Mechanism of Interaction of Monovalent Ions with Phosphatidylcholine Lipid Membranes

Release rates of liposomal contents are controlled by kosmotropes and chaotropes

Contents release from redox-responsive liposomes is anion-specific. Liposomal contents release is initiated by the contact of apposed liposome bilayers having in their outer leaflet 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), whose presence is due to the redox-stimulated removal of a quinone propionic acid protecting group (Q) from Q-DOPE lipids. Contents release occurs upon the phase transition of DOPE from its lamellar liquid-crystalline phase (Lα) to its hexagonal-II inverted micelle (HII) phase. Contents release is slower in the presence of weakly hydrated chaotropic anions versus highly hydrated kosmotropic anions and is attributed to ion accumulation near the zwitterionic DOPE headgroups, in turn altering the headgroup hydration, as indicated by the Lα → HII phase transition temperature, TH, for DOPE. The results are significant, not only for mechanistic aspects of liposome contents release in DOPE-based systems but also for drug delivery applications wherein exist at drug targeting sites variations in the type and concentration of ions and neutral species.

Molecular dynamics simulations of ion conductance in field-stabilized nanoscale lipid electropores

Molecular dynamics (MD) simulations of electrophoretic transport of monovalent ions through field-stabilized electropores in POPC lipid bilayers permit systematic characterization of the conductive properties of lipid nanopores. The radius of the electropore can be controlled by the magnitude of the applied sustaining external electric field, which also drives the transport of ions through the pore. We examined pore conductances for two monovalent salts, NaCl and KCl, at physiological concentrations. Na(+) conductance is significantly less than K(+) and Cl(-) conductance and is a nonlinear function of pore radius over the range of pore radii investigated. The single pore electrical conductance of KCl obtained from MD simulation is comparable to experimental values measured by chronopotentiometry.

Specific binding of chloride ions to lipid vesicles and implications at molecular scale

Biological membranes composed of lipids and proteins are in contact with electrolytes like aqueous NaCl solutions. Based on molecular dynamics studies it is widely believed that Na(+) ions specifically bind to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes, whereas Cl(-) ions stay in solution. Here, we present a careful comparison of recent data from electrophoresis and isothermal titration calorimetry experiments as well as molecular dynamics simulations suggesting that in fact both ions show very similar affinities. The corresponding binding constants are 0.44(±0.05) M(-1) for Na(+) and 0.40(±0.04) M(-1) for Cl(-) ions. This is highlighted by our observation that a widely used simulation setup showing asymmetric affinities of Na(+) and Cl(-) for POPC bilayers overestimates the effect of NaCl on the electrophoretic mobility of a POPC membrane by an order of magnitude. Implications for previous simulation results on the effect of NaCl on polarization of interfacial water, transmembrane potentials, and mechanisms for ion transport through bilayers are discussed. Our findings suggest that a range of published simulations results on the interaction of NaCl with phosphocholine bilayers have to be reconsidered and revised and that force field refinements are necessary for reliable simulation studies of membranes at physiological conditions on a molecular level.

Macro- versus microscopic view on the electrokinetics of a water-membrane interface

Electrophoresis is an experimental method widely used to study electrostatic properties of interfaces. Here, we question the validity of the macroscopic theory for the planar geometry by Helmholtz and Smoluchowski by considering a POPC bilayer in an aqueous solution with 500 mM NaCl, using molecular dynamics simulations. We find that POPC shows positive electrophoretic mobility due to adsorption of sodium ions at the lipid headgroups. The theory assumes that the region in which the water density undergoes a transition from the bulk value to zero (interfacial width) is small compared to the Debye screening length. This separation of length scale is not fullfilled in the present case. Hence, contrasting the theory, we observe that the surface is not sharply defined, continuum hydrodynamics is not applicable, the effective viscosity in the double layer is increased compared to the bulk, and the zeta potential is dominated by the dipole potential. Our results might have widespread implications for interpretation of electrokinetic studies in general.

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

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

Free energetics and the role of water in the permeation of methyl guanidinium across the bilayer-water interface: insights from molecular dynamics simulations using charge equilibration potentials

Combining umbrella sampling molecular dynamics (MD) simulations, the weighted histogram analysis method (WHAM) for unbiasing probabilities, and polarizable charge equilibration force fields, we compute the potential of mean force for the reversible transfer of methyl guanidinium from bulk solution to the center of a model DPPC bilayer. A 5 kcal/mol minimum in the potential of mean force profile for membrane permeation suggests that the analogue will preferentially reside in the headgroup region of the lipid, qualitatively in agreement with previously published results. We find the potential of mean force for permeation to be approximately 28 kcal/mol (relative to the minimum in the headgroups), within the range of values reported for similar types of simulations using fixed-charge force fields. From analysis of the lipid structure, we find that the lipid deformation leads to a substantial destabilizing contribution to the free energy of the methyl guanidinium as it resides in the bilayer center, though this deformation allows more efficient stabilization by water defects and transient pores. Water in the bilayer core stabilizes the charged residue. The role of water in stabilizing or destabilizing the solute as it crosses the bilayer depends on bulk electrolyte concentration. In 1 M KCl solution, the water contribution to the potential of mean force is stabilizing over the entire range of the permeation coordinate, with the sole destabilizing force originating from the anionic species in solution. Conversely, methyl guanidinium experiences net destabilization from water in the absence of electrolyte. The difference in solvent contributions to permeation free energy is traced to a local effect arising from differences in water density in the bilayer-water solution interface, thus leading to starkly opposite net forces on the permeant. The origin of the local water density differential rests with the penetration of hydrated chloride anions into the solution-bilayer interface. Finally, water permeation into the bilayer is required for the deformation of individual lipid molecules and permeation of ions into the membrane. From simulations where water is first excluded from the bilayer center where methyl guanidinium is restrained and then, after equilibration, allowed to enter the bilayer, we find that in the absence of any water defects/permeation into the bilayer, the lipid headgroups do not follow the methyl guanidinium. Only when water enters the bilayer do we see deformation of individual lipid molecules to associate with the amino acid analogue at bilayer center.

LIPID11: a modular framework for lipid simulations using amber

Accurate simulation of complex lipid bilayers has long been a goal in condensed phase molecular dynamics (MD). Structure and function of membrane-bound proteins are highly dependent on the lipid bilayer environment and are challenging to study through experimental methods. Within Amber, there has been limited focus on lipid simulations, although some success has been seen with the use of the General Amber Force Field (GAFF). However, to date there are no dedicated Amber lipid force fields. In this paper we describe a new charge derivation strategy for lipids consistent with the Amber RESP approach and a new atom and residue naming and type convention. In the first instance, we have combined this approach with GAFF parameters. The result is LIPID11, a flexible, modular framework for the simulation of lipids that is fully compatible with the existing Amber force fields. The charge derivation procedure, capping strategy, and nomenclature for LIPID11, along with preliminary simulation results and a discussion of the planned long-term parameter development are presented here. Our findings suggest that LIPID11 is a modular framework feasible for phospholipids and a flexible starting point for the development of a comprehensive, Amber-compatible lipid force field.

Fluidity modulation of phospholipid bilayers by electrolyte ions: insights from fluorescence microscopy and microslit electrokinetic experiments

Fluidity and charging of supported bilayer lipid membranes (sBLMs) prepared from 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) were studied by fluorescence recovery after photobleaching (FRAP) and microslit electrokinetic measurements at varying pH and ionic composition of the electrolyte. Measurements in neutral electrolytes (KCl, NaCl) revealed a strong correlation between the membrane fluidity and the membrane charging due to unsymmetrical water ion adsorption (OH(-) ≫ H(3)O(+)). The membrane fluidity significantly decreased below the isoelectric point of 3.9, suggesting a phase transition in the bilayer. The interactions of both chaotropic anions and strongly kosmotropic cations with the zwitterionic lipids were found to be related with nearly unhindered lipid mobility in the acidic pH range. While for the chaotropic anions the observed effect correlates with the increased negative net charge at low pH, no correlation was found between the changes in the membrane fluidity and charge in the presence of kosmotropic cations. We discuss the observed phenomena with respect to the interaction of the electrolyte ions with the lipid headgroup and the influence of this process on the headgroup orientation and hydration as well as on the lipid packaging.

Aqueous solutions at the interface with phospholipid bilayers

In a sense, life is defined by membranes, because they delineate the barrier between the living cell and its surroundings. Membranes are also essential for regulating the machinery of life throughout many interfaces within the cell's interior. A large number of experimental, computational, and theoretical studies have demonstrated how the properties of water and ionic aqueous solutions change due to the vicinity of membranes and, in turn, how the properties of membranes depend on the presence of aqueous solutions. Consequently, understanding the character of aqueous solutions at their interface with biological membranes is critical to research progress on many fronts. The importance of incorporating a molecular-level description of water into the study of biomembrane surfaces was demonstrated by an examination of the interaction between phospholipid bilayers that can serve as model biological membranes. The results showed that, in addition to well-known forces, such as van der Waals and screened Coulomb, one has to consider a repulsion force due to the removal of water between surfaces. It was also known that physicochemical properties of biological membranes are strongly influenced by the specific character of the ions in the surrounding aqueous solutions because of the observation that different anions produce different effects on muscle twitch tension. In this Account, we describe the interaction of pure water, and also of aqueous ionic solutions, with model membranes. We show that a symbiosis of experimental and computational work over the past few years has resulted in substantial progress in the field. We now better understand the origin of the hydration force, the structural properties of water at the interface with phospholipid bilayers, and the influence of phospholipid headgroups on the dynamics of water. We also improved our knowledge of the ion-specific effect, which is observed at the interface of the phospholipid bilayer and aqueous solution, and its connection with the Hofmeister series. Nevertheless, despite substantial progress, many issues remain unresolved. Thus, for example, we still cannot satisfactorily explain the force of interaction between phospholipid bilayers immersed in aqueous solutions of NaI. Although we try to address many issues here, the scope of the discussion is limited and does not cover such important topics as the influence of ionic solutions on phases of bilayers, the influence of salts on the properties of Langmuir monolayers containing lipid molecules, or the influence of aqueous solutions on bilayers containing mixtures of lipids. We anticipate that the future application of more powerful experimental techniques, in combination with more advanced computational hardware, software, and theory, will produce molecular-level information about these important topics and, more broadly, will further illuminate our understanding of interfaces between aqueous solutions and biological membranes.

Molecular dynamics simulation of hydrated DPPC monolayers using charge equilibration force fields

We present results of molecular dynamics simulations of a model DPPC-water monolayer using charge equilibration (CHEQ) force fields, which explicitly account for electronic polarization in a classical treatment of intermolecular interactions. The surface pressure, determined as the difference between the monolayer and pure water surface tensions at 323 K, is predicted to be 22.92 ±1.29 dyne/cm, just slightly below the broad range of experimental values reported for this system. The surface tension for the DPPC-water monolayer is predicted to be 42.35 ±1.16 dyne/cm, in close agreement with the experimentally determined value of 40.9 dyne/cm. This surface tension is also consistent with the value obtained from DPPC monolayer simulations using state-of-the-art nonpolarizable force fields. The current results of simulations predict a monolayer-water potential difference relative to the pure water-air interface of 0.64 ±0.02 Volts, an improved prediction compared to the fixed-charge CHARMM27 force field, yet still overestimating the experimental range of 0.3 to 0.45 Volts. As the charge equilibration model is a purely charge-based model for polarization, the current results suggest that explicitly modeled polarization effects can offer improvements in describing interfacial electrostatics in such systems.

Force spectroscopy reveals the effect of different ions in the nanomechanical behavior of phospholipid model membranes: the case of potassium cation

How do metal cations affect the stability and structure of phospholipid bilayers? What role does ion binding play in the insertion of proteins and the overall mechanical stability of biological membranes? Investigators have used different theoretical and microscopic approaches to study the mechanical properties of lipid bilayers. Although they are crucial for such studies, molecular-dynamics simulations cannot yet span the complexity of biological membranes. In addition, there are still some experimental difficulties when it comes to testing the ion binding to lipid bilayers in an accurate way. Hence, there is a need to establish a new approach from the perspective of the nanometric scale, where most of the specific molecular phenomena take place. Atomic force microscopy has become an essential tool for examining the structure and behavior of lipid bilayers. In this work, we used force spectroscopy to quantitatively characterize nanomechanical resistance as a function of the electrolyte composition by means of a reliable molecular fingerprint that reveals itself as a repetitive jump in the approaching force curve. By systematically probing a set of bilayers of different composition immersed in electrolytes composed of a variety of monovalent and divalent metal cations, we were able to obtain a wealth of information showing that each ion makes an independent and important contribution to the gross mechanical resistance and its plastic properties. This work addresses the need to assess the effects of different ions on the structure of phospholipid membranes, and opens new avenues for characterizing the (nano)mechanical stability of membranes.

Specific interactions of sodium salts with alanine dipeptide and tetrapeptide in water: insights from molecular dynamics

We examine computationally the dipeptide and tetrapeptide of alanine in pure water and solutions of sodium chloride (NaCl) and iodide (NaI), with salt concentrations up to 3 M. Enhanced sampling of the configuration space is achieved by the replica exchange method. In agreement with other works, we observe preferential sodium interactions with the peptide carbonyl groups, which are enhanced in the NaI solutions due to the increased affinity of the less hydrophilic iodide anion for the peptide methyl side-chains and terminal blocking groups. These interactions have been associated with a decrease in the helicities of more complex peptides. In our simulations, both salts have a small effect on the dipeptide, but consistently stabilize the intramolecular hydrogen-bonding interactions and "α-helical" conformations of the tetrapeptide. This behavior, and an analysis of the intermolecular interaction energies show that ion-peptide interactions, or changes in the peptide hydration due to salts, are not sufficient determining factors of the peptide conformational preferences. Additional simulations suggest that the observed stabilizing effect is not due to the employed force-field, and that it is maintained in short peptides but is reversed in longer peptides. Thus, the peptide conformational preferences are determined by an interplay of energetic and entropic factors, arising from the peptide sequence and length and the composition of the solution.

Validating affinities for ion-lipid association from simulation against experiment

Understanding biological membranes at physiological conditions requires comprehension of the interaction of lipid bilayers with sodium and potassium ions. These cations are adsorbed at palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers as indicated from previous studies. Here we compare the affinity of Na(+) and K(+) for POPC in molecular dynamics (MD) simulations with recent data from electrophoresis experiments and isothermal calorimetry (ITC) at neutral pH. NaCl and KCl were described using GROMOS or parameters matching solution activities on the basis of Kirkwood-Buff theory (KBFF), and K(+) was also described using parameters by Dang et al., all in conjunction with the Berger parameters for the lipids and the SPC water model. Apparent binding constants of GROMOS-Na(+) and KBFF-K(+) are the same within error and in good agreement with values from ITC. Although these force fields yield the same number of bound ions per number of lipids for Na(+) and K(+), they give a larger number of Na(+) ions per surface area compared to K(+), in agreement with the electrophoresis experiments, because Na(+) causes a stronger reduction in the area per lipid than K(+). The intrinsic binding constants, on the other hand, are reproduced by Dang-K(+) but overestimated by GROMOS-Na(+) and KBFF-K(+). That no ion force field reproduces the intrinsic and the apparent binding constant simultaneously arises from the fact that in MD simulations, implicitly meant to mimic neutral pH, pure PC is usually modeled with zero surface charge. In contrast, POPC at neutral conditions in experiment carries a low but significant negative surface charge and is uncharged only at acidic pH as indicated from electrophoretic mobilities. Implications for future simulation and experimental studies are discussed.

Electrohydrodynamics of soft polyelectrolyte multilayers: point of zero-streaming current

We report a comprehensive formalism for the electrokinetics (streaming current, I(str)) at soft multilayered polyelectrolyte films. These assemblies generally consist of a succession of permeable diffuse layers that differ in charge density, thickness, and hydrodynamic softness. The model, which extends one that we recently reported for the electrokinetics of monolayered soft thin films (Langmuir 2010, 26, 18169-18181), is valid without any restriction in the number and thickness of layers, or in the degree of dissociation and density of ionizable groups they carry. It further covers the limiting cases of hard and free draining films and correctly compares to semianalytical expressions derived for I(str) under conditions where the Debye-Hückel approximation applies. The flexibility of the theory is illustrated by simulations of I(str) for a two-layer assembly of cationic and anionic polymers over a large range of pH values and electrolyte concentrations. On this basis, it is shown that the point of zero streaming current (PZSC) of soft multilayered interphases, defined by the pH value where I(str) = 0, generally depends on the concentration of the (indifferent) electrolyte. The magnitude and direction of the shift in PZSC with varying salinity are intrinsically governed by the dissymmetry in protolytic characteristics and density of dissociable groups within each layer constituting the film, together with the respective film thickness and hydrodynamic softness. The fundamental effects covered by the theory are illustrated by streaming current measurements performed on two practically relevant systems, a polyelectrolyte bilayer prepared from poly(ethylene imine) (PEI) and poly(acrylic acid) (PAA) and a polymer-cushioned (PEI) bilayer lipid membrane.

Interactions of molecular ions with model phospholipid membranes

The affinities of a series of biologically relevant ions for a hydrated phospholipid membrane were investigated using molecular dynamics simulation. Interactions of molecular ions, such as guanidinium, tetramethylammonium, and thiocyanate with the bilayer were computationally characterized for the first time. Simulations reveal strong ion specificity. On one hand, ions like guanidinium and thiocyanate adsorb relatively strongly to the headgroup region of the membrane. On the other hand, potassium or chloride interact very weakly with the phospholipids and merely act as neutralizing counterions. Calculations also show that these ions affect differently biophysical properties of the membrane, such as lipid diffusion, headgroup hydration and tilt angle.

Intramolecular hydrogen bonding in articaine can be related to superior bone tissue penetration: a molecular dynamics study

Local anesthetics (LAs) are drugs that cause reversible loss of nociception during surgical procedures. Articaine is a commonly used LA in dentistry that has proven to be exceptionally effective in penetrating bone tissue and induce anesthesia on posterior teeth in maxilla and mandibula. In the present study, our aim was to gain a deeper understanding of the penetration of articaine through biological membranes by studying the interactions of articaine with a phospholipid membrane. Our approach involves Langmuir monolayer experiments combined with molecular dynamics simulations. Membrane permeability of LAs can be modulated by pH due to a titratable amine group with a pKa value close to physiological pH. A change in protonation state is thus known to act as a lipophilicity switch in LAs. Our study shows that articaine has an additional unique lipophilicity switch in its ability to form an intramolecular hydrogen bond. We suggest this intramolecular hydrogen bond as a novel and additional solvent-dependent mechanism for modulation of lipophilicity of articaine which may enhance its diffusion through membranes and connective tissue.

Direct imaging of salt effects on lipid bilayer ordering at sub-molecular resolution

The interactions of salts with lipid bilayers are known to alter the properties of membranes and therefore influence their structure and dynamics. Sodium and calcium cations penetrate deeply into the headgroup region and bind to the lipids, whereas potassium ions only loosely associate with lipid molecules and mostly remain outside of the headgroup region. We investigated a dipalmitoylphosphatidylcholine (DPPC) bilayer in the gel phase in the presence of all three cations with a concentration of Ca²+ ions an order of magnitude smaller than the Na+ and K+ ions. Our findings indicate that the area per unit cell does not significantly change in these three salt solutions. However the lipid molecules do re-order non-isotropically under the influence of the three different cations. We attribute this reordering to a change in the highly directional intermolecular interactions caused by a variation in the dipole-dipole bonding arising from a tilt of the headgroup out of the membrane plane. Measurements in different NaCl concentrations also show a non-isotropic re-ordering of the lipid molecules.

Partitioning and localization of environment-sensitive 2-(2'-pyridyl)- and 2-(2'-pyrimidyl)-indoles in lipid membranes: a joint refinement using fluorescence measurements and molecular dynamics simulations

Fluorescence of environment-sensitive dyes is widely applied to monitor local structure and solvation dynamics of biomolecules. It has been shown that, in comparison with a parent indole fluorophore, fluorescence of 2-(2'-pyridyl)-5-methylindole (5M-PyIn-0) and 2-[2'-(4',6'-dimethylpyrimidyl)]-indole (DMPmIn-0) is remarkably sensitive to hydrogen bonding with protic partners. Strong fluorescence, observed for these compounds in nonpolar and polar aprotic solvents, is efficiently quenched in aqueous solution. This study demonstrates that 5M-PyIn-0 and DMPmIn-0, which are almost nonemitting in aqueous solution, become highly fluorescent upon titrating with phospholipid vesicles. The fluorescence enhancement is accompanied by a significant blue shift of emission maximum. The Gibbs free energy of membrane partitioning, measured by the increase in the steady-state fluorescence intensities during transfer from an aqueous environment to a lipid bilayer, is very favorable for both compounds, being in a range from -7.1 to -8.0 kcal/mol and depending only slightly on lipid composition of the membrane. The fluorescence enhancement upon membrane partitioning is indicative of the loss of the specific hydrogen-bonding interactions between the excited fluorophore and water molecules, causing efficient fluorescence quenching in bulk water. This conclusion is supported by atomistic molecular dynamics (MD) simulations, demonstrating that both 5M-PyIn-0 and DMPmIn-0 bind rapidly and partition deeply into a lipid bilayer. MD simulations also show a rapid, nanosecond-scale decrease in the probability of solute-solvent hydrogen bonding during passive diffusion of the probe molecules from bulk water into a lipid bilayer. At equilibrium conditions, both 5M-PyIn-0 and DMPmIn-0 prefer deep localization within the hydrophobic, water-free region of the bilayer. A free energy profile of penetration across a bilayer estimated using MD umbrella sampling shows that both indole derivatives favor residence in a rather wide potential energy well located 10-15 Å from the bilayer center.