Bilayer Thickness Research Articles

Notes on molecular fragmentation and parameter settings for a dissipative particle dynamics study of a C10E4/water mixture with lamellar bilayer formation

The influence of molecular fragmentation and parameter settings on a mesoscopic dissipative particle dynamics (DPD) simulation of lamellar bilayer formation for a C10E4/water mixture is studied. A “bottom-up” decomposition of C10E4 into the smallest fragment molecules (particles) that satisfy chemical intuition leads to convincing simulation results which agree with experimental findings for bilayer formation and thickness. For integration of the equations of motion Shardlow’s S1 scheme proves to be a favorable choice with best overall performance. Increasing the integration time steps above the common setting of 0.04 DPD units leads to increasingly unphysical temperature drifts, but also to increasingly rapid formation of bilayer superstructures without significantly distorted particle distributions up to an integration time step of 0.12. A scaling of the mutual particle–particle repulsions that guide the dynamics has negligible influence within a considerable range of values but exhibits apparent lower thresholds beyond which a simulation fails. Repulsion parameter scaling and molecular particle decomposition show a mutual dependence. For mapping of concentrations to molecule numbers in the simulation box particle volume scaling should be taken into account. A repulsion parameter morphing investigation suggests to not overstretch repulsion parameter accuracy considerations.

Construction of self-assembled bilayer core–shell V2O3 microspheres as absorber with superior microwave absorption performance

The design and preparation of heterogeneous structures of dielectric materials has been the mainstream direction for the construction of superior microwave absorption materials (MAMs). We report a facile and efficient procedure combination of hydrothermal process and subsequent heat treatment for successfully prepared bilayer core–shell structure self-assembled V2O3 microspheres (BCSV). The microstructure, defects, dielectric properties and microwave absorption (MA) properties of BCSV were systematically investigated, and the effect of bilayer core–shell structure on the MA properties was discussed. By varying the heat treatment temperature, it is feasible to regulate the thickness of V2O3 bilayer and its unique structure defects, hence enhancing the attenuation and multiple polarization loss of electromagnetic waves inside the microspheres. Self-assembled V2O3 microspheres with bilayer core–shell structure exhibit high-performance MA property. The reflection loss (RL) gets to − 67.12 dB at 11.69 GHz covering the whole X-band after heat treatment at 600 °C, and the broad effective absorption bandwidth is 5.49 GHz with a thickness of 2.20 mm. The conductivity loss, multiple polarization loss and dielectric loss are ascribed to the specific bilayer core–shell structure. Thus, our work provides a good perspective on how to create vanadium oxide-based MAMs with effective absorption and broad bandwidth.

Dependence of protein-induced lipid bilayer deformations on protein shape.

Membrane proteins typically deform the surrounding lipid bilayer membrane, which can play an important role in the function, regulation, and organization of membrane proteins. Membrane elasticity theory provides a beautiful description of protein-induced lipid bilayer deformations, in which all physical parameters can be directly determined from experiments. While analytic solutions of protein-induced elastic bilayer deformations are most easily developed for proteins with approximately circular cross sections, structural biology has shown that membrane proteins come in a variety of distinct shapes, with often considerable deviations from a circular cross section. We develop here a boundary value method (BVM) that permits the construction of analytic solutions of protein-induced elastic bilayer deformations for protein shapes with arbitrarily large deviations from a circular cross section, for constant as well as variable boundary conditions along the bilayer-protein interface. We apply this BVM to protein-induced lipid bilayer thickness deformations. Our BVM reproduces available analytic solutions for proteins with circular cross sectionand yields, for proteins with noncircular cross section, excellent agreement with numerical, finite element solutions. On this basis, we formulate a simple analytic approximation of the bilayer thickness deformation energy associated with general protein shapes and show that, for modest deviations from rotational symmetry, this analytic approximation is in good agreement with BVM solutions. Using the BVM, we survey the dependence of protein-induced elastic bilayer thickness deformations on protein shape, and thus explore how the coupling of protein shape and bilayer thickness deformations affects protein oligomerization and transitions in protein conformational state.

Molecular dynamics study of the influence of cholesterol on the rigidity of an asymmetric membrane of red blood cells.

Analysis of the mechanical properties of the plasma membrane of red blood cell (RBC) is useful for the future exploration of cell-targeted methods for treatment of Alzheimer's disease. This preliminary study aimed to observe the effects cholesterol and lipid concentration/asymmetry have on the membrane rigidity and bilayer thickness. We focused on the structure-property relationship in the plasma membranes via the analysis of bending rigidity as a function of the molar concentration of the cholesterol and lipids in the plasma membrane. The plasma membrane of RBC consists mainly of cholesterol, phosphatidylcholine (PC), sphingomyelin (SM), and proteins. Cholesterol plays a key role in the plasma membrane by maintaining its viscoelasticity and structure via modulating the fluidity and rigidity of the membrane. To analyze these properties, the coarse-grained (CG) molecular dynamics (MD) simulations were performed. The membrane building included cholesterol molar concentration ranging from 10% to 30%. To replicate the asymmetry of the cell SM and PC, the molar concentrations of lipids were varied in the range of 10%-20% and 50%-80%, respectively. The initial membrane configuration was generated using CHARMM-GUI (online graphical user interface tool). The MD simulations were performed with the Gromacs (molecular dynamic simulation program) software using all-atom and coarse-grained approaches. The aggregation of cholesterol at various molar concentrations was observed along with the fluctuation of rigidity, which could be tentatively attributed to the lipid types used in the study, the ratio of cholesterol and lipids, and the use of water as a solvent for both the inner and outer leaflet. In the future we will focus on the all-atom study of the tail interactions between cholesterol and lipids in order to identify the leading factors contributing to the bending rigidity.

Structural Insights into Polymer-Bounded Lipid Nanodiscs.

Membrane proteins are an essential part of signaling and transport processes and are targeted by multiple drugs. To isolate and investigate them in their native state, polymer-bounded nanodiscs have become valuable tools. In this study, we investigate the lipid model system dimyristoyl-phosphocholine (DMPC) with the nanodisc-forming copolymers styrene maleic acid (SMA) and diisobutylene maleic acid (DIBMA). Using small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS), we studied the influence of polymer concentration and temperature on the nanodisc structure. In Tris buffer, the size of nanodiscs formed with SMA is smaller compared to DIBMA at the same polymer ratio. In both cases, the size decreases monotonically with increasing polymer concentration, and this effect is more pronounced when using SMA. Measurements at temperatures (T) between 5 and 30 °C in phosphate buffer showed an incomplete solubilization at high T even at polymer/lipid ratios above that required for complete lipid solubilization. For DIBMA, the nanodiscs developed at lower temperatures are stable and the net repulsion increases, while for SMA, the individual nanodiscs possess smaller sizes and are less affected by T. However, using DLS, one can observe SMA agglomerates at low T. Interestingly, for both polymers, no drastic changes of the observable parameters (radius and bilayer thickness) are seen upon cooling, which would indicate a sharp (first-order) phase transition from liquid-crystalline to gel, but only gradual changes. Hence, we conclude that the transition from a gel toward a liquid-crystalline lipid phase proceeds over a broad T-range compared to a continuous lipid bilayer. These results can pave the way toward the development of better protocols for studying membrane proteins stabilized in this type of membrane mimics.

Untangling the rivalry between cholesterol and antimicrobial peptides.

Antimicrobial peptides (AMPs) are promising candidates for next generation antimicrobial therapeutics. In mammalian cell membranes cholesterol plays a key regulatory function in antibiotic drug resistance and the immune response. However, the mechanism of action and the selectivity of AMPs towards bacterial membranes are not yet well understood. We hypothesized that differences in biophysical properties of mammalian versus bacterial membranes give rise to differences in peptide-membrane interactions which underlie the AMP mechanism and selectivity. Solid-state 2H NMR spectroscopy was employed to study di-monounsaturated phosphatidylcholine and phosphatidylethanolaminelipids (DOPC versus DOPE) bilayers in the presence of the antimicrobial peptide LL37 (2−4 mol%) and cholesterol (20−30% mol) as model systems mimicking mammalian and bacterial membranes. The lipid systems were probed with POPC-d31 (10 mol%) where the residual quadrupolar couplings yielded segmental order parameters (SCD) for the individual acyl segments. The LL37-peptide decreased the quadrupolar splittings indicating it increased the area per lipid, while cholesterol showed the opposite. The mean-torque model was used to calculate the key bilayer properties of area per lipid and bilayer thickness [2]. The model-free functional dependence of spin-lattice relaxation rates on SCD (square-law plots) indicated greater bending rigidity with cholesterol for both DOPC and DOPE membranes. By contrast LL37 decreased the bending rigidity of the membranes. Additionally all-atom molecular dynamics simulations for bilayers comprising POPE/DMPG/cardiolipin (90:5:5) and DMPC/cholesterol (90:10) were conducted with LL37 peptides on the bilayer surface to capture the molecular-level interactions. We observed partial insertion of the LL37-peptides in the POPE system, while the peptides were repelled from the membrane in the DMPC/cholesterol system. These observations reveal the surprising rivalry between cholesterol and AMPs which underlies AMP mechanism and selectivity.

Packing and elasticity in membrane mechanical properties.

Here we address how membrane properties involving head group size, acyl chain unsaturation, and lipid packing affect their structure and dynamics. Strong out-of-plane proteolipid couplings considered by the flexible surface model (FSM) affect protein function through elastic forces described by the spontaneous curvature and the bending rigidity. We hypothesized that the mechanical properties and the molecular packing resulting from different lipid types are captured by introducing a perdeuterated POPC-d31 probe lipid. Di-monounsaturated phosphatidylcholine and phosphatidylethanolamine (DOPC, DOPE), and different ratios of DOPC/DOPE mixtures were studied using MD simulations combined with experimental solid-state 2H NMR spectroscopy. Residual quadrupolar couplings obtained from de-Paked powder-type spectra yielded order parameter (SCD) profiles for the individual acyl segments giving insights into average membrane structural properties such as area per lipid and bilayer thickness [1]. The corresponding spin-lattice relaxation rates (R1Z) for the resolved peaks followed a theoretical square-law dependence on SCD providing the bending rigidity [2]. Furthermore, MD simulations for the lipid systems were carried out and order parameters were calculated and compared against the NMR-observed SCD values. Utilization of the probe lipid POPC-d31 (1−10 mol%)affected the lamellar to hexagonal phase transition temperature of DOPE lipid systems but did not affect the quadrupolar splittings of DOPE and DOPC systems. Higher bending rigidity was observed for DOPE membranes versus DOPC supporting the idea that the membrane bending rigidity is primarily governed by increased lipid packing and is related to the area-compressibility modulus. Use of a probe lipid to capture membrane mechanical properties of complex lipid systems allow investigation of the relationship of membrane mechanics in vital biological functions.

Evidence for long-term potentiation in lipid membranes.

Biological supramolecular assemblies, such as phospholipid bilayer membranes, have been used to demonstrate signal processing via short-term synaptic plasticity (STP) in the form of paired pulse facilitation (PPF) and depression (PPD), thus showing that model membranes have the potential to emulate the brain's efficiency and flexible cognitive capabilities. However, STP memory states in lipid bilayers are volatile and cannot be stored or accessed over relevant periods of time, a central requirement for learning. Using droplet interface bilayers (DIBs) composed of lipids and hexadecane, we apply an electrical stimulation protocol featuring repetitive voltage cycling in the form of a sinusoidal waveform. We show that DIBs displaying memcapacitance can also exhibit long-term synaptic plasticity in the form of long-term potentiation (LTP) associated with capacitive energy storage in the phospholipid bilayer. The time scales for the physical changes associated with LTP range between minutes to hours and are substantially longer than previous STP studies, where stored energy dissipated after only a few seconds. While STP behavior is the result of changes in bilayer geometry associated with reversible changes in bilayer area and thickness, LTP is the result of molecular and structural changes due to the zwitterionic lipid headgroups and the dielectric properties of the lipid bilayer that result from the buildup of an increasingly asymmetric charge distribution between the bilayer interfaces.

The role of the membrane in driving CLC-ec1 dimerization.

Most membrane proteins oligomerize in the membrane. In the case of the bacterial chloride/proton antiporter CLC-ec1, the role of highly stable homodimers is not understood since individual CLC-ec1 protomers can maintain the chloride/proton transport stoichiometry. In a previous study, we used experimental and computational approaches to show that in C16:0/C18:1 palmitoyl-oleoyl lipid membranes, the monomeric state of CLC-ec1 creates a membrane defect that is ≈ 8 Å thinner than the bulk. However, in the dimer state, the membrane is restored to a near-native bilayer thickness and structure. We hence proposed that the dimerization is driven by the larger free-energy cost of lipid solvation of the dissociated monomers. Here, we carry out coarse-grained molecular dynamics simulations with umbrella sampling and free energy perturbation methods to quantify the membrane's contribution to free energy of dimerization. In the case of CLC-ec1, addition of 30% C12:0 di-lauryl chain lipids stabilizes the free energy of the monomeric state by ≈ 2 kcal/mole, in qualitative agreement with our experimental observations. Therefore, we have a physical model of how changes in lipid composition can regulate self-assembly reactions of membrane proteins in membranes. This work was partially performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-840782.

Sterol physical properties and relationships to endo-/exocytosis.

Multiple diseases arise from sterol homeostasis disruption and are characterized at the molecular level by depletion or accumulation of sterol species. One such disease is Smith-Lemli-Opitz syndrome (SLOS) that results from genetic mutations in a particular cholesterol synthesis enzyme, which leads to depletion of cholesterol and accumulation of 7-dehydrocholesterol (7DHC). Recent in vivo work demonstrated that depleting/enriching specific sterol species modulated clathrin-mediated endocytosis (CME) that might contribute to the cellular phenotypes observed in disease states. The CME results were explained by softening mechanisms involving sterol lateral redistribution, but it remains a question as to why this occurs. Here, we take the outlook that differences in Gaussian curvature modulus and local lipid composition destabilize narrow necks in fission, reducing the membrane's apparent rigidity. Using membrane fluctuation analysis, we quantify sterol physical properties that partially define the fusion/fission energetic pathway, for example: the compressibility modulus, bending modulus, leaflet neutral surface, leaflet spontaneous curvature, bilayer thickness preference, and thickness contribution to the Gaussian modulus. Using these quantities, we make predictions on where sterol species will redistribute on strongly curved membranes. Finally, we use all-atom simulations of strongly curved fusion pore and hemifusion diaphragm geometries to quantify the sterol redistribution and compare to hypotheses. Comparing cholesterol and 7DHC physical properties, for example, the key statistically significant quantity is the thickness contribution to the Gaussian modulus, and we predicted that 7DHC would be less excluded from the fusion pore than cholesterol. All-atom molecular dynamics simulations of the fusion pore confirmed this hypothesis. The result correlates to in vivo experiments that showed reduced CME with 7DHC compared to cholesterol (p < 0.10). Other tests are ongoing.

Surface-functionalized polystyrene nanoparticles alter transmembrane potential via ion-selective pores maintaining global bilayer integrity.

Although nanoplastics have well-known toxic effects towards the environment and living organisms, their molecular toxicity mechanisms, including the nature of nanoparticle-cell membrane interactions, are still under investigation. We employ dynamic light scattering (DLS), quartz crystal microbalance with dissipation monitoring (QCM-D) and electrophysiology to investigate the interaction between polystyrene nanoparticles (PS NPs) and phospholipid membranes. Our results show that PS NPs adsorb onto lipid bilayers creating soft inhomogeneous films that include disordered defects. PS NPs form an integral part of the generated channels so that the surface functionalization and charge of the NP determine the pore conductive properties. The large difference in size between nanoparticle diameter and lipid bilayer thickness (∼60 nm vs ∼5 nm) suggests a particular and complex lipid-NP assembly that is able to maintain overall membrane integrity. In view of that, we suggest that nanoparticle-induced toxicity in cells could operate in more subtle ways than membrane disintegration, such as inducing lipid reorganization and transmembrane ionic fluxes that disrupt the membrane potential.

Nanoscale membrane dynamics in chain asymmetric lipid bilayers.

Lipids that have two tails of different lengths are found throughout biomembranes in nature, yet the effects of this asymmetry on the membrane properties are not well understood, especially the membrane dynamics. Accommodating such a difference in tail length could significantly impact the membrane dynamics and elastic properties depending on how the longer tail is situated within the bilayer. To probe these effects, here we study the nanoscale bending fluctuations in model chain-asymmetric 14:0-18:0 PC (MSPC) and 18:0-14:0 PC (SMPC) lipid bilayers using neutron spin echo (NSE) spectroscopy. We find that despite the partial interdigitation that is known to persist in the fluid phase of these membranes, the collective fluctuations are enhanced on timescales of tens of nanoseconds, and the chain-asymmetric lipid bilayers are softer than an analogous chain-symmetric lipid with the same average number of carbons in the acyl tails, di-16:0 PC (DPPC). Quantitative comparison of the NSE results suggests that the enhanced bending fluctuations at the nanosecond timescales are consistent with experimental and computational studies that showed the compressibility moduli of chain-asymmetric lipid membranes are 20 % to 40% lower than chain-symmetric lipid membranes. These studies add to growing evidence that the partial interdigitation in chain-asymmetric lipid membranes in the fluid phase is highly dynamic and influences membrane dynamic processes from the molecular to mesoscopic length scales without significantly changing the bilayer thickness or area per lipid.

Factors That Control the Force Needed to Unfold a Membrane Protein in Silico Depend on the Mode of Denaturation.

Single-molecule force spectroscopy methods, such as AFM and magnetic tweezers, have proved extremely beneficial in elucidating folding pathways for soluble and membrane proteins. To identify factors that determine the force rupture levels in force-induced membrane protein unfolding, we applied our near-atomic-level Upside molecular dynamics package to study the vertical and lateral pulling of bacteriorhodopsin (bR) and GlpG, respectively. With our algorithm, we were able to selectively alter the magnitudes of individual interaction terms and identify that, for vertical pulling, hydrogen bond strength had the strongest effect, whereas other non-bonded protein and membrane-protein interactions had only moderate influences, except for the extraction of the last helix where the membrane-protein interactions had a stronger influence. The up-down topology of the transmembrane helices caused helices to be pulled out as pairs. The rate-limiting rupture event often was the loss of H-bonds and the ejection of the first helix, which then propagated tension to the second helix, which rapidly exited the bilayer. The pulling of the charged linkers across the membrane had minimal influence, as did changing the bilayer thickness. For the lateral pulling of GlpG, the rate-limiting rupture corresponded to the separation of the helices within the membrane, with the H-bonds generally being broken only afterward. Beyond providing a detailed picture of the rupture events, our study emphasizes that the pulling mode greatly affects the factors that determine the forces needed to unfold a membrane protein.

Changes in Electrical Capacitance of Cell Membrane Reflect Drug Partitioning-Induced Alterations in Lipid Bilayer

The plasma membrane is a lipid bilayer that establishes the outer boundary of a living cell. The composition of the lipid bilayer influences the membrane's biophysical properties, including fluidity, thickness, permeability, phase behavior, charge, elasticity, and formation of flat sheet or curved structures. Changes in the biophysical properties of the membrane can be occasioned when new entities, such as drug molecules, are partitioned in the bilayer. Therefore, assessing drugs for their effect on the biophysical properties of the lipid bilayer of a cell membrane is critical to understanding specific and non-specific drug action. Previously, we reported a non-invasive technique for real-time characterization of cellular dielectric properties, such as membrane capacitance and cytoplasmic conductivity. In this study, we discuss the potential application of the technique in assessing the biophysical properties of the cell membrane in response to interaction with amiodarone compared to aspirin/acetylsalicylic acid and glucose. Amiodarone is a potent drug used to treat cardiac arrhythmia, but it also exerts various non-specific effects. Compared to aspirin and glucose, we measured a rapid and higher magnitude increase in membrane capacitance on cells under amiodarone treatment. Increased membrane capacitance induced by aspirin and glucose quickly returned to baseline in 15 s, while amiodarone-induced increased capacitance sustained and decreased slowly, approaching baseline or another asymptotic limit in ~2.5 h. Because amiodarone has a strong lipid partitioning property, we reason that drug partitioning alters the lipid bilayer context and subsequently reduces bilayer thickness, leading to an increase in the electrical capacitance of the cell membrane. The presented microfluidic system promises a new approach to assess drug-membrane interactions and delineate specific and non-specific actions of the drug on cells.

Incorporation and localisation of alkanes in a protomembrane model by neutron diffraction

Protomembranes at the origin of life were likely composed of short-chain lipids, readily available on the early Earth. Membranes formed by such lipids are less stable and more permeable under extreme conditions, so a novel membrane architecture was suggested to validate the accuracy of this assumption. The model membrane includes the presence of a layer of alkanes in the mid-plane of the protomembrane in between the two monolayer leaflets and lying perpendicular to the lipid acyl chains. Here, we investigated such a possibility experimentally for membranes formed by the short-chain phospholipid 1,2-didecanoyl-sn-glycero-3-phophocholine, including or not the alkanes eicosane, squalane or triacontane by means of neutron membrane diffraction and contrast variation. We found strong indications for incorporation of two of the three alkanes in the membrane mid-plane through the determination of neutron scattering length density profiles with hydrogenated vs deuterated alkanes and membrane swelling at various relative humidities indicating a slightly increased bilayer thickness when the alkanes are incorporated into the bilayers. The selectivity of the incorporation points out the role of the length of the n-alkanes with respect to the capacity of the membrane to incorporate them.

Nanoscale Bending Dynamics in Mixed-Chain Lipid Membranes

Lipids that have two tails of different lengths are found throughout biomembranes in nature, yet the effects of this asymmetry on the membrane properties are not well understood, especially when it comes to the membrane dynamics. Here we study the nanoscale bending fluctuations in model mixed-chain 14:0–18:0 PC (MSPC) and 18:0–14:0 PC (SMPC) lipid bilayers using neutron spin echo (NSE) spectroscopy. We find that despite the partial interdigitation that is known to persist in the fluid phase of these membranes, the collective fluctuations are enhanced on timescales of tens of nanoseconds, and the chain-asymmetric lipid bilayers are softer than an analogous chain-symmetric lipid bilayer with the same average number of carbons in the acyl tails, di-16:0 PC (DPPC). Quantitative comparison of the NSE results suggests that the enhanced bending fluctuations at the nanosecond timescales are consistent with experimental and computational studies that showed the compressibility moduli of chain-asymmetric lipid membranes are 20% to 40% lower than chain-symmetric lipid membranes. These studies add to growing evidence that the partial interdigitation in mixed-chain lipid membranes is highly dynamic in the fluid phase and impacts membrane dynamic processes from the molecular to mesoscopic length scales without significantly changing the bilayer thickness or area per lipid.

Investigating the cut-off effect of n-alcohols on lipid movement: a biophysical study.

Cellular membranes are responsible for absorbing the effects of external perturbants for the cell's survival. Such perturbants include small ubiquitous molecules like n-alcohols which were observed to exhibit anesthetic capabilities, with this effect tapering off at a cut-off alcohol chain length. To explain this cut-off effect and complement prior biochemical studies, we investigated a series of n-alcohols (with carbon lengths 2-18) and their impact on several bilayer properties, including lipid flip-flop, intervesicular exchange, diffusion, membrane bending rigidity and more. To this end, we employed an array of biophysical techniques such as time-resolved small angle neutron scattering (TR-SANS), small angle X-ray scattering (SAXS), all atomistic and coarse-grained molecular dynamics (MD) simulations, and calcein leakage assays. At an alcohol concentration of 30 mol% of the overall lipid content, TR-SANS showed 1-hexanol (C6OH) increased transverse lipid diffusion, i.e. flip-flop. As alcohol chain length increased from C6 to C10 and longer, lipid flip-flop slowed by factors of 5.6 to 32.2. Intervesicular lipid exchange contrasted these results with only a slight cut-off at alcohol concentrations of 30 mol% but not 10 mol%. SAXS, MD simulations, and leakage assays revealed changes to key bilayer properties, such as bilayer thickness and fluidity, that correlate well with the effects on lipid flip-flop rates. Finally, we tie our results to a defect-mediated pathway for alcohol-induced lipid flip-flop.

Structural insights on ionizable Dlin-MC3-DMA lipids in DOPC layers by combining accurate atomistic force fields, molecular dynamics simulations and neutron reflectivity.

Ionizable lipids such as the promising Dlin-MC3-DMA (MC3) are essential for the successful design of lipid nanoparticles (LNPs) as drug delivery agents. Combining molecular dynamics simulations with experimental data, such as neutron reflectivity experiments and other scattering techniques, is essential to provide insights into the internal structure of LNPs, which is not fully understood to date. However, the accuracy of the simulations relies on the choice of force field parameters and high-quality experimental data is indispensable to verify the parametrization. For MC3, different parameterizations in combination with the CHARMM and the Slipids force fields have recently emerged. Here, we complement the existing efforts by providing parameters for cationic and neutral MC3 compatible with the AMBER Lipid17 force field. Subsequently, we carefully assess the accuracy of the different force fields by providing a direct comparison to neutron reflectivity experiments of mixed lipid bilayers consisting of MC3 and DOPC at different pHs. At low pH (cationic MC3) and at high pH (neutral MC3) the newly developed MC3 parameters in combination with AMBER Lipid17 for DOPC give good agreement with the experiments. Overall, the agreement is similar compared to the Park-Im parameters for MC3 in combination with the CHARMM36 force field for DOPC. The Ermilova-Swenson MC3 parameters in combination with the Slipids force field underestimate the bilayer thickness. While the distribution of cationic MC3 is very similar, the different force fields for neutral MC3 reveal distinct differences ranging from strong accumulation in the membrane center (current MC3/AMBER Lipid17 DOPC), over mild accumulation (Park-Im MC3/CHARMM36 DOPC) to surface accumulation (Ermilova-Swenson MC3/Slipids DOPC). These pronounced differences highlight the importance of accurate force field parameters and their experimental validation.

Effect of oxidation on POPC lipid bilayers: anionic carboxyl group plays a major role.

Phospholipids with unsaturated acyl chains are major targets of reactive oxygen species leading to formation of oxidized lipids. Oxidized phospholipids have a pronounced role in cell membrane damage. We investigated the effect of oxidation on physiological properties of phospholipid bilayers using atomistic molecular dynamics simulations. We studied phospholipid bilayer systems of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and its two stable oxidized products, 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PoxnoPC) and 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC). Structural properties of the POPC lipid bilayer upon the addition of PoxnoPC or PazePC with concentration ranging from 10% to 30% were described. The key finding is that PazePC lipids bend their polar tails toward the bilayer-water interface whereas PoxnoPC lipids orient their tail toward the bilayer interior. The bilayer thickness decreases such that the thickness reduction in bilayers containing PazePC is stronger than in bilayers containing PoxnoPC. The average area per lipid decreases with a stronger effect in bilayers containing PoxnoPC. The addition of PoxnoPC makes both POPC acyl chains slightly more ordered whereas the addition of PazePC reduces the order in the two POPC acyl chains. These structural changes lead to an enhancement in the permeabilities of the bilayers containing these two oxidized products depending on the type, and the amount of oxidation. This enhancement can be achieved with a lower concentration of PazePC (10% or 15%), whereas a higher concentration of PoxnoPC (20%) is required to achieve an apparent enhancement in permeability. While the permeability of bilayers containing PazePC is higher than bilayers containing PoxnoPC in the 10-20% concentration range, by increasing the concentration of the oxidized products to higher than 20%, permeability of the bilayers containing PazePC is reduced such that it is slightly smaller than those containing PoxnoPC.

Protein-induced transformation of unilamellar to multilamellar vesicles triggered by a polysaccharide

We report on the morphological transitions of didodecyldimethylammonium bromide (DDAB) cationic vesicles and hybrid DDAB/hyaluronic acid (HA) vesicles upon addition of BSA at pH 7 where BSA is overall negatively charged. Small angle neutron scattering (SANS) is used to extract the size distributions of the nanovesicles, the thickness of the DDAB bilayers and their lamellarity. Although the HA-decorated DDAB vesicles contain the negatively charged polysaccharide the interaction with BSA appears to be more intense in comparison to bare vesicles. Characteristic peaks in the SANS patterns indicate the presence of multilamellar interfaces while the formation of multilamellar vesicles induced by BSA depends on the amount of added HA. Consequently, higher lamellarities are observed at higher BSA contents. This work demonstrates a simple methodology to tune the encapsulation of globular proteins in vesicular nanoassemblies by affecting their lamellarity and has direct implications on the application of vesicles and liposomes in protein delivery.