Salt screening and specific ion adsorption determine neutral-lipid membrane interactions

Variable Non-Gaussian Transport of Nanoplastic on Supported Lipid Bilayers in Saline Conditions

Nanoplastic-lipid interaction is vital to understanding the nanoscale mechanism of plastic adsorption and aggregation on a lipid membrane surface. However, a single-particle mechanistic picture of the nanoplastic transport process on a lipid surface remains unclear. Here, we report a salt-dependent non-Gaussian transport mechanism of polystyrene particles on a supported 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) lipid bilayer surface. Particle stickiness on the POPC surface increases with salt concentration, where the particles stay longer at the surface and diffuse to shorter distances. Additionally, a non-Gaussian diffusion state dominates the transport process at high salt concentrations. Our current study provides insight into the transport mechanism of polystyrene (PS) particles on supported lipid membranes, which is essential to understanding fundamental questions regarding the adsorption mechanisms of nanoplastics on lipid surfaces.

Water Structuring Induces Nonuniversal Hydration Repulsion between Polar Surfaces: Quantitative Comparison between Molecular Simulations, Theory, and Experiments

Polar surfaces in water typically repel each other at close separations, even if they are charge-neutral. This so-called hydration repulsion balances the van der Waals attraction and gives rise to a stable nanometric water layer between the polar surfaces. The resulting hydration water layer is crucial for the properties of concentrated suspensions of lipid membranes and hydrophilic particles in biology and technology, but its origin is unclear. It has been suggested that surface-induced molecular water structuring is responsible for the hydration repulsion, but a quantitative proof of this water-structuring hypothesis is missing. To gain an understanding of the mechanism causing hydration repulsion, we perform molecular simulations of different planar polar surfaces in water. Our simulated hydration forces between phospholipid bilayers agree perfectly with experiments, validating the simulation model and methods. For the comparison with theory, it is important to split the simulated total surface interaction force into a direct contribution from surface-surface molecular interactions and an indirect water-mediated contribution. We find the indirect hydration force and the structural water-ordering profiles from the simulations to be in perfect agreement with the predictions from theoretical models that account for the surface-induced water ordering, which strongly supports the water-structuring hypothesis for the hydration force. However, the comparison between the simulations for polar surfaces with different headgroup architectures reveals significantly different decay lengths of the indirect water-mediated hydration-force, which for laterally homogeneous water structuring would imply different bulk-water properties. We conclude that laterally inhomogeneous water ordering, induced by laterally inhomogeneous surface structures, shapes the hydration repulsion between polar surfaces in a decisive manner. Thus, the indirect water-mediated part of the hydration repulsion is caused by surface-induced water structuring but is surface-specific and thus nonuniversal.

Electrostatic Potentials Caused by the Release of Protons from Photoactivated Compound Sodium 2-Methoxy-5-nitrophenyl Sulfate at the Surface of Bilayer Lipid Membrane

Lateral transport and release of protons at the water-membrane interface play crucial roles in cell bioenergetics. Therefore, versatile techniques need to be developed for investigating as well as clarifying the main features of these processes at the molecular level. Here, we experimentally measured the kinetics of binding of protons released from the photoactivated compound sodium 2-methoxy-5-nitrophenyl sulfate (MNPS) at the surface of a bilayer lipid membrane (BLM). We developed a theoretical model of this process describing the damage of MNPS coupled with the release of the protons at the membrane surface, as well as the exchange of MNPS molecules and protons between the membrane and solution. We found that the total change in the boundary potential difference across the membrane, ∆ϕb, is the sum of opposing effects of adsorption of MNPS anions and release of protons at the membrane-water interface. Steady-state change in the ∆ϕb due to protons decreased with the concentration of the buffer and increased with the pH of the solution. The change in the concentration of protons evaluated from measurements of ∆ϕb was close to that in the unstirred water layer near the BLM. This result, as well as rate constants of the proton exchange between the membrane and the bulk solution, indicated that the rate-limiting step of the proton surface to bulk release is the change in the concentration of protons in the unstirred layer. This means that the protons released from MNPS remain in equilibrium between the BLM surface and an adjacent water layer.

Passive and reversible area regulation of supported lipid bilayers in response to fluid flow

Biological and model membranes are frequently subjected to fluid shear stress. However, membrane mechanical responses to flow remain incompletely described. This is particularly true of membranes supported on a solid substrate, and the influences of membrane composition and substrate roughness on membrane flow responses remain poorly understood. Here, we combine microfluidics, fluorescence microscopy, and neutron reflectivity to explore how supported lipid bilayer patches respond to controlled shear stress. We demonstrate that lipid membranes undergo a significant, passive, and partially reversible increase in membrane area due to flow. We show that these fluctuations in membrane area can be constrained, but not prevented, by increasing substrate roughness. Similar flow-induced changes to membrane structure may contribute to the ability of living cells to sense and respond to flow.

Hofmeister Series: From Aqueous Solution of Biomolecules to Single Cells and Nanovesicles

Hofmeister effects play a critical role in numerous physicochemical and biological phenomena, including the solubility and/or accumulation of proteins, the activities of enzymes, ion transport in biochannels, the structure of lipid bilayers, and the dynamics of vesicle opening and exocytosis. This minireview focuses on how ionic specificity affects the physicochemical properties of biomolecules to regulate cellular exocytosis, vesicular content, and nanovesicle opening. We summarize recent progress in further understanding Hofmeister effects on biomacromolecules and their applications in biological systems. These important steps have increased our understanding of the Hofmeister effects on cellular exocytosis, vesicular content, and nanovesicle opening. Increasing evidence is firmly establishing that the ions along the Hofmeister series play an important role in living organisms that has often been ignored.

Location of dopamine in lipid bilayers and its relevance to neuromodulator function

Dopamine (DA) is a neurotransmitter that also acts as a neuromodulator, with both functions being essential to brain function. Here, we present the first experimental measurement of DA location in lipid bilayers using x-ray diffuse scattering, solid-state deuterium NMR, and electron paramagnetic resonance. We find that the association of DA with lipid headgroups as seen in electron density profiles leads to an increase of intermembrane repulsion most likely due to electrostatic charging. DA location in the lipid headgroup region also leads to an increase of the cross-sectional area per lipid without affecting the bending rigidity significantly. The order parameters measured by solid-state deuterium NMR decrease in the presence of DA for the acyl chains of PC and PS lipids, consistent with an increase in the area per lipid due to DA. Most importantly, these results support the hypothesis that three-dimensional diffusion of DA to target membranes could be followed by relatively more efficient two-dimensional diffusion to receptors within those membranes.

Non-Invasive, Reliable, and Fast Quantification of DNA Loading on Gold Nanoparticles by a One-Step Optical Measurement

An exquisite, versatile, and reproducible quantification of DNA loading on gold nanoparticles (Au NPs) has long been pursued because this loading influences the analytical, therapeutic, and self-assembly behaviors of DNA-Au NPs. Nevertheless, the existing methods used thus far rely solely on the invasive detachment and subsequent spectroscopic quantification of DNA, which are error-prone and highly dependent on trained personnel. Here, we present a non-invasive optical framework that can determine the number of DNA strands on Au NPs by versatile one-step measurement of the visible absorption spectra of DNA-Au NP solutions without any invasive modifications or downstream processes. Using effective medium theory in conjunction with electromagnetic numerical calculation, the change in DNA loading density, resulting from varying the ion concentration, Au NP size, DNA strand length, and surrounding temperature, can be tracked in situ merely by the one-step measurement of visible absorption spectra, which is otherwise impossible to achieve. Moreover, the simplicity and robustness of this method promote reproducible DNA loading quantification regardless of experimental adeptness, which is in stark contrast with existing invasive and multistep methods. Overall, the optical framework outlined in this work can contribute to democratizing research on DNA-Au NPs and facilitating their rapid adoption in transformative applications.

Cations Do Not Alter the Membrane Structure of POPC—A Lipid With an Intermediate Area

Combining small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and densitometric measurements, we have studied the interactions of the divalent cations Ca²⁺ and Mg²⁺ with the lipid vesicles prepared of a mixed-chain palmitoyl-oleoyl-phosphatidylcholine (POPC) at 25°C. The structural parameters of the POPC bilayer, such as the bilayer thickness, lateral area, and volume per lipid, displayed no changes upon the ion addition at concentrations up to 30 mM and minor changes at > 30 mM Ca²⁺ and Mg²⁺, while some decrease in the vesicle radius was observed over the entire concentration range studied. This examination allows us to validate the concept of lipid–ion interactions governed by the area per lipid suggested previously and to propose the mixed mode of those interactions that emerge in the POPC vesicles. We speculate that the average area per POPC lipid that corresponds to the cutoff length of lipid–ion interactions generates an equal but opposite impact on ion bridges and separate lipid–ion pairs. As a result of the dynamic equilibrium, the overall structural properties of bilayers are not affected. As the molecular mechanism proposed is affected by the structural properties of a particular lipid, it might help us to understand the fundamentals of processes occurring in complex multicomponent membrane systems.

Dielectric Constant of Aqueous Solutions of Proteins and Organic Polymers from Molecular Dynamics Simulations

The dielectric constant of water/oligomer mixtures, spanning the range from pure water to pure oligomeric melts, is investigated using molecular dynamics (MD) simulations. As prototypical water-soluble organic substances we consider neutral poly-glycine, poly-ethylene glycol and charged monomeric propanic acid. As the water content is reduced, the dielectric constant decreases but does not follow an ideal mixing behavior. The deviations from ideal mixing originate primarily in the non-linear relation between the oligomer mass fraction and collective polarization effects. We find that the dielectric constant is dominated by water polarization, even if the oligomer mass fraction exceeds 50%. By a double extrapolation of the MD simulation results to the limit of vanishing water fraction and to the limit of infinite oligomeric chain length, we estimate the orientational contribution to the dielectric constant of the pure polymeric melts. By this procedure, we obtain ε = 17 {plus minus} 2 for polyglycine and ε = 1 {plus minus} 0.3 for polyethylene glycol. The large difference is rationalized by polarization correlations of glycine units. Interestingly, we find constant temperature simulations to outperform replica exchange simulations in terms of equilibration speed.

Design of Lipid-Based Nanocarriers via Cation Modulation of Ethanol-Interdigitated Lipid Membranes

Short-chain alcohols (i.e., ethanol) can induce membrane interdigitation in saturated-chain phosphatidylcholines (PCs). In this process, alcohol molecules intercalate between phosphate heads, increasing lateral separation and favoring hydrophobic interactions between opposing acyl chains, which interpenetrate forming an interdigitated phase. Unraveling mechanisms underlying the interactions between ethanol and model lipid membranes has implications for cell biology, biochemistry, and for the formulation of lipid-based nanocarriers. However, investigations of ethanol-lipid membrane systems have been carried out in deionized water, which limits their applicability. Here, using a combination of small- and wide-angle X-ray scattering, small-angle neutron scattering, and all-atom molecular dynamics simulations, we analyzed the effect of varying CaCl₂ and NaCl concentrations on ethanol-induced interdigitation. We observed that while ethanol addition leads to the interdigitation of bulk phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers in the presence of CaCl₂ and NaCl regardless of the salt concentration, the ethanol-induced interdigitation of vesicular DPPC depends on the choice of cation and its concentration. These findings unravel a key role for cations in the ethanol-induced interdigitation of lipid membranes in either bulk phase or vesicular form.

Hydration Forces Dominate Surface Charge Dependent Lipid Bilayer Interactions under Physiological Conditions

Lipid bilayer interactions are essential to a vast range of biological functions, such as intracellular transport mechanisms. Surface charging mediated by concentration dependent ion adsorption and desorption on lipid headgroups alters electric double layers as well as van der Waals and steric hydration forces of interacting bilayers. Here, we directly measure bilayer interactions during charge modulation in a symmetrically polarized electrochemical three-mirror interferometer surface forces apparatus. We quantify polarization and concentration dependent hydration and electric double layer forces due to cation adsorption/desorption. Our results demonstrate that exponential hydration layer interactions effectively describe surface potential dependent surface forces due to cation adsorption at high salt concentrations. Hence, electric double layers of lipid bilayers are exclusively dominated by inner Helmholtz charge regulation under physiological conditions. These results are important for rationalizing bilayer behavior under physiological conditions, where charge and concentration modulation may act as biological triggers for function and signaling.

Osmotic Transport at the Aqueous Graphene and hBN Interfaces: Scaling Laws from a Unified, First-Principles Description

Osmotic transport in nanoconfined aqueous electrolytes provides alternative venues for water desalination and "blue energy" harvesting. The osmotic response of nanofluidic systems is controlled by the interfacial structure of water and electrolyte solutions in the so-called electrical double layer (EDL), but a molecular-level picture of the EDL is to a large extent still lacking. Particularly, the role of the electronic structure has not been considered in the description of electrolyte/surface interactions. Here, we report enhanced sampling simulations based on ab initio molecular dynamics, aiming at unravelling the free energy of prototypical ions adsorbed at the aqueous graphene and hBN interfaces, and its consequences on nanofluidic osmotic transport. Specifically, we predicted the zeta potential, the diffusio-osmotic mobility, and the diffusio-osmotic conductivity for a wide range of salt concentrations from the ab initio water and ion spatial distributions through an analytical framework based on Stokes equation and a modified Poisson-Boltzmann equation. We observed concentration-dependent scaling laws, together with dramatic differences in osmotic transport between the two interfaces, including diffusio-osmotic flow and current reversal on hBN but not on graphene. We could rationalize the results for the three osmotic responses with a simple model based on characteristic length scales for ion and water adsorption at the surface, which are quite different on graphene and on hBN. Our work provides fundamental insights into the structure and osmotic transport of aqueous electrolytes on 2D materials and explores alternative pathways for efficient water desalination and osmotic energy conversion.

Large scale model lipid membrane movement induced by a cation switch

A biomembrane sample system where millimolar changes of cations induce reversible large scale (≥ 200 Å) changes in the membrane-to-surface distance is described. The system composes of a free-floating bilayer, formed adjacent to a self-assembled monolayer (SAM). To examine the membrane movements, differently charged floating bilayers in the presence and absence of Ca2+ and Na+, respectively, were examined using neutron reflectivity and quartz crystal microbalance measurements, alongside molecular dynamics simulations. In neutron reflectivity the variation of Ca2+ and Na+ concentration enabled precision manipulation of the membrane-to-surface distance. Simulations suggest that Ca2+ ions bridge between SAM and bilayer whereas the more diffuse binding of Na+, especially to bilayers, is unable to fully overcome the repulsion between anionic floating bilayer and anionic SAM. Reproduced neutron reflectivity results with quartz crystal microbalance demonstrate the potential of this easily producible sample system to become a standard analysis tool for e.g. investigating membrane binding effects, endocytosis and cell signaling.

Water at charged interfaces

The ubiquity of aqueous solutions in contact with charged surfaces and the realization that the molecular-level details of water-surface interactions often determine interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the molecular picture of the interfacial organization and preferential alignment of water molecules, as well as the structure of water molecules and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aqueous interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which molecular properties are relevant, such as interfacial chemical conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.

Charge Effects Provide Ångström-Level Control of Lipid Bilayer Morphology on Titanium Dioxide Surfaces

Interfaces between molecular organic architectures and oxidic substrates are a central feature of biosensors and applications of biomimetics in science and technology. For phospholipid bilayers, the large range of pH- and ionic strength-dependent surface charge densities adopted by titanium dioxide and other oxidic surfaces leads to a rich landscape of phenomena that provides exquisite control of membrane interactions with such substrates. Using neutron reflectometry measurements, we report sharp, reversible transitions that occur between closely surface-associated and weakly coupled states. We show that these states arise from a complex interplay of the tunable length scale of electrostatic interactions with the length scale arising from other forces that are independent of solution conditions. A generalized free energy potential, with its inputs only derived from established measurements of surface and bilayer properties, quantitatively describes these and previously reported observations concerning the unbinding of bilayers from supporting substrates.

Relative humidity in droplet and airborne transmission of disease

A large number of infectious diseases are transmitted by respiratory droplets. How long these droplets persist in the air, how far they can travel, and how long the pathogens they might carry survive are all decisive factors for the spread of droplet-borne diseases. The subject is extremely multifaceted and its aspects range across different disciplines, yet most of them have only seldom been considered in the physics community. In this review, we discuss the physical principles that govern the fate of respiratory droplets and any viruses trapped inside them, with a focus on the role of relative humidity. Importantly, low relative humidity-as encountered, for instance, indoors during winter and inside aircraft-facilitates evaporation and keeps even initially large droplets suspended in air as aerosol for extended periods of time. What is more, relative humidity affects the stability of viruses in aerosol through several physical mechanisms such as efflorescence and inactivation at the air-water interface, whose role in virus inactivation nonetheless remains poorly understood. Elucidating the role of relative humidity in the droplet spread of disease would permit us to design preventive measures that could aid in reducing the chance of transmission, particularly in indoor environment.

Spectroscopically Quantifying the Influence of Salts on Nonionic Surfactant Chemical Potentials and Micelle Formation

The influence of two salts (NaSCN and Na₂SO₄) on the micellization of a nonionic surfactant (1,2-hexanediol) is quantified using Raman multivariate curve resolution spectroscopy, combined with a generalized theoretical analysis of the corresponding chemical potential changes. Although the SCN^- and SO₄^2- anions are on opposite ends of the Hofmeister series, they are both found to lower the critical micelle concentration. Our combined spectroscopic and theoretical analysis traces these observations to the fact that in both salt solutions the ions have a greater affinity for (or are less strongly expelled from) the hydration shell of the micelle than the free surfactant monomer, as quantified using the corresponding chemical potentials and Wyman-Tanford coefficients. This probe-free experimental and theoretical analysis strategy may readily be extended to micelle formation processes involving other surfactants, salts, and cosolvents, as well as to other sorts of aggregation and binding processes.

Ca2+-mediated enhancement of anesthetic diffusion across phospholipid multilamellar systems

Although sharing common properties with other divalent cations, calcium ions induce fine-tuned electrostatic effects essential in many biological processes. Not only related with protein structure or ion channels, calcium is also determinant for other biomolecules such as lipids or even drugs. Cellular membranes are the first interaction barriers for drugs. Depending on their hydrophilic, hydrophobic or amphipathic properties, they have to overcome such barriers to permeate and diffuse through inner lipid bilayers, cells or even tissues. In this context, the role of calcium in the permeation of cationic amphiphilic drugs (CADs) through lipid membranes is not well understood. We combine differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) to investigate the effect of Ca²⁺ on the interlamellar diffusion kinetics of the local anesthetic tetracaine (TTC) in multilamellar artificial membrane systems. Our DSC results show the interesting phenomenon that TTC diffusion can be modified in two different ways in the presence of Ca²⁺. Furthermore, TTC diffusion exhibits a thermal-dependent membrane interaction in the presence of Ca²⁺. The FTIR results suggest the presence of ion-dipole interactions between Ca²⁺ and the carbonyl group of TTC, leading us to hypothesize that Ca²⁺ destabilizes the hydration shell of TTC, which in turn diffuses deeper into the multilamellar lipid structures. Our results demonstrate the relevance of the Ca²⁺ ion in the drug permeation and diffusion through lipid bilayers.

Morphologies of Vesicle Doublets: Competition among Bending Elasticity, Surface Tension, and Adhesion

To study the mechanical laws governing the form of multicellular organisms, we examine the morphology of adhering vesicle doublets as the simplest model system. We monitor the morphological transformations of doublets induced by changes of adhesion strength and volume/area ratio, which are controlled by intermembrane interactions and thermal area expansion, respectively. When we increase the temperature in the weak adhesion regime, a dumbbell flat-contact doublet is transformed to a parallel-prolate doublet, whereas in the strong adhesion regime, heating transforms the dumbbell flat-contact doublet into a spherical sigmoid-contact doublet. We reproduce the observed doublet morphologies by numerically minimizing the total energy, including the contact-potential adhesion term as well as the surface and bending terms, using the Surface Evolver package. From the reproduced morphologies, we extract the adhesion strength, the surface tension, and the volume/area ratio of the vesicles, which reveals the detailed mechanisms of the morphological transitions in doublets.

Anomaly of the dielectric function of water under confinement and its role in van der Waals interactions

We present a theoretical calculation of the changes in the Hamaker constant due to the anomalous reduction of the static dielectric function of water. Under confinement, the dielectric function of water decreases from a bulk value of 80 down to 2. If the confining walls are made of a dielectric material, the Hamaker constant reduces by almost 90%. However, if the confinement is realized with metallic plates, there is little change in the Hamaker constant. Additionally, we show that confinement can be used to decreases the Debye screening length without changing the salt concentration. This in turn is used to change the Hamaker constant in the presence of electrolytes.

Bridges of Calcium Bicarbonate Tightly Couple Dipolar Lipid Membranes

The interaction between lipid membranes and ions is associated with a range of key physiological processes. Most earlier studies have focused on the interaction of lipids with cations, while the specific effects of the anions have been largely overlooked. Owing to dissolved atmospheric carbon dioxide, bicarbonate is an important ubiquitous anion in aqueous media. In this paper, we report on the effect of bicarbonate anions on the interactions between dipolar lipid membranes in the presence of previously adsorbed calcium cations. Using a combination of solution X-ray scattering, osmotic stress, and molecular dynamics simulations, we followed the interactions between 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) lipid membranes that were dialyzed against CaCl2 solutions in the presence and absence of bicarbonate anions. Calcium cations adsorbed onto DLPC membranes, charge them, and lead to their swelling. In the presence of bicarbonate anions, however, the calcium cations can tightly couple one dipolar DLPC membrane to the other and form a highly condensed and dehydrated lamellar phase with a repeat distance of 3.45 ± 0.02 nm. Similar tight condensation and dehydration has only been observed between charged membranes in the presence of multivalent counterions. Bridging between bilayers by calcium bicarbonate complexes induced this arrangement. Furthermore, in this condensed phase, lipid molecules and adsorbed ions were arranged in a two-dimensional oblique lattice.

Counteranions in the Stimulation Solution Alter the Dynamics of Exocytosis Consistent with the Hofmeister Series

We show that the Hofmeister series of ions can be used to explain the cellular changes in exocytosis observed by single-cell amperometry for different counteranions. The formation, expansion, and closing of the membrane fusion pore during exocytosis was found to be strongly dependent on the counteranion species in solution. With stimulation of chaotropic anions (e.g., ClO₄^–), the expansion and closing time of the fusion pore are longer, suggesting chaotropes can extend the duration of exocytosis compared with kosmotropic anions (e.g., Cl^–). At a concentration of 30 mM, the two parameters (e.g., t_1/2 and t_fall) that define the duration of exocytosis vary with the Hofmeister series (Cl^– < Br^– < NO₃^– ≤ ClO₄^– < SCN^–). More interestingly, fewer (e.g., N_foot/N_events) and smaller (e.g., I_foot) prespike events are observed when chaotropes are counterions in the stimulation solution, and the values can be sorted by the reverse Hofmeister series (Cl^– ≥ Br^– > NO₃^– > ClO₄^– > SCN^–). Based on ion specificity, an adsorption-repulsion mechanism, we suggest that the exocytotic Hofmeister series effect originates from a looser swelling lipid bilayer structure due to the adsorption and electrostatic repulsion of chaotropes on the hydrophobic portion of the membrane. Our results provide a chemical link between the Hofmeister series and the cellular process of neurotransmitter release via exocytosis and provide a better physical framework to understand this important phenomenon.

Strong Differential Monovalent Anion Selectivity in Narrow Diameter Carbon Nanotube Porins

Inner pores of carbon nanotubes combine extremely fast water transport and ion selectivity that could potentially be useful for high-performance water desalination and separation applications. We used dye-quenching halide assays and stopped-flow spectrometry to determine intrinsic permeability of three small monovalent halide anions (chloride, bromide, iodide) and one pseudohalide anion (thiocyanate) through narrow 0.8 nm diameter carbon nanotube porins (CNTPs). These measurements revealed unexpectedly strong differential ion selectivity with permeabilities of different ions varying by up to 2 orders of magnitude. Removal of the negative charge from the nanotube entrance increased anion permeability by only a relatively small factor, indicating that electrostatic repulsion was not a major determinant of CNTP selectivity. First principle molecular dynamics simulations revealed that the origin of this strong differential ion selectivity is partial dehydration of anions upon entry into the narrow CNTP channels.

Normal and shear forces between boundary sphingomyelin layers under aqueous conditions

Sphingomyelin is one of the predominant phospholipid groups in synovial joints, where lipids have been strongly implicated in the boundary lubrication of articular cartilage; however, little attention has been paid to its lubrication behavior. In this study, we demonstrate that sphingomyelin is an excellent boundary lubricant by measuring the normal and shear forces between sphingomyelin-layer-coated surfaces with a surface force balance under aqueous conditions. Slightly negatively-charged egg sphingomyelin vesicles were adsorbed on mica either by calcium bridging or by charge screening with high concentration monovalent salt. The normal force profiles between opposing egg sphingomyelin layers (vesicles or bilayers) show long-ranged weak repulsion and short-ranged strong repulsion on approaching. Friction coefficients, calculated from the highest load, were (7.2 ± 1.7) × 10-4 at contact stresses of 9.1 ± 0.7 MPa across 0.3 mM liposome dispersion in 0.03 mM Ca2+, and (0.8-3.5) × 10-3 at contact stresses of 7.6 ± 0.8 MPa across 0.3 mM liposome dispersion in 150 mM NaNO3. Similar or slightly lower friction coefficients of (5.3 ± 0.8) × 10-4 at 9.8 ± 0.2 MPa were obtained by replacing the liposome dispersion in 0.03 mM Ca2+ by water. Such low friction coefficients, attributed to the hydration lubrication mechanism, are comparable to those of phosphatidylcholine lipids, which have been widely recognized as excellent aqueous biolubricants. Therefore, we believe that sphingomyelin, in parallel with phosphatidylcholine, contributes to the remarkably good boundary lubrication in synovial joints.

Unveiling the Interstitial Pressure between Growing Ice Crystals during Ice-Templating Using a Lipid Lamellar Probe

What is the pressure generated by ice crystals during ice-templating? This work addresses this crucial question by estimating the pressure exerted by oriented ice columns on a supramolecular probe composed of a lipid lamellar hydrogel during directional freezing. This process, also known as freeze-casting, has emerged as a unique processing technique for a broad class of organic, inorganic, soft, and biological materials. Nonetheless, the pressure exerted during and after crystallization between two ice columns is not known, despite its importance with respect to the fragility of the frozen material, especially for biological samples. By using the lamellar period of a glycolipid lamellar hydrogel as a common probe, we couple data obtained from ice-templated-resolved in situ synchrotron small-angle X-ray scattering (SAXS) with data obtained from controlled adiabatic desiccation experiments. We estimate the pressure to vary between 1 ± 10% kbar at -15 °C and 3.5 ± 20% kbar at -60 °C.

Primary and Secondary Hydration Forces between Interdigitated Membranes Composed of Bolaform Microbial Glucolipids

To better understand lipid membranes in living organisms, the study of intermolecular forces using the osmotic pressure technique applied to model lipid membranes has constituted the ground knowledge in the field of biophysics since four decades. However, the study of intermolecular forces in lipid systems other than phospholipids, like glycolipids, has gained a certain interest only recently. Even in this case, the work generally focuses on the study of membrane glycolipids, but little is known on new forms of non-membrane functional compounds, like microbial bolaform glycolipids. This work explores, through the osmotic stress method involving an adiabatic humidity chamber coupled to neutron diffraction, the short-range (<2 nm) intermolecular forces of membranes entirely composed of interdigitated glucolipids. Experiments are performed at pH 6 when the glucolipid is partially negatively charged and for which we explore the effect of low (16 mM) and high (100 mM) ionic strength. We find that this system is characterized by primary and secondary hydration regimes insensitive and sensitive to ionic strength, respectively, and with typical decay lengths of λ_H1 = 0.37 ± 0.12 nm and λ_H2 = 1.97 ± 0.78 nm.

How is the interaction of a chloride channel blocker with phospholipids influenced by divalent metal ions? Effect of unsaturation on the lipid side chain

The present study reveals the effect of various divalent ions (Ca²⁺, Mg²⁺and Zn²⁺) on the binding interaction of a prospective chloride channel blocker, 9-methylanthroate (9MA), with liposome membranes, namely, dimyristoylphosphatidylcholine (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The liposome membranes DMPC and POPC differ in the unsaturation of the side-chain. The drug (9MA) is found to experience a greater degree of partitioning into the POPC lipid bilayer (containing unsaturated side-chain) in comparison to DMPC (containing saturated side-chain). The stronger 9MA-POPC binding interaction is found to be only nominally perturbed by the presence of metal salts. On the contrary, the 9MA-DMPC binding interaction is found to be significantly perturbed by the presence of metal salts and is manifested on the environment-responsive spectroscopic properties of the drug. The steady-state and picosecond-resolved fluorescence spectroscopic results reveal the effect of metal ions on DMPC bilayer to follow the trend Ca²⁺ < Mg²⁺ < Zn²⁺. This is also quantified by evaluating the partition coefficient of the drug into DMPC lipid in the presence of various divalent ions which is found to follow the same sequence. The degree of penetration of these cations has been rationalized on the basis of adsorption of cations on DMPC headgroup region resulting in dehydration of the headgroup along with shrinking of it.

Self-Assembled Fluid Phase Floating Membranes with Tunable Water Interlayers

We present a reliable method for the fabrication of fluid phase, unsaturated lipid bilayers by self-assembly onto charged Self-Assembled Monolayer (SAM) surfaces with tunable membrane to surface aqueous interlayers. Initially, the formation of water interlayers between membranes and charged surfaces was characterized using a comparative series of bilayers deposited onto charged, self-assembled monolayers by sequential layer deposition. Using neutron reflectometry, a bilayer to surface water interlayer of ∼8 Å was found between the zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane and an anionic carboxyl terminated grafted SAM with the formation of this layer attributed to bilayer repulsion by hydration water on the SAM surface. Furthermore, we found we could significantly reduce the technical complexity of sample fabrication through self-assembly of planar membranes onto the SAM coated surfaces. Vesicle fusion onto carboxyl-terminated monolayers yielded high coverage (>95%) bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) which floated on a 7-11 Å solution interlayer between the membrane and the surface. The surface to membrane distance was then tuned via the addition of 200 mM NaCl to the bulk solution immersing a POPC floating membrane, which caused the water interlayer to swell reversibly to ∼33 Å. This study reveals that biomimetic membrane models can be readily self-assembled from solution onto functionalized surfaces without the use of polymer supports or tethers. Once assembled, surface to membrane distance can be tailored to the experimental requirements using physiological concentrations of electrolytes. These planar bilayers only very weakly interact with the substrate and are ideally suited for use as biomimetic models for accurate in vitro biochemical and biophysical studies, as well as for technological applications, such as biosensors.

On the Mechanism of Bilayer Separation by Extrusion, or Why Your LUVs Are Not Really Unilamellar

Extrusion through porous filters is a widely used method for preparing biomimetic model membranes. Of primary importance in this approach is the efficient production of single bilayer (unilamellar) vesicles that eliminate the influence of interlamellar interactions and strictly define the bilayer surface area available to external reagents such as proteins. Submicroscopic vesicles produced using extrusion are widely assumed to be unilamellar, and large deviations from this assumption would impact interpretations from many model membrane experiments. Using three probe-free methods-small angle X-ray and neutron scattering and cryogenic electron microscopy-we report unambiguous evidence of extensive multilamellarity in extruded vesicles composed of neutral phosphatidylcholine lipids, including for the common case of neutral lipids dispersed in physiological buffer and extruded through 100-nm diameter pores. In such preparations, only ∼35% of lipids are externally accessible and this fraction is highly dependent on preparation conditions. Charged lipids promote unilamellarity as does decreasing solvent ionic strength, indicating the importance of electrostatic interactions in determining the lamellarity of extruded vesicles. Smaller extrusion pore sizes also robustly increase the fraction of unilamellar vesicles, suggesting a role for membrane bending. Taken together, these observations suggest a mechanistic model for extrusion, wherein the formation of unilamellar vesicles involves competition between bilayer bending and adhesion energies. The findings presented here have wide-ranging implications for the design and interpretation of model membrane studies, especially ensemble-averaged observations relying on the assumption of unilamellarity.

The Hofmeister series: Specific ion effects in aqueous polymer solutions

Specific ion effects in aqueous polymer solutions have been under active investigation over the past few decades. The current state-of-the-art research is primarily focused on the understanding of the mechanisms through which ions interact with macromolecules and affect their solution stability. Hence, we herein first present the current opinion on the sources of ion-specific effects and review the relevant studies. This includes a summary of the molecular mechanisms through which ions can interact with polymers, quantification of the affinity of ions for the polymer surface, a thermodynamic description of the effects of salts on polymer stability, as well as a discussion on the different forces that contribute to ion-polymer interplay. Finally, we also highlight future research issues that call for further scrutiny. These include fundamental questions on the mechanisms of ion-specific effects and their correlation with polymer properties as well as a discussion on the specific ion effects in more complex systems such as mixed electrolyte solutions.

Understanding Calcium-Mediated Adhesion of Nanomaterials in Reservoir Fluids by Insights from Molecular Dynamics Simulations

Interest in nanomaterials for subsurface applications has grown markedly due to their successful application in a variety of disciplines, such as biotechnology and medicine. Nevertheless, nanotechnology application in the petroleum industry presents greater challenges to implementation because of the harsh conditions (i.e. high temperature, high pressure, and high salinity) that exist in the subsurface that far exceed those present in biological applications. The most common subsurface nanomaterial failures include colloidal instability (aggregation) and sticking to mineral surfaces (irreversible retention). We previously reported an atomic force microscopy (AFM) study on the calcium-mediated adhesion of nanomaterials in reservoir fluids (S. L. Eichmann and N. A. Burnham, Sci. Rep. 7, 11613, 2017), where we discovered that the functionalized and bare AFM tips showed mitigated adhesion forces in calcium ion rich fluids. Herein, molecular dynamics reveal the molecular-level details in the AFM experiments. Special attention was given to the carboxylate-functionalized AFM tips because of their prominent ion-specific effects. The simulation results unambiguously demonstrated that in calcium ion rich fluids, the strong carboxylate-calcium ion complexes prevented direct carboxylate-calcite interactions, thus lowering the AFM adhesion forces. We performed the force measurement simulations on five representative calcite crystallographic surfaces and observed that the adhesion forces were about two to three fold higher in the calcium ion deficient fluids compared to the calcium ion rich fluids for all calcite surfaces. Moreover, in calcium ion deficient fluids, the adhesion forces were significantly stronger on the calcite surfaces with higher calcium ion exposures. This indicated that the interactions between the functionalized AFM tips and the calcite surfaces were mainly through carboxylate interactions with the calcium ions on calcite surfaces. Finally, when analyzing the order parameters of the tethered functional groups, we observed significantly different behavior of the alkanethiols depending on the absence or presence of calcium ions. These observations agreed well with AFM experiments and provided new insights for the competing carboxylate/calcite/calcium ion interactions.

Influence of Trivalent Metal Ions on Lipid Vesicles: Gelation and Fusion Phenomena

In this contribution, we report the interaction of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) lipid vesicles with a series of trivalent metal ions of the same group, namely, Al³⁺, Ga³⁺, and In³⁺, to get a distinct view of the effect of size, effective charge, and hydration free energy of these metal ions on lipid vesicles. We employed steady-state and time-resolved spectroscopic techniques including time-resolved anisotropy measurement, confocal imaging, and dynamic light scattering (DLS) measurement to probe the interaction. Our study reveals that all of the three trivalent metal ions induce gelation in lipid vesicles by removing water molecules from the interfacial region. The extent of gelation induced by the metal ions follows the order of In³⁺ > Ga³⁺ ≥ Al³⁺. We explain this observation in light of different free-energy terms. Notably, the degree of interaction for trivalent metal ions is higher as compared to that for divalent metal ions at physiological pH (pH ∼ 7.0). Most importantly, we observe that unlike divalent metal ions, trivalent metal ions dehydrate the lipid vesicles even at lower pH. The DLS measurement and confocal imaging indicate that In³⁺ causes significant aggregation or fusion of the PC vesicles, while Al³⁺ and Ga³⁺ did not induce any aggregation at the experimental concentration. We employ Derjaguin-Landau-Vervey-Overbeek (DLVO) theory to explain the aggregation phenomena induced by In³⁺.

Long-lived ionic nano-domains can modulate the stiffness of soft interfaces

Metal ions underpin countless processes at bio-interfaces, including maintaining electroneutrality, modifying mechanical properties and driving bioenergetic activity. These processes are typically described by ions behaving as independently diffusing point charges. Here we show that Na+ and K+ ions instead spontaneously form correlated nanoscale networks that evolve over seconds at the interface with an anionic bilayer in solution. Combining single-ion level atomic force microscopy and molecular dynamic simulations we investigate the configuration and dynamics of Na+, K+, and Rb+ at the lipid surface. We identify two distinct ionic states: the well-known direct electrostatic interaction with lipid headgroups and a water-mediated interaction that can drive the formation of remarkably long-lived ionic networks which evolve over many seconds. We show that this second state induces ionic network formation via correlative ion-ion interactions that generate an effective energy well of -0.4kBT/ion. These networks locally reduce the stiffness of the membrane, providing a spontaneous mechanism for tuning its mechanical properties with nanoscale precision. The ubiquity of water-mediated interactions suggest that our results have far-reaching implications for controlling the properties of soft interfaces.

Influence of polar co-solutes and salt on the hydration of lipid membranes

The influence of the co-solutes TMAO, urea, and NaCl on the hydration repulsion between lipid membranes is investigated in a combined experimental/simulation approach.

Interaction between nanoparticles and charged phospholipid membranes

Charged lipids in cell membranes and subcellular organelles are arranged in the form of a bilayer with the hydrocarbon tails sequestered away from the water and the polar head groups exposed to the aqueous environment. Most of them bear net negative charges leading to the negatively charged cell membranes. Charged lipid-lipid and lipid-protein interactions are generally dynamic and heavily depend on their local molecular concentrations. To examine the electrostatic properties of charged lipid layers in contact with an electrolyte solution, we incorporate the single chain mean field theory with Poisson-Boltzmann theory to explore the equilibrium structure of charged phospholipid membranes. Using the three bead coarse-grained model we reproduced the essential equilibrium properties of the charged phospholipid bilayer. We also investigate the influence of the mobile ions on the thickness of the layer, the area per lipid (APL), and the electrostatic potential of the membrane. Then we investigate the attraction-repulsion property of two charged nanoparticles which are stuck on the charged lipid molecules surrounded with mobile ions. After that we simulated the interaction between the Pleckstrin homology domain (PH domain) of Akt and the cytoplasmic membrane. Taking into account the electrostatic interaction, we observe the structure changes of the membrane at different concentrations of mobile ions in its equilibrium state. Also we discuss the influence of mobile ions on the size of the pore opened in the membrane by the charged protein. Such an observation may shed light on the activation of oncogenic Akt (or protein kinase B) around the membrane at the molecular level.

The effect of H3O+ on the membrane morphology and hydrogen bonding of a phospholipid bilayer

At the 2017 meeting of the Australian Society for Biophysics, we presented the combined results from two recent studies showing how hydronium ions (H₃O⁺) modulate the structure and ion permeability of phospholipid bilayers. In the first study, the impact of H₃O⁺ on lipid packing had been identified using tethered bilayer lipid membranes in conjunction with electrical impedance spectroscopy and neutron reflectometry. The increased presence of H₃O⁺ (i.e. lower pH) led to a significant reduction in membrane conductivity and increased membrane thickness. A first-order explanation for the effect was assigned to alterations in the steric packing of the membrane lipids. Changes in packing were described by a critical packing parameter (CPP) related to the interfacial area and volume and shape of the membrane lipids. We proposed that increasing the concentraton of H₃O⁺ resulted in stronger hydrogen bonding between the phosphate oxygens at the water-lipid interface leading to a reduced area per lipid and slightly increased membrane thickness. At the meeting, a molecular model for these pH effects based on the result of our second study was presented. Multiple μs-long, unrestrained molecular dynamic (MD) simulations of a phosphatidylcholine lipid bilayer were carried out and showed a concentration dependent reduction in the area per lipid and an increase in bilayer thickness, in agreement with experimental data. Further, H₃O⁺ preferentially accumulated at the water-lipid interface, suggesting the localised pH at the membrane surface is much lower than the bulk bathing solution. Another significant finding was that the hydrogen bonds formed by H₃O⁺ ions with lipid headgroup oxygens are, on average, shorter in length and longer-lived than the ones formed in bulk water. In addition, the H₃O⁺ ions resided for longer periods in association with the carbonyl oxygens than with either phosphate oxygen in lipids. In summary, the MD simulations support a model where the hydrogen bonding capacity of H₃O⁺ for carbonyl and phosphate oxygens is the origin of the pH-induced changes in lipid packing in phospholipid membranes. These molecular-level studies are an important step towards a better understanding of the effect of pH on biological membranes.

Effect of Zwitterionic Phospholipid on the Interaction of Cationic Membranes with Monovalent Sodium Salts

Cationic lipids have attracted much attention because of their potential for biomedical applications, such as gene delivery. The gene transfection efficiency of cationic lipids is greatly influenced by the counterions as well as salt ions. We have systematically investigated the interaction of different monovalent sodium salts with positively charged membrane, composed of 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and DOTAP, using dynamic light scattering, zeta potential, isothermal titration calorimetry (ITC), and fluorescence spectroscopy techniques. Our results reveal that the affinity of anions with cationic membranes follows the sequence I^- ≫ Br^- > Cl^- according to descending order of their sizes and is consistent with the Hofmeister series. Interestingly, the electrostatic behavior of the DOTAP membrane in the presence of monovalent anions differs significantly from the DOPC/DOTAP membrane. This difference is due to the strong interplay between phosphocholine and trimethylammonium-propane (TAP) headgroups leading to the reorientation of the TAP group in the membrane. The binding constant of anions, derived from zeta potential and ITC is in agreement with the affinity of anions mentioned above. Among all anions, I^- shows strongest affinity, as evidenced from the rapid increase in hydrodynamic radius which eventually leads to the formation of large aggregates. The fluorescence spectroscopy of a lypophilic probe Nile red in the presence of cationic vesicles containing ions complements the I^- adsorption onto the membrane. Nonlinear Stern-Volmer plot, consisting of accessible and inaccessible Nile red to I^- is consistent with the zeta potential as well as ITC results.

Tailoring van der Waals dispersion interactions with external electric charges

van der Waals (vdW) dispersion interactions strongly impact the properties of molecules and materials. Often, the description of vdW interactions should account for the coupling with pervasive electric fields, stemming from membranes, ionic channels, liquids, or nearby charged functional groups. However, this quantum-mechanical effect has been omitted in atomistic simulations, even in widely employed electronic-structure methods. Here, we develop a model and study the effects of an external charge on long-range vdW correlations. We show that a positive external charge stabilizes dispersion interactions, whereas a negative charge has an opposite effect. Our analytical results are benchmarked on a series of (bio)molecular dimers and supported by calculations with high-level correlated quantum-chemical methods, which estimate the induced dispersion to reach up to 35% of intermolecular binding energy (4 kT for amino-acid dimers at room temperature). Our analysis bridges electrostatic and electrodynamic descriptions of intermolecular interactions and may have implications for non-covalent reactions, exfoliation, dissolution, and permeation through biological membranes.

Drastic Swelling of Lipid Oligobilayers by Polyelectrolytes: A Potential Molecular Model for the Internal Structure of Lubricating Films in Mammalian Joints

Osteoarthritis is the most common arthropathy in western civilization. It is primarily caused by the degeneration of lipid-coated cartilage, leading to increased friction in joints. Hyaluronic acid (HA), a negatively charged polysaccharide and the main component of the synovial fluid, is held responsible for joint lubrication. It is believed that HA, adsorbed to the lipid-coated cartilage, forms a protective layer against wear. Studies have shown that the concentration and molecular weight (MW) of HA are reduced in joints suffering from osteoarthritis. On the basis of these observations, local joint injections of HA or mixtures of HA and surface-active phospholipids (SAPLs) have been applied as medical cures to restore the functionality of the joints in a procedure called viscosupplementation. However, this cure is still disputed, and no consensus has been reached with respect to optimum HA concentration and MW. To provide detailed insight in the structural rearrangement of lipid films upon contact with HA or polymeric analogues, we studied the interaction of the polyelectrolyte poly(allylamine hydrochloride) (PAH) with surface-bound oligobilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by neutron reflectivity (NR) and ellipsometry. Using this model system, we found a drastic swelling of the lipid films as a function of PAH concentration, whose strength compares to that in previous studies on HA incubation. In contrast, no significant dependence of film thickness on PAH MW was observed. A detailed picture of the film architecture was developed which inter alia shows that charged PAH is adsorbed to the lipid headgroups, leading to electrostatic repulsion. The swelling behavior is well explained by the equilibrium of Coulomb and van der Waals interactions in a DLVO-based model. Our detailed structural analysis of the PAH/lipid interfacial layer may help to elucidate the mechanisms of viscosupplementation and derive a structure-function relationship for the lubricating interface in mammalian joints.

Spectroscopic evidence for hydration and dehydration of lipid bilayers upon interaction with metal ions: a new physical insight

In this manuscript, we investigate the interactions of different metal ions with zwitterionic phospholipid bilayers of different chain lengths using the well-known membrane probe PRODAN and steady state and time resolved fluorescence spectroscopy.

Synthesis, supramolecular organization and thermotropic phase behaviour of N-acyltris(hydroxymethyl)aminomethane

Herein, we reported the supramolecular organization of N-acyltris(hydroxymethyl)aminomethane (NATM) in the solid state as well as in aqueous solution. Single crystal X-ray diffraction revealed that NATM adopts a fully interdigitized structure. The thermodynamic parameters associated with thermotropic phase behaviour of NATM was determined by differential scanning calorimetry. The molecular packing and phase state of the NATM analyzed by laurdan and prodan fluorescence supports the formation of an interdigitized phase in aqueous solution. The potential application of the self-assembled NATM vesicles was demonstrated through entrapping model drug, Rhodamine B.

Self assembly of N-acyltris(hydroxymethyl)aminomethane into interdigitized vesicles.

The effect of hydronium ions on the structure of phospholipid membranes

This work studies the mechanisms by which hydronium ions modulate the structure of phospholipid bilayers.