and the activity of incorporated transmembrane proteins and peptides is discussed.

Comparison of [corrected] actin- and glass-supported phospholipid bilayer diffusion coefficients

The formation of biomimetic lipid membranes has the potential to provide insights into cellular lipid membrane dynamics. The construction of such membranes necessitates not only the utilization of appropriate lipids, but also physiologically relevant substrate/support materials. The substrate materials employed have been shown to have demonstrable effects on the behavior of the overlying lipid membrane, and thus must be studied before use as a model cushion support. To our knowledge, we report the formation and investigation of a novel actin protein-supported lipid membrane. Specifically, inner leaflet lateral mobility of globular actin-supported DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) bilayers, deposited via the Langmuir-Blodgett/Langmuir Schaefer methodology, was investigated by z-scan fluorescence correlation spectroscopy across a temperature range of 20-44°C. The actin substrate was found to decrease the diffusion coefficient when compared to an identical membrane supported on glass. The depression of the diffusion coefficient occurred across all measured temperatures. These results indicated that the actin substrate exerted a direct effect on the fluidity of the lipid membrane and highlighted the fact that the choice of substrate/support is critical in studies of model lipid membranes.

Supported Lipid Bilayers for the Generation of Dynamic Cell-Material Interfaces

Supported lipid bilayers (SLB) offer unique possibilities for studying cellular membranes and have been used as a synthetic architecture to interact with cells. Here, the state-of-the-art in SLB-based technology is presented. The fabrication, analysis, characteristics and modification of SLBs are described in great detail. Numerous strategies to form SLBs on different substrates, and the means to patteren them, are described. The use of SLBs as model membranes for the study of membrane organization and membrane processes in vitro is highlighted. In addition, the use of SLBs as a substratum for cell analysis is presented, with discrimination between cell-cell and cell-extracellular matrix (ECM) mimicry. The study is concluded with a discussion of the potential for in vivo applications of SLBs.

Potential Controls the Interaction of Liposomes with Octadecanol-Modified Au Electrodes: An in Situ AFM Study

The formation of supported lipid bilayers using liposomes requires interaction with the solid surface, rupture of the liposome, and spreading to cover the surface with a lipid bilayer. This can result in a less-than-uniform coating of the solid surface. Presented is a method that uses the electrochemical poration of an adsorbed lipid-like layer on a Au electrode to control the interaction of 100 nm DOPC liposomes. An octadecanol-coated Au-on-mica surface was imaged using tapping-mode AFM during the application of potential in the presence or absence of liposomes. When the substrate potential was made negative enough, defects formed in the adsorbed layer and new taller features were observed. More features were observed and existing features increased in size with time spent at this negative poration potential. The new features were 1.8-2.0 nm higher than the octadecanol-coated gold surface, half the thickness of a DOPC bilayer. These features were not observed in the absence of liposomes when undergoing the same potential perturbation. In the presence of liposomes, the application of a poration potential was needed to initiate the formation of these taller features. Once the applied potential was removed, the features stopped growing and no new regions were observed. The size of these new regions was consistent with the footprint of a flattened 100 nm liposome. It is speculated that the DOPC liposomes were able to interact with the defects and became soluble in the octadecanol, creating a taller region that was limited in size to the liposome that adsorbed and became incorporated. This AFM study confirms previous in situ fluorescence measurements of the same system and illustrates the use of a potential perturbation to control the formation of these regions of increased DOPC content.

Selective Binding of DNA Origami on Biomimetic Lipid Patches

Arrays of biomimetic lipid patches for studying the binding of DNA origami structures can be tailored in size, shape, and composition with the aid of lipid-dip pen nanolithography. This approach allows for analysis of the effects of lipid composition with high throughput which could be applied for the targeted presentation of functional DNA origami structures on surfaces.

Macroscopic and Nanoscopic Heterogeneous Structures in a Three-Component Lipid Bilayer Mixtures Determined by Atomic Force Microscopy

Much of lipid raft properties can be inferred from phase behavior of multicomponent lipid membranes. We use liquid compatible atomic force microscopy (AFM) to study a three-component system composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), egg sphingomyelin (eSM), and cholesterol. Specifically, we obtain macroscopic and nanoscopic heterogeneous structures in a broad compositional space of DOPC/eSM/cholesterol (23 °C). In the macroscopic liquid coexisting region, we quantify area fraction of the coexisting phases and determine a set of thermodynamic tie-lines. When lipid compositions are near the critical point, we obtain fluctuation-like nanoscopic structures. We also use AFM height images to explore the hypothetical three-phase coexisting region. Finally, we use fluorescence microscopy to compare the phase behavior from our AFM measurements to that in free-floating giant unilamellar vesicles (GUVs). Our results highlight the role of lipid composition in mediating lipid domain formation and stability.

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

Visually detecting nanoscopic structures in lipid membranes is important for elucidating lipid-lipid interactions, which are suggested to play a role in mediating membrane rafts. We use solution atomic force microscopy (AFM) to study lateral and normal organization in multicomponent lipid membranes supported by mica substrate. Nanoscopic heterogeneity is observed in a three-component system composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/brain-sphingomyelin (bSM)/cholesterol (Chol). We find sub-ten-nanometer correlation lengths that are used to describe membrane lateral organization. In addition, we find that the correlation length is independent on cholesterol concentration, while the height fluctuation (variation) is not. To explore the mechanism that controls the size of membrane heterogeneity, we extend our study to a four-component system composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/POPC/bSM/Chol. By systematically adjusting the relative amount of DOPC and POPC, we obtain macroscopic-to-nanoscopic size transition of membrane heterogeneity. In contrast to the results from vesicle based fluorescence microscopy, we find that the structural transition is continuous both in the lateral and normal directions. We compare our nanoscopic structures to two theoretical models, and find that both the critical fluctuations and the nanodomain models are not sufficient to account for our solution AFM data. Finally, we propose a nanoheterogeneity model that could serve as the organization principle of the observed nanoscopic structures in multicomponent lipid membranes.

Insertion and self-diffusion of a monotopic protein, the Aquifex aeolicus sulfide quinone reductase, in supported lipid bilayers

Monotopic proteins constitute a class of membrane proteins that bind tightly to cell membranes, but do not span them. We present a FRAPP (Fluorescence Recovery After Patterned Photobleaching) study of the dynamics of a bacterial monotopic protein, SQR (sulfide quinone oxidoreductase) from the thermophilic bacteria Aquifex aeolicus, inserted into two different types of lipid bilayers (EggPC: L-α-phosphatidylcholine (Egg, Chicken) and DMPC: 1,2-dimyristoyl-sn-glycero-3-phosphocholine) supported on two different types of support (mica or glass). It sheds light on the behavior of a monotopic protein inside the bilayer. The insertion of SQR is more efficient when the bilayer is in the fluid phase than in the gel phase. We observed diffusion of the protein, with no immobile fraction, and deduced from the diffusion coefficient measurements that the resulting inserted object is the same whatever the incubation conditions, i.e. homogeneous in terms of oligomerization state. As expected, the diffusion coefficient of the SQR is smaller in the gel phase than in the fluid phase. In the supported lipid bilayer, the diffusion coefficient of the SQR is smaller than the diffusion coefficient of phospholipids in both gel and fluid phase. SQR shows a diffusion behavior different from the transmembrane protein α-hemolysin, and consistent with its monotopic character. Preliminary experiments in the presence of the substrate of SQR, DecylUbiquinone, an analogue of quinone, component of transmembrane electrons transport systems of eukaryotic and prokaryotic organisms, have been carried out. Finally, we studied the behavior of SQR, in terms of insertion and diffusion, in bilayers formed with lipids from Aquifex aeolicus. All the conclusions that we have found in the biomimetic systems applied to the biological system.

A ring to rule them all: the effect of cyclopropane Fatty acids on the fluidity of lipid bilayers

Cyclopropane fatty acids are widespread in bacteria. As their concentration increases on exposure to hostile environments, they have been proposed to protect membranes. Here, the effect of cyclopropane and unsaturated fatty acids, both in cis and trans configurations, on the packing, order, and fluidity of lipid bilayers is explored using molecular dynamics simulations. It is shown that cyclopropane fatty acids disrupt lipid packing, favor the occurrence of gauche defects in the chains, and increase the lipid lateral diffusion, suggesting that they enhance fluidity. At the same time, they generally induce a greater degree of order than unsaturated fatty acids of the same configuration and limit the rotation about the bonds surrounding the cyclopropane ring. This indicates that cyclopropane fatty acids may fulfill a dual function: stabilizing membranes against adverse conditions while simultaneously promoting their fluidity. Marked differences in the effect of cis- and trans-monocyclopropanated fatty acids were also observed, suggesting that they may play alternative roles in membranes.

Reconstitution of proteins on electroformed giant unilamellar vesicles

In vitro reconstitution of simplified biological systems from molecular parts has proven to be a powerful method for investigating the biochemical and biophysical principles underlying cellular processes. In recent years, there has been a growing interest in reconstitution of protein-membrane interactions to understand the critical role played by membranes in organizing molecular-scale events into micron-scale patterns and protrusions. However, while all reconstitution experiments depend on identifying and isolating an essential set of soluble biomolecules, such as proteins, DNA, and RNA, reconstitution of membrane-based processes involves the additional challenge of forming and working with lipid bilayer membranes with composition, fluidity, and mechanical properties appropriate for the process at hand. Here we discuss a selection of methods for forming synthetic lipid bilayer membranes and present a versatile electroformation protocol that our lab uses for reconstituting proteins on giant unilamellar vesicles. This synthetic membrane-based approach to reconstitution offers the ability to study protein organization and activity at membranes under more cell-like conditions, addressing a central challenge to accomplishing the grand goal of "building the cell."

Single lipid bilayer deposition on polymer surfaces using bicelles

A lipid bilayer was deposited on a 3 μm diameter polystyrene (PS) bead via hydrophobic anchoring of bicelles containing oxyamine-bearing cholesteric moieties reacting with the aldehyde functionalized bead surface. Discoidal bicelles were formed by mixing dimyristoylphosphatidylcholine (DMPC), dihexanoylphosphatidylcholine (DHPC), dimyristoyltrimethylammonium propane (DMTAP), and the oxyamine-terminated cholesterol derivative, cholest-5-en-3β-oxy-oct-3,6-oxa-an-8-oxyamine (CHOLOA), in the molar ratio DMPC/DHCP/DMTAP/CHOLOA (1/0.5/0.01/0.05) in water. Upon exposure to aldehyde-bearing PS beads, a stable single lipid bilayer coating rapidly formed at the bead surface. Fluorescence recovery after photobleaching demonstrated that the deposited lipids fused into an encapsulating lipid bilayer. Electrospray ionization mass spectrometry showed that the short chain lipid DHPC was entirely absent from the PS adherent lipid coating. Fluorescence quenching measurements proved that the coating was a single lipid bilayer. The bicelle coating method is thus simple and robust, can be modified to include membrane-associated species, and can be adapted to coat any number of different surfaces.

Lateral diffusion of proteins on supported lipid bilayers: additive friction of synaptotagmin 7 C2A-C2B tandem domains

The synaptotagmin (Syt) family of proteins contains tandem C2 domains, C2A and C2B, which bind membranes in the presence of Ca²⁺ to trigger vesicle fusion during exocytosis. Despite recent progress, the role and extent of interdomain interactions between C2A and C2B in membrane binding remain unclear. To test whether the two domains interact on a planar lipid bilayer (i.e., experience thermodynamic interdomain contacts), diffusion of fluorescent-tagged C2A, C2B, and C2AB domains from human Syt7 was measured using total internal reflection fluorescence microscopy with single-particle tracking. The C2AB tandem exhibits a lateral diffusion constant approximately half the value of the isolated single domains and does not change when additional residues are engineered into the C2A–C2B linker. This is the expected result if C2A and C2B are separated when membrane-bound; theory predicts that C2AB diffusion would be faster if the two domains were close enough together to have interdomain contact. Stopped-flow measurements of membrane dissociation kinetics further support an absence of interdomain interactions, as dissociation kinetics of the C2AB tandem remain unchanged when rigid or flexible linker extensions are included. Together, the results suggest that the two C2 domains of Syt7 bind independently to planar membranes, in contrast to reported interdomain cooperativity in Syt1.

Acceleration of Lateral Equilibration in Mixed Lipid Bilayers Using Replica Exchange with Solute Tempering

The lateral heterogeneity of cellular membranes plays an important role in many biological functions such as signaling and regulating membrane proteins. This heterogeneity can result from preferential interactions between membrane components or interactions with membrane proteins. One major difficulty in molecular dynamics simulations aimed at studying the membrane heterogeneity is that lipids diffuse slowly and collectively in bilayers, and therefore, it is difficult to reach equilibrium in lateral organization in bilayer mixtures. Here, we propose the use of the replica exchange with solute tempering (REST) approach to accelerate lateral relaxation in heterogeneous bilayers. REST is based on the replica exchange method but tempers only the solute, leaving the temperature of the solvent fixed. Since the number of replicas in REST scales approximately only with the degrees of freedom in the solute, REST enables us to enhance the configuration sampling of lipid bilayers with fewer replicas, in comparison with the temperature replica exchange molecular dynamics simulation (T-REMD) where the number of replicas scales with the degrees of freedom of the entire system. We apply the REST method to a cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer mixture and find that the lateral distribution functions of all molecular pair types converge much faster than in the standard MD simulation. The relative diffusion rate between molecules in REST is, on average, an order of magnitude faster than in the standard MD simulation. Although REST was initially proposed to study protein folding and its efficiency in protein folding is still under debate, we find a unique application of REST to accelerate lateral equilibration in mixed lipid membranes and suggest a promising way to probe membrane lateral heterogeneity through molecular dynamics simulation.

Diffusion in phospholipid bilayer membranes: dual-leaflet dynamics and the roles of tracer-leaflet and inter-leaflet coupling

A variety of observations-sometimes controversial-have been made in recent decades when attempting to elucidate the roles of interfacial slip on tracer diffusion in phospholipid membranes. Evans-Sackmann theory (1988) has furnished membrane viscosities and lubrication-film thicknesses for supported membranes from experimentally measured lateral diffusion coefficients. Similar to the Saffman and Delbrück model, which is the well-known counterpart for freely supported membranes, the bilayer is modelled as a single two-dimensional fluid. However, the Evans-Sackman model cannot interpret the mobilities of monotopic tracers, such as individual lipids or rigidly bound lipid assemblies; neither does it account for tracer-leaflet and inter-leaflet slip. To address these limitations, we solve the model of Wang and Hill, in which two leaflets of a bilayer membrane, a circular tracer and supports are coupled by interfacial friction, using phenomenological friction/slip coefficients. This furnishes an exact solution that can be readily adopted to interpret the mobilities of a variety of mosaic elements-including lipids, integral monotopic and polytopic proteins, and lipid rafts-in supported bilayer membranes.

Preparation of mica supported lipid bilayers for high resolution optical microscopy imaging

Supported lipid bilayers (SLBs) are widely used as a model for studying membrane properties (phase separation, clustering, dynamics) and its interaction with other compounds, such as drugs or peptides. However SLB characteristics differ depending on the support used. Commonly used techniques for SLB imaging and measurements are single molecule fluorescence microscopy, FCS and atomic force microscopy (AFM). Because most optical imaging studies are carried out on a glass support, while AFM requires an extremely flat surface (generally mica), results from these techniques cannot be compared directly, since the charge and smoothness properties of these materials strongly influence diffusion. Unfortunately, the high level of manual dexterity required for the cutting and gluing thin slices of mica to the glass slide presents a hurdle to routine use of mica for SLB preparation. Although this would be the method of choice, such prepared mica surfaces often end up being uneven (wavy) and difficult to image, especially with small working distance, high numerical aperture lenses. Here we present a simple and reproducible method for preparing thin, flat mica surfaces for lipid vesicle deposition and SLB preparation. Additionally, our custom made chamber requires only very small volumes of vesicles for SLB formation. The overall procedure results in the efficient, simple and inexpensive production of high quality lipid bilayer surfaces that are directly comparable to those used in AFM studies.

TrackArt: the user friendly interface for single molecule tracking data analysis and simulation applied to complex diffusion in mica supported lipid bilayers

Background

Single molecule tracking (SMT) analysis of fluorescently tagged lipid and protein probes is an attractive alternative to ensemble averaged methods such as fluorescence correlation spectroscopy (FCS) or fluorescence recovery after photobleaching (FRAP) for measuring diffusion in artificial and plasma membranes. The meaningful estimation of diffusion coefficients and their errors is however not straightforward, and is heavily dependent on sample type, acquisition method, and equipment used. Many approaches require advanced computing and programming skills for their implementation.

Findings

Here we present TrackArt software, an accessible graphic interface for simulation and complex analysis of multiple particle paths. Imported trajectories can be filtered to eliminate spurious or corrupted tracks, and are then analyzed using several previously described methodologies, to yield single or multiple diffusion coefficients, their population fractions, and estimated errors. We use TrackArt to analyze the single-molecule diffusion behavior of a sphingolipid analog SM-Atto647N, in mica supported DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) bilayers. Fitting with a two-component diffusion model confirms the existence of two separate populations of diffusing particles in these bilayers on mica. As a demonstration of the TrackArt workflow, we characterize and discuss the effective activation energies required to increase the diffusion rates of these populations, obtained from Arrhenius plots of temperature-dependent diffusion. Finally, TrackArt provides a simulation module, allowing the user to generate models with multiple particle trajectories, diffusing with different characteristics. Maps of domains, acting as impermeable or permeable obstacles for particles diffusing with given rate constants and diffusion coefficients, can be simulated or imported from an image. Importantly, this allows one to use simulated data with a known diffusion behavior as a comparison for results acquired using particular algorithms on actual, “natural” samples whose diffusion behavior is to be extracted. It can also serve as a tool for demonstrating diffusion principles.

Conclusions

TrackArt is an open source, platform-independent, Matlab-based graphical user interface, and is easy to use even for those unfamiliar with the Matlab programming environment. TrackArt can be used for accurate simulation and analysis of complex diffusion data, such as diffusion in lipid bilayers, providing publication-quality formatted results.

Surface charge effects on the 2D conformation of supercoiled DNA

We have adsorbed plasmid pUc19 DNA on a supported bilayer. By varying the fraction of cationic lipids in the membrane, we have tuned the surface charge. Plasmid conformations were imaged by Atomic Force Microscopy (AFM). We performed two sets of experiments: deposition from salt free solution on charged bilayers and deposition from salty solutions on neutral bilayers. Both sets show similar trends: at low surface charge density or low bulk salt concentration, the internal electrostatic repulsion forces plasmids to adopt completely opened structures, while at high surface charge density or higher bulk salt concentration, usual supercoiled plectonemes are observed. We experimentally demonstrate the equivalence of surface screening by mobile interfacial charges and bulk screening from salt ions. At low to medium screening, the electrostatic repulsion at plasmid crossings is predominant, leading to a number of crossovers decreasing linearly with the characteristic screening length. We compare our data with an analytical 2D-equilibrated model developed recently for the system and extract the DNA effective charge density when strands are adsorbed at the surface.

Diffusion coefficients in leaflets of bilayer membranes

We study diffusion coefficients of liquid domains by explicitly taking into account the two-layered structure called leaflets of the bilayer membrane. In general, the velocity fields associated with each leaflet are different and the layers sliding past each other cause frictional coupling. We obtain analytical results of diffusion coefficients for a circular liquid domain in a leaflet, and quantitatively study their dependence on the interleaflet friction. We also show that the diffusion coefficients diverge in the absence of coupling between the bilayer and solvents, even when the interleaflet friction is taken into account. In order to corroborate our theory, the effect of the interleaflet friction on the correlated diffusion is examined.

Ripple formation in unilamellar-supported lipid bilayer revealed by FRAPP

The mechanisms of formation and conditions of the existence of the ripple phase are fundamental thermodynamic questions with practical implications for medicine and pharmaceuticals. We reveal a new case of ripple formation occurring in unilamellar-supported bilayers in water, which results solely from the bilayer/support interaction, without using lipid mixtures or specific ions. This ripple phase is detected by FRAPP using diffusion coefficient measurements as a function of temperature: a diffusivity plateau is observed. It occurs in the same temperature range where ripple phase existence has been observed using other methods. When AFM experiments are performed in the appropriate temperature range the ripple phase is confirmed.

Electric migration of α-hemolysin in supported n-bilayers: a model for transmembrane protein microelectrophoresis

Proteome analysis involves separating proteins as a preliminary step toward their characterization. This paper reports on the translational migration of a model transmembrane protein (α-hemolysin) in supported n-bilayers (n, the number of bilayers, varies from 1 to around 500 bilayers) when an electric field parallel to the membrane plane is applied. The migration changes in direction as the charge on the protein changes its sign. Its electrophoretic mobility is shown to depend on size and charge. The electrophoretic mobility varies as 1/R(2), with R the equivalent geometric radius of the embedded part of the protein. Measuring mobilities at differing pH in our system enables us to determine the pI and the charge of the protein. Establishing all these variations points to the feasibility of electrophoretic transport of a charged object in this medium and is a first step toward electrophoretic separation of membrane proteins in n-bilayer systems.

Single-particle tracking reveals switching of the HIV fusion peptide between two diffusive modes in membranes

Fusion of the HIV membrane with that of a target T cell is an essential first step in the viral infection process. Here we describe single-particle tracking (SPT) studies of a 16-amino-acid peptide derived from the HIV fusion protein (FP16), as it interacts with a supported lipid bilayer. FP16 was found to spontaneously insert into and move within the bilayer with two different modes of diffusion, a fast mode with a diffusion coefficient typical of protein motion in membranes and a much slower one. We observed transitions between the two modes: slow peptides were found to speed up, and fast peptides could slow down. Hidden Markov model analysis was employed as a method for the identification of the two modes in single-molecule trajectories and analysis of their interconversion rates. Surprisingly, the diffusion coefficients of the two modes were found to depend differently on solution viscosity. Thus, whereas the fast diffusive mode behaved as predicted by the Saffman-Delbrück theory, the slow mode behaved according to the Stokes-Einstein relation. To further characterize the two diffusive modes, FP16 molecules were studied in bilayers cooled through their liquid crystalline-to-gel phase transition. Our analysis suggested that the slow diffusive mode might originate from the formation of large objects, such as lipid domains or local protrusions, which are induced by the peptides and move together with them.

Phospholipid diffusion coefficients of cushioned model membranes determined via z-scan fluorescence correlation spectroscopy

Model cellular membranes enable the study of biological processes in a controlled environment and reduce the traditional challenges associated with live or fixed cell studies. However, model membrane systems based on the air/water or oil/solution interface do not allow for incorporation of transmembrane proteins or for the study of protein transport mechanisms. Conversely, a phospholipid bilayer deposited via the Langmuir-Blodgett/Langmuir-Schaefer method on a hydrogel layer is potentially an effective mimic of the cross section of a biological membrane and facilitates both protein incorporation and transport studies. Prior to application, however, such membranes must be fully characterized, particularly with respect to the phospholipid bilayer phase transition temperature. Here we present a detailed characterization of the phase transition temperature of the inner and outer leaflets of a chitosan supported model membrane system. Specifically, the lateral diffusion coefficient of each individual leaflet has been determined as a function of temperature. Measurements were performed utilizing z-scan fluorescence correlation spectroscopy (FCS), a technique that yields calibration-free diffusion information. Analysis via the method of Wawrezinieck and co-workers revealed that phospholipid diffusion changes from raftlike to free diffusion as the temperature is increased-an insight into the dynamic behavior of hydrogel supported membranes not previously reported.

Effect of ionic strength on dynamics of supported phosphatidylcholine lipid bilayer revealed by FRAPP and Langmuir-Blodgett transfer ratios

To determine how lipid bilayer/support interactions are affected by ionic strength, we carried out lipid diffusion coefficient measurements by fluorescence recovery after patterned photobleaching (FRAPP) and transfer ratio measurements using a Langmuir balance on supported bilayers of phosphatidylcholine lipids. The main effect of increasing ionic strength is shown to be enhanced diffusion of the lipids due to a decrease in the electrostatic interaction between the bilayer and the support. We experimentally confirm that the two main parameters governing bilayer behavior are electrostatic interaction and bilayer/support distance. Both these parameters can therefore be used to vary the potential that acts on the bilayer. Additionally, our findings show that FRAPP is an extremely sensitive tool to study interaction effects: here, variations in diffusion coefficient as well as the presence or absence of leaflet decoupling.

Protein-phospholipid interactions in nonclassical protein secretion: problem and methods of study

Extracellular proteins devoid of signal peptides use nonclassical secretion mechanisms for their export. These mechanisms are independent of the endoplasmic reticulum and Golgi. Some nonclassically released proteins, particularly fibroblast growth factors (FGF) 1 and 2, are exported as a result of their direct translocation through the cell membrane. This process requires specific interactions of released proteins with membrane phospholipids. In this review written by a cell biologist, a structural biologist and two membrane engineers, we discuss the following subjects: (i) Phenomenon of nonclassical protein release and its biological significance; (ii) Composition of the FGF1 multiprotein release complex (MRC); (iii) The relationship between FGF1 export and acidic phospholipid externalization; (iv) Interactions of FGF1 MRC components with acidic phospholipids; (v) Methods to study the transmembrane translocation of proteins; (vi) Membrane models to study nonclassical protein release.

Fusion of bolaamphiphile micelles: a method to prepare stable supported biomimetic membranes

Supported biomimetic membranes (SBMs) on solid substrates have been commonly prepared from vesicle-forming double-tail lipids, such as zwitterionic phospholipids, using the method of vesicle fusion. Here we report on the preparation of SBMs on silica surfaces via a similar process of "micelle fusion" from a cationic single-tail bolaamphiphile GLH-20 that forms spherical and elongated thread-like micelles in solution. We demonstrate that, in contrast to zwitterionic phospholipids, GLH-20 self-assembles into a stable contiguous SBM at both low and high ionic strengths. The cationic charge of GLH-20 promotes the formation of a stable SBM through enhanced double-layer interactions with the negatively charged silica surface. It is also shown that spinach aquaporin PM-28 was successfully incorporated within bolaamphiphile SBM in a manner similar to SBMs prepared by vesicle/proteoliposome fusion; thereby the inherent curvature of the micelle surface does not inhibit protein reconstitution. The results suggest that SBMs based on charged bolaamphiphiles might be an attractive platform for applications such as water purification and biosensors, where the stability and low defect rate of SBMs in diverse conditions are crucial for achieving desired performance.

Lipid rearrangement in DSPC/DMPC bilayers: a neutron reflectometry study

Lipid translocation in membranes is still far from being understood and well characterized for natural cell membranes as well as for simpler bilayer model systems. Several discrepancies with respect to its occurrence and its characteristic time scale are present in the literature. In the current work, the structural changes induced by lipid rearrangement in a distearoyl-/dimyristoyl-phosphocholine binary lipid system have been addressed by means of neutron reflectivity. It has been shown that a fast, spontaneous compositional reorganization with lipid transfer between the two leaflets of the bilayer takes place only when the lipid species are both in the fluid phase. This process has been identified as the so-called lipid flip-flop. Moreover, the influence of the preparation protocol on the structural properties of the system has been investigated.

Beyond Saffman-Delbruck approximation: a new regime for 2D diffusion of α-hemolysin complexes in supported lipid bilayer

Cell mechanisms are actively modulated by membrane dynamics. We studied the dynamics of a first-stage biomimetic system by Fluorescence Recovery After Patterned Photobleaching. Using this simple biomimetic system, constituted by α -hemolysin from Staphylococcus aureus inserted as single heptameric pore or complexes of pores in a glass-supported DMPC bilayer, we observed true diffusion behavior, with no immobile fraction. We find two situations: i) when incubation is shorter than 15 hours, the protein inserts as a heptameric pore and diffuses roughly three times more slowly than its host lipid bilayer; ii) incubation longer than 15 hours leads to the formation of larger complexes which diffuse more slowly. Our results indicate that, while the Saffman-Delbruck model adequately describes the diffusion coefficient D for small radii, D of the objects decreases as 1/R(2) for the size range explored in this study. Additionally, in the presence of inserted proteins, the gel-to-fluid transition of the supported bilayer as well as a temperature shift in the gel-to-fluid transition are observed.

Lipid Bilayers: The Effect of Force Field on Ordering and Dynamics

The sensitivity of the structure and dynamics of a fully hydrated pure bilayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in molecular dynamics simulations to changes in force-field and simulation parameters has been assessed. Three related force fields (the Gromos 54A7 force field, a Gromos 53A6-derived parameter set and a variant of the Berger parameters) in combination with either particle-mesh Ewald (PME) or a reaction field (RF) were compared. Structural properties such as the area per lipid, carbon-deuterium order parameters, electron density profile and bilayer thicknesses, are reproduced by all the parameter sets within the uncertainty of the available experimental data. However, there are clear differences in the ordering of the glycerol backbone and choline headgroup, and the orientation of the headgroup dipole. In some cases, the degree of ordering was reminiscent of a liquid-ordered phase. It is also shown that, although the lateral diffusion of the lipids in the plane of the bilayer is often used to validate lipid force fields, because of the uncertainty in the experimental measurements and the fact that the lateral diffusion is dependent on the choice of the simulation conditions, it should not be employed as a measure of quality. Finally, the simulations show that the effect of small changes in force-field parameters on the structure and dynamics of a bilayer is more significant than the treatment of the long-range electrostatic interactions using RF or PME. Overall, the Gromos 54A7 best reproduced the range of experimental data examined.

Asymmetric distribution of anionic phospholipids in supported lipid bilayers

Lipid bilayers with a controlled content of anionic lipids are a prerequisite for the quantitative study of hydrophobic-electrostatic interactions of proteins with lipid bilayers. Here, the asymmetric distribution of zwitterionic and anionic lipids in supported lipid bilayers is studied by neutron reflectometry. We prepare POPC/POPS (3:1) unilamellar vesicles in a high-salt-concentration buffer. Initially, no fusion of the vesicles to a SiO(2) surface is observed over hours and days. Once the isotonic buffer is exchanged with hypotonic buffer, vesicle fusion and bilayer formation occur by osmotic shock. Neutron reflectivity on the bilayers formed this way reveals the presence of anionic lipids (d(31)-POPS) in the outer bilayer leaflet only, and no POPS is observed in the leaflet facing the SiO(2) substrate. We argue that this asymmetric distribution of POPS is induced by the electrostatic repulsion of the phosphatidylserines from the negatively charged hydroxy surface groups of the silicon block. Such bilayers with controlled and high contents of anionic lipids in the outer leaflet are versatile platforms for studying anionic lipid protein interactions that are key elements in signal transduction pathways in the cytoplasmic leaflet of eukaryotic cells.

Preparation of supported lipid membranes for aquaporin Z incorporation

There has been a recent surge of interest to mimic the performance of natural cellular membranes by incorporating water channel proteins-aquaporins (AQPs) into various ultrathin films for water filtration applications. To make biomimetic membranes one of the most crucial steps is preparing a defect-free platform for AQPs incorporation on a suitable substrate. In this study two methods were used to prepare supported lipid membranes on NF membrane surfaces under a benign pH condition of 7.8. One method was direct vesicle fusion on a hydrophilic membrane NF-270; the other was vesicle fusion facilitated by hydraulic pressure on a modified hydrophilic NF-270 membrane whose surface has been spin-coated with positively charged lipids. Experiments revealed that the supported lipid membrane without AQPs prepared by the spin coating plus vesicle fusion had a much lower defect density than that prepared by vesicle fusion alone. It appears that the surface roughness and charge are the main factors determining the quality of the supported lipid membrane. Aquaporin Z (AqpZ) proteins were successfully incorporated into 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes and its permeability was measured by the stopped-flow experimental procedure. However, after the proteoliposomes have been fused onto the modified substrate, the AqpZ function in the resultant membrane was not observed and AFM images showed distinct aggregations of unfused proteoliposomes or AqpZ proteins on the substrate surface. It is speculated that the inhibition of AqpZ function may be caused by the low lipid mobility on the NF membrane surface. Further investigations to evaluate and optimize the structure-performance relationship are required.

Hydration effects on membrane structure probed by single molecule orientations

Single molecule fluorescence measurements are used to probe the structural changes in glass-supported DPPC bilayers as a function of relative humidity (RH). Defocused polarized total internal reflection fluorescence microscopy is employed to determine the three-dimensional orientation of the fluorescent lipid analogue BODIPY-PC, doped into DPPC membranes in trace amounts. Supported DPPC bilayers formed using vesicle fusion and Langmuir-Blodgett/Langmuir-Schäfer (LB/LS) transfer are compared and show similar trends as a function of relative humidity. Population histograms of the emission dipole tilt angle reveal bimodal distributions as observed previously for BODIPY-PC in DPPC. These distributions are dominated by large populations of BODIPY-PC molecules with emission dipoles oriented parallel (≥81°) and normal (≤10°) to the membrane plane, with less than 25% oriented at intermediate tilts. As the relative humidity is increased from 13% to 95%, the population of molecules oriented normal to the surface decreases with a concomitant increase in those oriented parallel to the surface. The close agreement in trends observed for bilayers formed from vesicle fusion and LB/LS transfer supports the assignment of an equivalent surface pressure of 23 mN/m for bilayers formed from vesicle fusion. At each RH condition, a small population of BODIPY-PC dye molecules are laterally mobile in both bilayer preparations. This population exponentially increases with RH but never exceeds 6% of the total population. Interestingly, even under conditions where there is little lateral diffusion, fluctuations in the single molecule orientations can be observed which suggests there is appreciable freedom in the acyl chain region. Dynamic measurements of single molecule orientation changes, therefore, provide a new view into membrane properties at the single molecule level.

Concentration dependence of lipopolymer self-diffusion in supported bilayer membranes

Self-diffusion coefficients of poly(ethylene glycol)2k-derivatized lipids (DSPE-PEG2k-CF) in glass-supported DOPC phospholipid bilayers are ascertained from quantitative fluorescence recovery after photobleaching (FRAP). We developed a first-order reaction-diffusion model to ascertain the bleaching constant, mobile fraction and lipopolymer self-diffusion coefficient D(s) at concentrations in the range c ≈ 0.5-5 mol%. In contrast to control experiments with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt) (DOPE-NBD) in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the lipopolymer self-diffusion coefficient decreases monotonically with increasing concentration, without a distinguishing mushroom-to-brush transition. Our data yield a correlation D(s) = D(0)/(1 + αc), where D(0) ≈ 3.36 µm(2) s(-1) and α ≈ 0.56 (with c expressed as a mole percent). Interpreting the dilute limit with the Scalettar-Abney-Owicki statistical mechanical theory for transmembrane proteins yields an effective disc radius a(e) ≈ 2.41 nm. On the other hand, the Bussell-Koch-Hammer theory, which includes hydrodynamic interactions, yields a(e) ≈ 2.92 nm. As expected, both measures are smaller than the Flory radius of the 2 kDa poly(ethylene glycol) (PEG) chains, R(F) ≈ 3.83 nm, and significantly larger than the nominal radius of the phospholipid heads, a(l) ≈ 0.46 nm. The diffusion coefficient at infinite dilution D(0) was interpreted using the Evans-Sackmann theory, furnishing an inter-leaflet frictional drag coefficient b(s) ≈ 1.33 × 10(8) N s m(-3). Our results suggest that lipopolymer interactions are dominated by the excluded volume of the PEG-chain segments, with frictional drag dominated by the two-dimensional bilayer hydrodynamics.

Supported lipid bilayers on mica and silicon oxide: comparison of the main phase transition behavior

The usual biophysical approach to the study of biological membranes is that of turning to model systems. From these models, general physical principles ruling the lateral membrane structure can be obtained. A promising model system is the supported lipid bilayer (SLB) which could foresee the simultaneous investigation of the structure and physical properties of lipid bilayers reconstituted with membrane proteins. A complete exploitation of the model system to retrieve biologically relevant information requires an in-depth knowledge of the possible effect that experimental parameters could have on the behavior of the SLB. Here we used atomic force microscopy (AFM) to study the effect of different types of substrates on the behavior of SLBs as far as their main phase transition is concerned. We found that different substrates (mica and silicon oxide) can affect in dissimilar ways the interleaflet coupling of the bilayer, which might represent a sort of lipid signaling allowing communication between receptors on the extracellular leaflet and cytoplasmic components. By decreasing the interaction between the SLB and the substrate the interleaflet coupling is preserved independently of the bilayer preparation strategy. Moreover, we investigated by time-lapse AFM an isothermal phase transition induced by a pH change on a SLB. We established that the presence of a pH gradient across the bilayer can weaken the strength of the interleaflet coupling which is present in symmetrical pH conditions.