Relations for lipid bilayers. Connection of electron density profiles to other structural quantities

Modulating a model membrane of sphingomyelin by a tricyclic antidepressant drug

Tricyclic medicine such as amitriptyline (AMT) hydrochloride, initially developed to treat depression, is also used to treat neuropathic pain, anxiety disorder, and migraines. The mechanism of functioning of this type of drugs is ambiguous. Understanding the mechanism is important for designing new drug molecules with higher pharmacological efficiency. Hence, in the present study, biophysical approaches have been taken to shed light on their interactions with a model cellular membrane of brain sphingomyelin in the form of monolayer and multi-lamellar vesicles. The surface pressure-area isotherm infers the partitioning of a drug molecule into the lipid monolayer at the air water interface, providing a higher surface area per molecule and reducing the in-plane elasticity. Further, the surface electrostatic potential of the lipid monolayer is found to increase due to the insertion of drug molecule. The interfacial rheology revealed a reduction of the in-plane viscoelasticity of the lipid film, which, depends on the adsorption of the drug molecule onto the film. Small-angle X-ray scattering (SAXS) measurements on multilamellar vesicles (MLVs) have revealed that the AMT molecules partition into the hydrophobic core of the lipid membrane, modifying the organization of lipids in the membrane. The modified physical state of less rigid membrane and the transformed electrostatics of the membrane could influence its interaction with synaptic vesicles and neurotransmitters making higher availability of the neurotransmitters in the synaptic cleft.

Structure of the gel phase of diC22:1PC lipid bilayers determined by x-ray diffraction

High-resolution x-ray data are reported for the ordered phases of long-chain di-monounsaturated C22:1 phosphocholine lipid bilayers. Similar to PC lipids that have saturated chains, diC22:1PC has a subgel phase and a gel phase, but dissimilarly, we find no ripple phase. Our quantitative focus is on the structure of the gel phase. We have recorded 17 lamellar orders, indicating a very well-ordered structure. Fitting to a model provides the phases of the orders. The Fourier construction of the electron density profile has two well-defined headgroup peaks and a very sharp and deep methyl trough. The wide-angle scattering exhibits two Bragg rods that provide the area per molecule. They have an intensity pattern quite different than that of lipids with saturated chains. Models of chain packing indicate that ground state chain configurations are tilted primarily toward next nearest neighbors with an angle that is also consistent with the modeling of the electron density profile. Wide-angle modeling also indicates broken mirror symmetry between the monolayers. Our wide-angle results and our electron density profile together leads to the hypothesis that the sn-1 and sn-2 chains have equivalent penetration depths in contrast to the gel phase structure of lipids with saturated hydrocarbon chains.

Structural and mechanical properties of the red blood cell's cytoplasmic membrane seen through the lens of biophysics

Red blood cells (RBCs) are the most abundant cell type in the human body and critical suppliers of oxygen. The cells are characterized by a simple structure with no internal organelles. Their two-layered outer shell is composed of a cytoplasmic membrane (RBC cm ) tethered to a spectrin cytoskeleton allowing the cell to be both flexible yet resistant against shear stress. These mechanical properties are intrinsically linked to the molecular composition and organization of their shell. The cytoplasmic membrane is expected to dominate the elastic behavior on small, nanometer length scales, which are most relevant for cellular processes that take place between the fibrils of the cytoskeleton. Several pathologies have been linked to structural and compositional changes within the RBC cm and the cell's mechanical properties. We review current findings in terms of RBC lipidomics, lipid organization and elastic properties with a focus on biophysical techniques, such as X-ray and neutron scattering, and Molecular Dynamics simulations, and their biological relevance. In our current understanding, the RBC cm 's structure is patchy, with nanometer sized liquid ordered and disordered lipid, and peptide domains. At the same time, it is surprisingly soft, with bending rigidities κ of 2-4 kBT. This is in strong contrast to the current belief that a high concentration of cholesterol results in stiff membranes. This extreme softness is likely the result of an interaction between polyunsaturated lipids and cholesterol, which may also occur in other biological membranes. There is strong evidence in the literature that there is no length scale dependence of κ of whole RBCs.

Cochleate drug delivery systems: An approach to their characterization

Cochleate systems formed from phospholipids have very useful properties as drug delivery systems with sustained release capabilities, which are able to improve bioavailability and efficacy, reduce toxicity and increase the shelf-life of encapsulated molecules. These nanometric or micrometric structures are usually obtained after interaction of negatively charged liposomes with a positively charged bridging agent. Many different methods are now available to prepare cochleates and there are also numerous techniques that can be used to characterize them, some of which can be easily applied while others require more sophisticated equipment or analysis. The present review describes the important features of this drug delivery system; including their structural properties and potential applications, as well as a brief account of methods for their preparation and an extensive description of the techniques used for their characterization. This information could guide formulators in their choice of methods of characterization that would be best suited to their needs in terms of time, precision and technological difficulty.

Blood bank storage of red blood cells increases RBC cytoplasmic membrane order and bending rigidity

Blood banks around the world store blood components for several weeks ensuring its availability for transfusion medicine. Red blood cells (RBCs) are known to undergo compositional changes during storage, which may impact the cells' function and eventually the recipients' health. We extracted the RBC's cytoplasmic membrane (RBCcm) to study the effect of storage on the membranes' molecular structure and bending rigidity by a combination of X-ray diffraction (XRD), X-ray diffuse scattering (XDS) and coarse grained Molecular Dynamics (MD) simulations. Blood was stored in commercial blood bags for 2 and 5 weeks, respectively and compared to freshly drawn blood. Using mass spectrometry, we measured an increase of fatty acids together with a slight shift towards shorter tail lengths. We observe an increased fraction (6%) of liquid ordered (lo) domains in the RBCcms with storage time, and an increased lipid packing in these domains, leading to an increased membrane thickness and membrane order. The size of both, lo and liquid disordered (ld) lipid domains was found to decrease with increased storage time by up to 25%. XDS experiments reveal a storage dependent increase in the RBCcm's bending modulus κ by a factor of 2.8, from 1.9 kBT to 5.3 kBT. MD simulations were conducted in the absence of proteins. The results show that the membrane composition has a small contribution to the increased bending rigidity and suggests additional protein-driven mechanisms.

Deconvolving signals of the stretched exponential form

A solution is presented for parametrically deconvolving an axisymmetric intrinsic field signal when it is expected to conform to the stretched exponential family of functions (SEF). Except for the Gaussian SEF, computable analytical models for the forward Abel transform of SEFs did not exist until recently. I will highlight a novel mathematical identity that has facilitated this calculation and show how to use the 2D models for the Abel transform to reconstruct the 3D signal to an accuracy of ∼10^-6 (or better) under noise-free conditions (possibly even with zero error). Several deconvolution techniques have tested their reconstructions using a noise-free projection of the Gaussian. Under similar conditions, our reconstruction produces errors ∼1950 times lower than 10 other techniques; we get significantly lower errors for other SEFs as well using fewer computing resources. Additionally, unlike other methods, our approach works on unequally spaced data and does not encounter the problem of increasing errors with radii in the outer parts as seen in all other methods. I will describe applications in the imaging of diverse astrophysical and biological systems where SEFs have been used and also highlight the possibility of using the projection of SEFs as basis functions in image deconvolution algorithms.

Computational analysis of deleterious single nucleotide polymorphisms in catechol O-Methyltransferase conferring risk to post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is one of the prevalent neurological disorder which is drawing increased attention over the past few decades. Major risk factors for PTSD can be categorized into environmental and genetic factors. Among the genetic risk factors, polymorphisms in the catechol-O-methyltransferase (COMT) gene is known to be associated with the risk for PTSD. In the present study, we analysed the impact of deleterious single nucleotide polymorphisms (SNPs) in the COMT gene conferring risk to PTSD using computational based approaches followed by molecular dynamic simulations. The data on COMT gene associated with PTSD were collected from several databases including Online Mendelian Inheritance in Man (OMIM) search. Datasets related to SNP were downloaded from the dbSNP database. To study the structural and dynamic effects of COMT wild type and mutant forms, we performed molecular dynamics simulations (MD simulations) at a time scale of 300 ns. Results from screening the SNPs using the computational tools SIFT and Polyphen-2 demonstrated that the SNP rs4680 (V158M) in COMT has a deleterious effect with phenotype in PTSD. Results from the MD simulations showed that there is some major fluctuations in the structural features including root mean square deviation (RMSD), radius of gyration (Rg), root mean square fluctuation (RMSF) and secondary structural elements including α-helices, sheets and turns between wild-type (WT) and mutant forms of COMT protein. In conclusion, our study provides novel insights into the deleterious effects and impact of V158M mutation on COMT protein structure which plays a key role in PTSD.

Effect of TMAO on the Structure and Phase Transition of Lipid Membranes: Potential Role of TMAO in Stabilizing Cell Membranes under Osmotic Stress

Extremophiles adopt strategies to deal with different environmental stresses, some of which are severely damaging to their cell membrane. To combat high osmotic stress, deep-sea organisms synthesize osmolytes, small polar organic molecules, like trimethylamine-N-oxide (TMAO), and incorporate them in the cell. TMAO is known to protect cells from high osmotic or hydrostatic pressure. Several experimental and simulation studies have revealed the roles of such osmolytes on stabilizing proteins. In contrast, the effect of osmolytes on the lipid membrane is poorly understood and broadly debated. A recent experiment has found strong evidence of the possible role of TMAO in stabilizing lipid membranes. Using the molecular dynamics (MD) simulation technique, we have demonstrated the effect of TMAO on two saturated fully hydrated lipid membranes in their fluid and gel phases. We have captured the impact of TMAO's concentration on the membrane's structural properties along with the fluid/gel phase transition temperatures. On increasing the concentration of TMAO, we see a substantial increase in the packing density of the membrane (estimated by area, thickness, and volume) and enhancement in the orientational order of lipid molecules. Having repulsive interaction with the lipid head group, the TMAO molecules are expelled away from the membrane surface, which induces dehydration of the lipid head groups, increasing the packing density. The addition of TMAO also increases the fluid/gel phase transition temperature of the membrane. All of these results are in close agreement with the experimental observations. This study, therefore, provides a molecular-level understanding of how TMAO can influence the cell membrane of deep-sea organisms and help in combating the osmotic stress condition.

Anesthetics significantly increase the amount of intramembrane water in lipid membranes

The potency of anesthesia was directly linked to the partitioning of the drug molecules in cell membranes by Meyer and Overton. Many molecules interact with lipid bilayers and lead to structural and functional changes. It remains an open question which change in membrane properties is responsible for a potential anesthetic effect or if anesthetics act by binding to direct targets. We studied the effect of ethanol, diethyl ether and isoflurane on the water distribution in lipid bilayers by combining all-atom molecular dynamics simulations and neutron diffraction experiments. The simulations show strong membrane-drug interactions with partitioning coefficients of 38%, 92% and 100% for ethanol, diethyl ether and isoflurane, respectively, and provide evidence for an increased water partitioning in the membrane core. The amount of intramembrane water molecules was experimentally determined by selectively deuterium labeling lipids, anesthetic drug and water molecules in neutron diffraction experiments. Four additional water molecules per lipid were observed in the presence of ethanol. Diethyl ether and isoflurane were found to significantly increase the amount of intramembrane water by 25% (8 water molecules). This increase in intramembrane water may contribute to the non-specific interactions between anesthetics and lipid membranes.

Stabilization of Lipid Membranes through Partitioning of the Blood Bag Plasticizer Di-2-ethylhexyl phthalate (DEHP)

The safe storage of blood is of fundamental importance to health care systems all over the world. Currently, plastic bags are used for the collection and storage of donated blood and are typically made of poly(vinyl chloride) (PVC) plasticized with di-2-ethylhexyl phthalate (DEHP). DEHP is known to migrate into packed red blood cells (RBC) and has been found to extend their shelf life. It has been speculated that DEHP incorporates itself into the RBC membrane and alters membrane properties, thereby reducing susceptibility to hemolysis and morphological deterioration. Here, we used high-resolution X-ray diffraction and molecular dynamics (MD) simulations to study the interaction between DEHP and model POPC lipid membranes at high (9 mol %) and low (1 mol %) concentrations of DEHP. At both concentrations, DEHP was found to spontaneously partition into the bilayer. At high concentrations, DEHP molecules were found to aggregate in the aqueous phase before inserting as clusters into the membrane. The presence of DEHP in the bilayers resulted in subtle, yet statistically significant, alterations in several membrane properties in both the X-ray diffraction experiments and MD simulations. DEHP led to (1) an increase of membrane width and (2) an increase in the area per lipid. It was also found to (3) increase the deuterium order parameter, however, (4) decrease membrane orientation, indicating the formation of thicker, stiffer membranes with increased local curvature. The observed effects of DEHP on lipid bilayers may help to better understand its effect on RBC membranes in increasing the longevity of stored blood by improving membrane stability.

Membrane interactions of non-membrane targeting antibiotics: The case of aminoglycosides, macrolides, and fluoroquinolones

Numerous antibiotics are known to target intracellular pathways, such as protein translation or DNA replication. Membrane transporters typically regulate drug uptake; however, little is known about direct interactions between these antibiotics and the cell membranes. Here, we studied the interactions between different aminoglycosides (kanamycin, gentamicin, streptomycin, neomycin), macrolides (azithromycin, clarithromycin, erythromycin), and fluoroquinolones (ciprofloxacin, levofloxacin) with bacterial membrane mimics to determine drug partitioning and potential drug-induced membrane disruption. The antibiotics' exact location in the bilayers and their effect on membrane thickness and fluidity were determined from high-resolution X-ray diffraction. While the antibiotics did not change membrane thickness at low (1:100 drug/lipid) or high (1:10 drug/lipid) concentrations, they were found to increase membrane disorder in a dose-dependent manner. However, no membrane damage, such as membrane disruption or pore formation, was observed for any of the antibiotics. To note, all antibiotics partitioned into the lipid head groups, while macrolides and fluoroquinolones also partitioned into the bilayer core. The results suggest that the bacterial membrane is relatively inert in the direct mechanisms of actions of these antibiotics.

Hybrid Erythrocyte Liposomes: Functionalized Red Blood Cell Membranes for Molecule Encapsulation

The modification of erythrocyte membrane properties provides a new tool towards improved drug delivery and biomedical applications. The fabrication of hybrid erythrocyte liposomes is presented by doping red blood cell membranes with synthetic lipid molecules of different classes (PC, PS, PG) and different degrees of saturation (14:0, 16:0-18:1). The respective solubility limits are determined, and material properties of the hybrid liposomes are studied by a combination of X-ray diffraction, epi-fluorescent microscopy, dynamic light scattering (DLS), Zeta potential, UV-vis spectroscopy, and Molecular Dynamics (MD) simulations. Membrane thickness and lipid orientation can be tuned through the addition of phosphatidylcholine lipids. The hybrid membranes can be fluorescently labelled by incorporating Texas-red DHPE, and their charge modified by incorporating phosphatidylserine and phosphatidylglycerol. By using fluorescein labeled dextran as an example, it is demonstrated that small molecules can be encapsulated into these hybrid liposomes.

Application of small-angle neutron diffraction to the localization of general anesthetics in model membranes

We set out to explore the applicability of small-angle neutron diffraction (SAND) to the localization of biomembrane components by studying the general anesthetic n-decane in a model lipid bilayer system composed of dioleoyl-phosphocholine (DOPC). Samples in the form of planar membrane multilayers were hydrated by varied mixtures of deuterated and protonated water, and examined by the means of SAND. Neutron scattering length density (NSLD) profiles of the system were then reconstructed from the experimental data. We exploited the significantly different neutron scattering properties of hydrogen and deuterium atoms via labeling in addition to water contrast variation. Enhancing the signals from particular components of bilayer system led to a set of characteristic membrane profiles and from their comparison we localized n-decane molecules unequivocally in the bilayer's hydrocarbon chain region.

Membrane charge and lipid packing determine polymyxin-induced membrane damage

With the advent of polymyxin B (PmB) resistance in bacteria, the mechanisms for mcr-1 resistance are of crucial importance in the design of novel therapeutics. The mcr-1 phenotype is known to decrease membrane charge and increase membrane packing by modification of the bacterial outer membrane. We used X-ray diffraction, Molecular Dynamics simulations, electrochemistry, and leakage assays to determine the location of PmB in different membranes and assess membrane damage. By varying membrane charge and lipid tail packing independently, we show that increasing membrane surface charge promotes penetration of PmB and membrane damage, whereas increasing lipid packing decreases penetration and damage. The penetration of the PmB molecules is well described by a phenomenological model that relates an attractive electrostatic and a repulsive force opposing insertion due to increased membrane packing. The model applies well to several gram-negative bacterial strains and may be used to predict resistance strength.

Carbapenems and Lipid Bilayers: Localization, Partitioning, and Energetics

Carbapenems are broad-spectrum antibiotics used today to treat otherwise antibiotic resistant bacteria. As their target transpeptidase is located within the periplasm of the Gram-negative bacteria, they can participate in nonspecific interactions between the inner leaflet of the outer membrane and the outer leaflet of the inner membrane. We, therefore, studied the interaction of the four most clinically relevant carbapenems, namely, imipenem, doripenem, ertapenem, and meropenem, with model phospholipid bilayers made of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) using molecular dynamics (MD) simulations and X-ray diffraction at low and high concentration of the drugs corresponding to 1 and 8 mol % (with respect to the number of membrane lipids). Membrane solubility was found to decrease from imipenem to doripenem, ertapenem, and finally meropenem. At low concentrations, membrane insertion was found to be a two step process, where the drugs first adsorb to the lipid head groups before inserting through a rotation of the molecule. At higher drug concentrations, the molecules were found to form aggregates in the aqueous phase before making contact with the membranes and spontaneously inserting into the bilayers. Two populations of imipenem were found: in the headgroup at ∼17 Å from the bilayer center and an inserted population at z-values of about 7 Å. Other carbapenems were found to localize in the tail groups with meropenem at ∼10 Å, doripenem at ∼8 Å, and ertapenem at ∼8 Å. The observed membrane solubility of carbapenems can potentially impact the availability of the drug to the target penicillin-binding proteins, potentially affecting their clinical efficacy.

Membrane Cholesterol Reduces Polymyxin B Nephrotoxicity in Renal Membrane Analogs

Polymyxin B (PmB) is a "last-line" antibiotic scarcely used due to its nephrotoxicity. However, the molecular basis for antibiotic nephrotoxicity is not clearly understood. We prepared kidney membrane analogs of detergent-susceptible membranes, depleted of cholesterol, and cholesterol enriched, resistant membranes. In both analogs, PmB led to membrane damage. By combining x-ray diffraction, molecular dynamics simulations, and electrochemistry, we present evidence for two populations of PmB molecules: peptides that lie flat on the membranes, and an inserted state. In cholesterol depleted membranes, PmB forms clusters on the membranes leading to an indentation of the bilayers and increase in water permeation. The inserted peptides formed aggregates in the membrane core leading to further structural instabilities and increased water intake. The presence of cholesterol in the resistant membrane analogs led to a significant decrease in membrane damage. Although cholesterol did not inhibit peptide insertion, it minimized peptide clustering and water intake through stabilization of the bilayer structure and suppression of lipid and peptide mobility.

Alcohol Interactions with Lipid Bilayers

We investigate the structural changes to lipid membrane that ensue from the addition of aliphatic alcohols with various alkyl tail lengths. Small angle neutron diffraction from flat lipid bilayers that are hydrated through water vapor has been employed to eliminate possible artefacts of the membrane curvature and the alcohol's membrane-water partitioning. We have observed clear changes to membrane structure in both transversal and lateral directions. Most importantly, our results suggest the alteration of the membrane-water interface. The water encroachment has shifted in the way that alcohol loaded bilayers absorbed more water molecules when compared to the neat lipid bilayers. The experimental results have been corroborated by molecular dynamics simulations to reveal further details. Namely, the order parameter profiles have been fruitful in correlating the mechanical model of structural changes to the effect of anesthesia.

Calcium and Zinc Differentially Affect the Structure of Lipid Membranes

Interactions of calcium (Ca²⁺) and zinc (Zn²⁺) cations with biomimetic membranes made of dipalmitoylphosphatidylcholine (DPPC) were studied by small angle neutron diffraction (SAND). Experiments show that the structure of these lipid bilayers is differentially affected by the two divalent cations. Initially, both Ca²⁺ and Zn²⁺ cause DPPC bilayers to thicken, while further increases in Ca²⁺ concentration result in the bilayer thinning, eventually reverting to having the same thickness as pure DPPC. The binding of Zn²⁺, on the other hand, causes the bilayers to swell to a maximum thickness, and the addition of more Zn²⁺ does not result in a further thickening of the membrane. Agreement between our results obtained using oriented planar membranes and those from vesicular samples implies that the effect of cations on bilayer thickness is the result of electrostatic interactions, rather than geometrical constraints due to bilayer curvature. This notion is further reinforced by MD simulations. Finally, the radial distribution functions reveal a strong interaction between Ca²⁺ and the phosphate oxygens, while Zn²⁺ shows a much weaker binding specificity.

The Molecular Structure of Human Red Blood Cell Membranes from Highly Oriented, Solid Supported Multi-Lamellar Membranes

We prepared highly oriented, multi-lamellar stacks of human red blood cell (RBC) membranes applied on silicon wafers. RBC ghosts were prepared by hemolysis and applied onto functionalized silicon chips and annealed into multi-lamellar RBC membranes. High resolution X-ray diffraction was used to determine the molecular structure of the stacked membranes. We present direct experimental evidence that these RBC membranes consist of nanometer sized domains of integral coiled-coil peptides, as well as liquid ordered (l_o) and liquid disordered (l_d) lipids. Lamellar spacings, membrane and hydration water layer thicknesses, areas per lipid tail and domain sizes were determined. The common drug aspirin was added to the RBC membranes and found to interact with RBC membranes and preferably partition in the head group region of the l_o domain leading to a fluidification of the membranes, i.e., a thinning of the bilayers and an increase in lipid tail spacing. Our results further support current models of RBC membranes as patchy structures and provide unprecedented structural details of the molecular organization in the different domains.

Reduction in Tension and Stiffening of Lipid Membranes in an Electric Field Revealed by X-Ray Scattering

The effect of ac electric fields on the elasticity of supported lipid bilayers is investigated at the microscopic level using grazing incidence synchrotron x-ray scattering. A strong decrease in the membrane tension up to 1  mN/m and a dramatic increase of its effective rigidity up to 300  k_{B}T are observed for local electric potentials seen by the membrane ≲1  V. The experimental results are analyzed using detailed electrokinetic modeling and nonlinear Poisson-Boltzmann theory. Based on a modeling of the electromagnetic stress, which provides an accurate description of the bilayer separation versus pressure curves, we show that the decrease in tension results from the amplification of charge fluctuations on the membrane surface whereas the increase in bending rigidity results from the direct interaction between charges in the electric double layer. These effects eventually lead to a destabilization of the bilayer and vesicle formation. Similar effects are expected at the tens of nanometers length scale in cell membranes with lower tension, and could explain a number of electrically driven processes.

The Lipid Bilayer Provides a Site for Cortisone Crystallization at High Cortisone Concentrations

Cortisone is an injected anti-inflammatory drug that can cause painful side effects known as "steroid flares" which are caused by cortisone crystallizing at the injection site. We used molecular dynamics simulations and X-ray diffraction to study the interaction of cortisone with model lipid membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at drug concentrations from 0 mol% to 50 mol%. Cortisone was found to partition in the lipid bilayer and locate in the hydrophilic to hydrophobic interface of the membranes. Cortisone strongly affects the integrity of the membrane, as quantified by a decreased membrane thickness, increased area per lipid, and decreased lipid tail order parameters. At cortisone concentrations of more than 20 mol%, signals from crystallized cortisone were observed. These crystallites are embedded in the bilayers and orient with the membranes. While the cortisone molecules align parallel to the bilayers at low concentrations, they start to penetrate the hydrophobic core at higher concentrations. Trans-membrane crystallites start to nucleate when the membrane thickness has decreased such that cortisone molecules in the different leaflets can find partners from the opposite leaflet resulting in a non-zero density of cortisone molecules in the bilayer center. We suggest that the lipid bilayer provides a site for cortisone crystallization.

Anomalous and anisotropic nanoscale diffusion of hydration water molecules in fluid lipid membranes

We have studied nanoscale diffusion of membrane hydration water in fluid-phase lipid bilayers made of 1,2-dimyristoyl-3-phosphocholine (DMPC) using incoherent quasi-elastic neutron scattering. Dynamics were fit directly in the energy domain using the Fourier transform of a stretched exponential. By using large, 2-dimensional detectors, lateral motions of water molecules and motions perpendicular to the membranes could be studied simultaneously, resulting in 2-dimensional maps of relaxation time, τ, and stretching exponent, β. We present experimental evidence for anomalous (sub-diffusive) and anisotropic diffusion of membrane hydration water molecules over nanometer distances. By combining molecular dynamics and Brownian dynamics simulations, the potential microscopic origins for the anomaly and anisotropy of hydration water were investigated. Bulk water was found to show intrinsic sub-diffusive motion at time scales of several picoseconds, likely related to caging effects. In membrane hydration water, however, the anisotropy of confinement and local dynamical environments leads to an anisotropy of relaxation times and stretched exponents, indicative of anomalous dynamics.

The organization of melatonin in lipid membranes

Melatonin is a hormone that has been shown to have protective effects in several diseases that are associated with cholesterol dysregulation, including cardiovascular disease, Alzheimer's disease, and certain types of cancers. We studied the interaction of melatonin with model membranes made of dimyristoylphosphatidylcholine (DMPC) at melatonin concentrations ranging from 0.5mol% to 30mol%. From 2-dimensional X-ray diffraction measurements, we find that melatonin induces a re-ordering of the lipid membrane that is strongly dependent on the melatonin concentration. At low melatonin concentrations, we observe the presence of melatonin-enriched patches in the membrane, which are significantly thinner than the lipid bilayer. The melatonin molecules were found to align parallel to the lipid tails in these patches. At high melatonin concentrations of 30mol%, we observe a highly ordered melatonin structure that is uniform throughout the membrane, where the melatonin molecules align parallel to the bilayers and one melatonin molecule associates with 2 lipid molecules. Understanding the organization and interactions of melatonin in membranes, and how these are dependent on the concentration, may shed light into its anti-amyloidogenic, antioxidative and photoprotective properties and help develop a structural basis for these properties.

Acetylsalicylic acid (ASA) increases the solubility of cholesterol when incorporated in lipid membranes

Cholesterol has been well established as a mediator of cell membrane fluidity. By interacting with lipid tails, cholesterol causes the membrane tails to be constrained thereby reducing membrane fluidity, well known as the condensation effect. Acetylsalicylic acid (ASA), the main ingredient in aspirin, has recently been shown to increase fluidity in lipid bilayers by primarily interacting with lipid head groups. We used high-resolution X-ray diffraction to study both ASA and cholesterol coexisting in model membranes of dimyristoylphosphatidylcholine (DMPC). While a high cholesterol concentration of 40 mol% cholesterol leads to the formation of immiscible cholesterol bilayers, as was reported previously, increasing the amount of ASA in the membranes between 0 to 12.5 mol% was found to significantly increase the fluidity of the bilayers and dissolve the cholesterol plaques. We, therefore, present experimental evidence for an interaction between cholesterol and ASA on the level of the cell membrane at elevated levels of cholesterol and ASA.

The interaction between amyloid-β peptides and anionic lipid membranes containing cholesterol and melatonin

One of the hallmarks of Alzheimer's disease is the formation of senile plaques, primarily consisting of amyloid- (A) peptides. Peptide-membrane and peptide-lipid interactions are thought to be crucial in this process. We studied the interaction of A and A peptides with anionic lipid membranes made of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphoserine (DMPS) using X-ray diffraction. We compare the experimentally determined electron densities in the gel state of the membranes with density calculations from peptide structures reported in the Protein Data Bank in order to determine the position of the peptide in the bilayers. The full length peptide A was found to embed in the hydrocarbon core of the anionic lipid bilayers. Two populations were found for the A peptide: (1) membrane-bound states in the hydrophilic head group region of the bilayers, where the peptides align parallel to the membranes, and (2) an embedded state in the bilayer center. Aging plays an important role in the development of Alzheimer's, in particular with respect to changes in cholesterol and melatonin levels in the brain tissue. Immiscible cholesterol plaques were created by addition of 30 mol% cholesterol to the anionic membranes. The A peptides were found to strongly interact with the lipid bilayers, displacing further cholesterol molecules into the plaques, effectively lowering the cholesterol concentration in the membranes and increasing the total fraction of cholesterol plaques. Addition of 30 mol% melatonin molecules to the anionic membranes drastically reduced the population of the membrane-embedded A state. These results present experimental evidence for an interaction between A peptides, melatonin and cholesterol in lipid membranes.

Membrane structure correlates to function of LLP2 on the cytoplasmic tail of HIV-1 gp41 protein

Mutation studies previously showed that the lentivirus lytic peptide (LLP2) sequence of the cytoplasmic C-terminal tail of the HIV-1 gp41 envelope protein inhibited viral-initiated T-cell death and T-cell syncytium formation, at which time in the HIV life cycle the gp41 protein is embedded in the T-cell membrane. In striking contrast, the mutants did not affect virion infectivity, during which time the gp41 protein is embedded in the HIV envelope membrane. To examine the role of LLP2/membrane interactions, we applied synchrotron x-radiation to determine structure of hydrated membranes. We focused on WT LLP2 peptide (+3 charge) and MX2 mutant (-1 charge) with membrane mimics for the T-cell and the HIV-1 membranes. To investigate the influence of electrostatics, cholesterol content, and peptide palmitoylation, we also studied three other LLP2 variants and HIV-1 mimics without negatively charged lipids or cholesterol as well as extracted HIV-1 lipids. All LLP2 peptides bound strongly to T-cell membrane mimics, as indicated by changes in membrane structure and bending. In contrast, none of the weakly bound LLP2 variants changed the HIV-1 membrane mimic structure or properties. This correlates well with, and provides a biophysical basis for, previously published results that reported lack of a mutant effect in HIV virion infectivity in contrast to an inhibitory effect in T-cell syncytium formation. It shows that interaction of LLP2 with the T-cell membrane modulates biological function.

X-ray structure, thermodynamics, elastic properties and MD simulations of cardiolipin/dimyristoylphosphatidylcholine mixed membranes

Cardiolipins (CLs) are important biologically for their unique role in biomembranes that couple phosphorylation and electron transport like bacterial plasma membranes, chromatophores, chloroplasts and mitochondria. CLs are often tightly coupled to proteins involved in oxidative phosphorylation. The first step in understanding the interaction of CL with proteins is to obtain the pure CL structure, and the structure of mixtures of CL with other lipids. In this work we use a variety of techniques to characterize the fluid phase structure, material properties and thermodynamics of mixtures of dimyristoylphosphatidylcholine (DMPC) with tetramyristoylcardiolipin (TMCL), both with 14-carbon chains, at several mole percentages. X-ray diffuse scattering was used to determine structure, including bilayer thickness and area/lipid, the bending modulus, KC, and SXray, a measure of chain orientational order. Our results reveal that TMCL thickens DMPC bilayers at all mole percentages, with a total increase of ∼6 Å in pure TMCL, and increases AL from 64 Å(2) (DMPC at 35 °C) to 109 Å(2) (TMCL at 50 °C). KC increases by ∼50%, indicating that TMCL stiffens DMPC membranes. TMCL also orders DMPC chains by a factor of ∼2 for pure TMCL. Coarse grain molecular dynamics simulations confirm the experimental thickening of 2 Å for 20mol% TMCL and locate the TMCL headgroups near the glycerol-carbonyl region of DMPC; i.e., they are sequestered below the DMPC phosphocholine headgroup. Our results suggest that TMCL plays a role similar to cholesterol in that it thickens and stiffens DMPC membranes, orders chains, and is positioned under the umbrella of the PC headgroup. CL may be necessary for hydrophobic matching to inner mitochondrial membrane proteins. Differential scanning calorimetry, SXray and CGMD simulations all suggest that TMCL does not form domains within the DMPC bilayers. We also determined the gel phase structure of TMCL, which surprisingly displays diffuse X-ray scattering, like a fluid phase lipid. AL=40.8 Å(2) for the ½TMCL gel phase, smaller than the DMPC gel phase with AL=47.2 Å(2), but similar to AL of DLPE=41 Å(2), consistent with untilted chains in gel phase TMCL.

Comparing simulations of lipid bilayers to scattering data: the GROMOS 43A1-S3 force field

Simulations of DOPC at T = 303 K were performed using the united atom force field 43A1-S3 at six fixed projected areas, A(P) = 62, 64, 66, 68, 70, and 72 Å(2), as well as a tensionless simulation that produced an average A(NPT) = 65.8 Å(2). After a small undulation correction for the system size consisting of 288 lipids, results were compared to experimental data. The best, and excellent, fit to neutron scattering data occurs at an interpolated A(N) = 66.6 Å(2) and the best, but not as good, fit to the more extensive X-ray scattering data occurs at A(X) = 68.7 Å(2). The distance ΔDB-H between the Gibbs dividing surface for water and the peak in the electron density profile agrees with scattering experiments. The calculated area compressibility K(A) = 277 ± 10 mN/m is in excellent agreement with the micromechanical experiment. The volume per lipid V(L) is smaller than volume experiments which suggests a workaround that raises all the areas by about 1.5%. Although A(X) ≠ A(N) ≠ A(NPT), this force field obtains acceptable agreement with experiment for A(L) = 67.5 Å(2) (68.5 Å(2) in the workaround), which we suggest is a better DOPC result from 43A1-S3 simulations than its value from the tensionless NPT simulation. However, nonsimulation modeling obtains better simultaneous fits to both kinds of scattering data, which suggests that the force fields can still be improved.

Gaussian deconvolution: a useful method for a form-free modeling of scattering data from mono- and multilayered planar systems

A new method for analysis of scattering data from lamellar bilayer systems is presented. The method employs a form-free description of the cross-section structure of the bilayer and the fit is performed directly to the scattering data, introducing also a structure factor when required. The cross-section structure (electron density profile in the case of X-ray scattering) is described by a set of Gaussian functions and the technique is termed Gaussian deconvolution. The coefficients of the Gaussians are optimized using a constrained least-squares routine that induces smoothness of the electron density profile. The optimization is coupled with the point-of-inflection method for determining the optimal weight of the smoothness. With the new approach, it is possible to optimize simultaneously the form factor, structure factor and several other parameters in the model. The applicability of this method is demonstrated by using it in a study of a multilamellar system composed of lecithin bilayers, where the form factor and structure factor are obtained simultaneously, and the obtained results provided new insight into this very well known system.

Interaction of aspirin (acetylsalicylic acid) with lipid membranes

We studied the interaction of Aspirin (acetylsalicylic acid) with lipid membranes using x-ray diffraction for bilayers containing up to 50 mol% of aspirin. From 2D x-ray intensity maps that cover large areas of reciprocal space we determined the position of the ASA molecules in the phospholipid bilayers and the molecular arrangement of the molecules in the plane of the membranes. We present direct experimental evidence that ASA molecules participate in saturated lipid bilayers of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and preferably reside in the head group region of the membrane. Up to 50 mol% ASA molecules can be dissolved in this type of bilayer before the lateral membrane organization is disturbed and the membranes are found to form an ordered, 2D crystal-like structure. Furthermore, ASA and cholesterol were found to co-exist in saturated lipid bilayers, with the ASA molecules residing in the head group region and the cholesterol molecules participating in the hydrophobic membrane core.

Structure and elasticity of lipid membranes with genistein and daidzein bioflavinoids using X-ray scattering and MD simulations

This work reports the effects of the bioflavinoids genistein and daidzein on lipid bilayers as determined by volume measurements, X-ray scattering, and molecular dynamics simulations. The experimental and simulated total molecular volumes were found to be in outstanding agreement with each other before the addition of genistein and daidzein and also after their addition. Both bioflavinoids inserted into the hydrocarbon region of both DOPC and diphytanoylPC near the carbonyls of the lipids and both decreased the bilayer thicknesses. The long axes of both bioflavinoids were oriented nearly parallel to the plane of the bilayer with their carbonyl groups preferentially pointed toward the proximal surface. A difference is that daidzein had a solubility limit of ∼0.14 mol fraction in DOPC (∼0.12 mol fraction in diphytanoylPC), whereas genistein was soluble at least to 0.20 mol fraction in both lipid membranes. Measurements of bending modulus K(C) and simulation results for area compressibility modulus K(A) indicate that both bioflavinoids soften bilayers.

HIV fusion peptide penetrates, disorders, and softens T-cell membrane mimics

This work investigates the interaction of N-terminal gp41 fusion peptide (FP) of human immunodeficiency virus type 1 (HIV-1) with model membranes in order to elucidate how FP leads to fusion of HIV and T-cell membranes. FP constructs were (i) wild-type FP23 (23 N-terminal amino acids of gp41), (ii) water-soluble monomeric FP that adds six lysines on the C-terminus of FP23 (FPwsm), and (iii) the C-terminus covalently linked trimeric version (FPtri) of FPwsm. Model membranes were (i) LM3 (a T-cell mimic), (ii) 1,2-dioleoyl-sn-glycero-3-phosphocholine, (iii) 1,2-dioleoyl-sn-glycero-3-phosphocholine/30 mol% cholesterol, (iv) 1,2-dierucoyl-sn-glycero-3-phosphocholine, and (v) 1,2-dierucoyl-sn-glycero-3-phosphocholine/30 mol% cholesterol. Diffuse synchrotron low-angle x-ray scattering from fully hydrated samples, supplemented by volumetric data, showed that FP23 and FPtri penetrate into the hydrocarbon region and cause membranes to thin. Depth of penetration appears to depend upon a complex combination of factors including bilayer thickness, presence of cholesterol, and electrostatics. X-ray data showed an increase in curvature in hexagonal phase 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, which further indicates that FP23 penetrates into the hydrocarbon region rather than residing in the interfacial headgroup region. Low-angle x-ray scattering data also yielded the bending modulus K(C), a measure of membrane stiffness, and wide-angle x-ray scattering yielded the S(xray) orientational order parameter. Both FP23 and FPtri decreased K(C) and S(xray) considerably, while the weak effect of FPwsm suggests that it did not partition strongly into LM3 model membranes. Our results are consistent with the HIV FP disordering and softening the T-cell membrane, thereby lowering the activation energy for viral membrane fusion.

Water replacement hypothesis in atomic details: effect of trehalose on the structure of single dehydrated POPC bilayers

We present molecular dynamics (MD) simulations to study the plausibility of the water replacement hypothesis (WRH) from the viewpoint of structural chemistry. A total of 256 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC) lipids were modeled for 400 ns at 11.7 or 5.4 waters/lipid. To obtain a single dehydrated bilayer relevant to the WRH, simulations were performed in the NP(xy)h(z)T ensemble with h(z) > 8 nm, allowing interactions between lipids in the membrane plane and preventing interactions between neighboring membranes via periodic boundary conditions. This setup resulted in a stable single bilayer in (or near) the gel state. Trehalose caused a concentration-dependent increase of the area per lipid (APL) accompanied by fluidizing the bilayer core. This mechanism has been suggested by the WRH. However, dehydrated bilayers in the presence of trehalose were not structurally identical to fully hydrated bilayers. The headgroup vector was in a more parallel orientation in dehydrated bilayers with respect to the bilayer plane and maintained this orientation in the presence of trehalose in spite of APL increase. The total dipole potential changed sign in dehydrated bilayers and remained slightly positive in the presence of trehalose. The model of a dehydrated bilayer presented here allows the study of the mechanisms of membrane protection against desiccation by different compounds.

Structural properties of phosphatidylcholine in a monolayer at the air/water interface: Neutron reflection study and reexamination of x-ray reflection measurements

Neutron reflectivities of phosphatidylcholine monolayers in the liquid condensed (LC) phase on ultrapure H(2)O and D(2)O subphases have been measured on a Langmuir film balance. Using a dedicated liquid surface reflectometer, reflectivities down to R = 10(-6) in the momentum transfer range Q(z) = 0-0.4 A(-1) were accessed.In a new approach, by refining neutron reflectivity data from chain-perdeuterated DPPC-d(62) in combination with x-ray measurements on the same monolayer under similar conditions it is shown that the two techniques mutually complement one another. This analysis leads to a detailed conception of the interface structure. It is found that in the LC phase (which is analogous to the L(beta), phase in vesicle dispersions) the head group is interpenetrated with subphase water (4 +/- 2.5 molecules per lipid) and the average tilt angle of the hydrophobic chains from the surface normal is 33 +/- 3 degrees.

Structure and water permeability of fully hydrated diphytanoylPC

Diphytanoylphosphatidylcholine (DPhyPC) is a branched chain lipid often used for model membrane studies, including peptide/lipid interactions, ion channels and lipid rafts. This work reports results of volume measurements, water permeability measurements P(f), X-ray scattering from oriented samples, and X-ray and neutron scattering from unilamellar vesicles at T=30 degrees C. We measured the volume/lipid V(L)=1426+/-1A(3). The area/lipid was found to be 80.5+/-1.5A(2) when both X-ray and neutron data were combined with the SDP model analysis (Kucerka, N., Nagle, J.F., Sachs, J.N., Feller, S.E., Pencer, J., Jackson, A., Katsaras, J., 2008. Lipid bilayer structure determined by the simultaneous analysis of neutron and X-ray scattering data. Biophys. J. 95, 2356-2367); this is substantially larger than the area of DOPC which has the largest area of the common linear chain lipids. P(f) was measured to be (7.0+/-1.0)x10(-3)cm/s; this is considerably smaller than predicted by the recently proposed 3-slab model (Nagle, J.F., Mathai, J.C., Zeidel, M.L., Tristram-Nagle, S., 2008. Theory of passive permeability through lipid bilayers. J. Gen. Physiol. 131, 77-85). This disagreement can be understood if there is a diminished diffusion coefficient in the hydrocarbon core of DPhyPC and that is supported by previous molecular dynamics simulations (Shinoda, W., Mikami, M., Baba, T., Hato, M., 2004. Molecular dynamics study on the effects of chain branching on the physical properties of lipid bilayers. 2. Permeability. J. Phys. Chem. B 108, 9346-9356). While the DPhyPC head-head thickness (D(HH)=36.4A), and Hamaker parameter (H=4.5x10(-21)J) were similar to the linear chain lipid DOPC, the bending modulus (K(C)=5.2+/-0.5x10(-21)J) was 30% smaller. Our results suggest that, from the biophysical perspective, DPhyPC belongs to a different family of lipids than phosphatidylcholines that have linear chain hydrocarbon chains.

Supported bilayers: combined specular and diffuse X-ray scattering

A method is proposed for the analysis of specular and off-specular reflectivity from supported lipid bilayers. Both thermal fluctuations and the "static" roughness induced by the substrate are carefully taken into account. Examples from supported bilayers and more complex systems comprising a bilayer adsorbed or grafted on the substrate and another "floating" bilayer are given. The combined analysis of specular and off-specular reflectivity allows the precise determination of the structure of adsorbed and floating bilayers, their tension, bending rigidity and interaction potentials. We show that this new method gives a unique opportunity to investigate phenomena like protrusion modes of adsorbed bilayers and opens the way to the investigation of more complex systems including different kinds of lipids, cholesterol or peptides.

Areas of monounsaturated diacylphosphatidylcholines

We have studied the structural properties of monounsaturated diacylphosphatidylcholine lipid bilayers (i.e., diCn:1PC, where n = 14, 16, 18, 20, 22, and 24 is the number of acyl chain carbons). High-resolution x-ray scattering data were analyzed in conjunction with contrast-varied neutron scattering data using a technique we recently developed. Analyses of the data show that the manner by which bilayer thickness increases with increasing n in monounsaturated diacylphosphatidylcholines is dependent on the double bond's position. For commonly available monounsaturated diacylphosphatidylcholines, this results in the nonlinear behavior of both bilayer thickness and lipid area, whereas for diC18:1PC bilayers, lipid area assumes a maximum value. It is worthwhile to note that compared to previous data, our results indicate that lipid areas are smaller by approximately 10%. This observation highlights the need to revisit lipid areas, as they are often used in comparisons with molecular dynamics simulations. Moreover, simulators are encouraged to compare their results not only to x-ray scattering data, but to neutron data as well.

Effects of ether vs. ester linkage on lipid bilayer structure and water permeability

The structure and water permeability of bilayers composed of the ether-linked lipid, dihexadecylphosphatidylcholine (DHPC), were studied and compared with the ester-linked lipid, dipalmitoylphosphaditdylcholine (DPPC). Wide angle X-ray scattering on oriented bilayers in the fluid phase indicate that the area per lipid A is slightly larger for DHPC than for DPPC. Low angle X-ray scattering yields A=65.1A(2) for DHPC at 48 degrees C. LAXS data provide the bending modulus, K(C)=4.2x10(-13)erg, and the Hamaker parameter H=7.2x10(-14)erg for the van der Waals attractive interaction between neighboring bilayers. For the low temperature phases with ordered hydrocarbon chains, we confirm the transition from a tilted L(beta') gel phase to an untilted, interdigitated L(beta)I phase as the sample hydrates at 20 degrees C. Our measurement of water permeability, P(f)=0.022cm/s at 48 degrees C for fluid phase DHPC is slightly smaller than that of DPPC (P(f)=0.027cm/s) at 50 degrees C, consistent with our triple slab theory of permeability.

Lipid bilayer structure determined by the simultaneous analysis of neutron and X-ray scattering data

Quantitative structures were obtained for the fully hydrated fluid phases of dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) bilayers by simultaneously analyzing x-ray and neutron scattering data. The neutron data for DOPC included two solvent contrasts, 50% and 100% D(2)O. For DPPC, additional contrast data were obtained with deuterated analogs DPPC_d62, DPPC_d13, and DPPC_d9. For the analysis, we developed a model that is based on volume probability distributions and their spatial conservation. The model's design was guided and tested by a DOPC molecular dynamics simulation. The model consistently captures the salient features found in both electron and neutron scattering density profiles. A key result of the analysis is the molecular surface area, A. For DPPC at 50 degrees C A = 63.0 A(2), whereas for DOPC at 30 degrees C A = 67.4 A(2), with estimated uncertainties of 1 A(2). Although A for DPPC agrees with a recently reported value obtained solely from the analysis of x-ray scattering data, A for DOPC is almost 10% smaller. This improved method for determining lipid areas helps to reconcile long-standing differences in the values of lipid areas obtained from stand-alone x-ray and neutron scattering experiments and poses new challenges for molecular dynamics simulations.

CRAC motif peptide of the HIV-1 gp41 protein thins SOPC membranes and interacts with cholesterol

This study uses low-angle (LAXS) and wide-angle (WAXS) X-ray synchrotron scattering, volume measurements and thin layer chromatography to determine the structure and interactions of SOPC, SOPC/cholesterol mixtures, SOPC/peptide and SOPC/cholesterol/peptide mixtures. N-acetyl-LWYIK-amide (LWYIK) represents the naturally-occurring CRAC motif segment in the pretransmembrane region of the gp41 protein of HIV-1, and N-acetyl-IWYIK-amide (IWYIK), an unnatural isomer, is used as a control. Both peptides thin the SOPC bilayer by approximately 3 A, and cause the area/unit cell (peptide+SOPC) to increase by approximately 9 A2 from the area/lipid of SOPC at 30 degrees C (67.0+/-0.9 A2). Model fitting suggests that LWYIK's average position is slightly closer to the bilayer center than IWYIK's, and both peptides are just inside of the phosphate headgroup. Both peptides increase the wide-angle spacing d of SOPC without cholesterol, whereas with 50% cholesterol LWYIK increases d but IWYIK decreases d. TLC shows that LWYIK is more hydrophobic than IWYIK; this difference persists in peptide/SOPC 1:9 mole ratio mixtures. Both peptides counteract the chain ordering effect of cholesterol to roughly the same degree, and both decrease KC, the bending modulus, thus increasing the SOPC membrane fluidity. Both peptides nucleate crystals of cholesterol, but the LWYIK-induced crystals are weaker and dissolve more easily.

Full q-space analysis of x-ray scattering of multilamellar membranes at liquid-solid interfaces

A method for analyzing the x-ray scattering of a stack of phospholipid membranes at the solid-liquid interface in excess water is presented. It is argued that bilayers near the substrate fluctuate less significantly than the ones away from it and, therefore, make a larger contribution to the specular reflection at the low index Bragg peaks. But the diffuse scattering due to thermal fluctuations corrupts the Bragg peaks at high angles so that the specular contribution is not clearly observable. In the midst the specular reflection and the diffuse scattering cannot be separated easily and must be analyzed simultaneously. The height-difference correlation function derived from a modified Caillé theory enables one to simulate the longitudinal and the transverse scans in the same theoretical framework to yield more reliable structural parameters. The theoretical apparatus is successfully applied to the experimental data on DOPC membranes.

Swelling of phospholipids by monovalent salt

Critical to biological processes such as membrane fusion and secretion, ion-lipid interactions at the membrane-water interface still raise many unanswered questions. Using reconstituted phosphatidylcholine membranes, we confirm here that multilamellar vesicles swell in salt solutions, a direct indication that salt modifies the interactions between neighboring membranes. By varying sample histories, and by comparing with data from ion carrier-containing bilayers, we eliminate the possibility that swelling is an equilibration artifact. Although both attractive and repulsive forces could be modified by salt, we show experimentally that swelling is driven primarily by weakening of the van der Waals attraction. To isolate the effect of salt on van der Waals interactions, we focus on high salt concentrations at which any possible electrostatic interactions are screened. By analysis of X-ray diffraction data, we show that salt does not alter membrane structure or bending rigidity, eliminating the possibility that repulsive fluctuation forces change with salt. By measuring changes in interbilayer separation with applied osmotic stress, we have determined, using the standard paradigm for bilayer interactions, that 1 M concentrations of KBr or KCl decrease the van der Waals strength by 50%. By weakening van der Waals attractions, salt increases energy barriers to membrane contact, possibly affecting cellular communication and biological signaling.