Hydration force between phosphatidylcholine surfaces in aqueous electrolyte solutions

Formation of Supported Lipid Bilayers (SLBs) from Buffers Containing Low Concentrations of Group I Chloride Salts

Supported lipid bilayers (SLBs) are a useful tool for studying the interactions between lipids and other biomolecules that make up a cell membrane. SLBs are typically formed by the adsorption and rupture of vesicles from solution. Although it is known that many experimental factors can affect whether SLB formation is successful, there is no comprehensive understanding of the mechanism. In this work, we have used a quartz crystal microbalance (QCM) to investigate the role of the salt in the buffer on the formation of phosphatidylcholine SLBs on a silicon dioxide (SiO₂) surface. We varied the concentration of sodium chloride in the buffer, from 5 to 150 mM, to find the minimum concentration of NaCl that was required for the successful formation of an SLB. We then repeated the experiments with other group I chloride salts (LiCl, KCl, and CsCl) and found that at higher salt concentrations (150 mM) SLB formation was successful for all of the salts used, and the degree of deformation of the adsorbed vesicles at the critical vesicle coverage was cation-dependent. The results showed that at an intermediate salt concentration (50 mM) the critical vesicle coverage was cation-dependent and at low salt concentrations (12.5 mM) the cation used determined whether SLB formation was successful. We found that the successful formation of SLBs could occur at lower electrolyte concentrations for KCl and CsCl than it did for NaCl. To understand these results, we calculated the magnitude of the vesicle-surface interaction energy using the Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended-DLVO theory. We managed to explain the results obtained at higher salt concentrations by including cation-dependent surface potentials in the calculations and at lower salt concentrations by the addition of a cation-dependent hydration force. These results showed that the way that different cations in solution affect the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-SiO₂ surface interaction energy depends on the ionic strength of the solution.

A Study of the Effects of Plasma Surface Treatment on Lipid Bilayers Self-Spreading on a Polydimethylsiloxane Substrate under Different Treatment Times

Plasma-treated poly(dimethylsiloxane) (PDMS)-supported lipid bilayers are used as functional tools for studying cell membrane properties and as platforms for biotechnology applications. Self-spreading is a versatile method for forming lipid bilayers. However, few studies have focused on the effect of plasma treatment on self-spreading lipid bilayer formation. In this paper, we performed lipid bilayer self-spreading on a PDMS surface with different treatment times. Surface characterization of PDMS treated with different treatment times is evaluated by AFM and SEM, and the effects of plasma treatment of the PDMS surface on lipid bilayer self-spreading behavior is investigated by confocal microscopy. The front-edge velocity of lipid bilayers increases with the plasma treatment time. By theoretical analyses with the extended-DLVO modeling, we find that the most likely cause of the velocity change is the hydration repulsion energy between the PDMS surface and lipid bilayers. Moreover, the growth behavior of membrane lobes on the underlying self-spreading lipid bilayer was affected by topography changes in the PDMS surface resulting from plasma treatment. Our findings suggest that the growth of self-spreading lipid bilayers can be controlled by changing the plasma treatment time.

Salt effects on lamellar repeat distance depending on head groups of neutrally charged lipids

Change in lamellar repeat distances of neutrally charged lipids upon addition of monovalent salts was measured with small-angle X-ray scattering for combinations of two lipids (PC and PE lipids) and six salts. Large dependence on lipid head group is observed in addition to those on added cation and anion. The ion and lipid dependences have little correlation with measured surface potentials of lipid membranes. These results indicate that the lamellar swelling by salt is not explained through balance among interactions considered previously (van der Waals interaction, electrostatic repulsion emerged by ion binding, etc.). It is suggested that effect of water structure, which is affected by not only ions but also lipid itself, should be taken into account for understanding membrane-membrane interactions, as in the Hofmeister effect.

Interaction of multiwalled carbon nanotubes with supported lipid bilayers and vesicles as model biological membranes

The influence of solution chemistry on the kinetics and reversibility of the deposition of multiwalled carbon nanotubes (MWNTs) on model biological membranes was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Supported lipid bilayers (SLBs) comprised of zwitterionic 1,2-dioleoyl-sn-glyero-3-phosphocholine (DOPC), as well as DOPC vesicles, were used as model cell membranes. Under neutral pH conditions, the deposition kinetics of MWNTs on SLBs increased with increasing electrolyte (NaCl and CaCl2) concentrations. In the presence of NaCl, favorable deposition was not achieved even at a concentration of 1 M, which is attributed to the presence of strong repulsive hydration forces due to the highly hydrophilic headgroups of SLBs. Conversely, favorable deposition was observed at CaCl2 concentrations above 0.5 mM when the charge of SLBs was reversed from negative to positive through the binding of Ca(2+) cations to the exposed phosphate headgroups. Favorable nanotube deposition was also observed at pH 2, at which the DOPC SLBs exhibited positive surface charge, since the isoelectric point of DOPC is ca. 4. When MWNTs on SLBs were rinsed with low ionic strength solutions at pH 7.3, only ca. 20% of deposited nanotubes were released, indicating that nanotube deposition was mostly irreversible. The deposition of MWNTs on DOPC vesicles under favorable deposition conditions did not result in any detectable leakage of solution from the vesicles, indicating that MWNTs did not severely disrupt the DOPC bilayers upon attachment.

Phosphatidylcholine — Mg2+ equilibria in a monolayer at the air/water interface

Abstract The interaction between Mg2+ and a phosphatidylcholine (lecithin, L) monolayer at the air/water interface was investigated. Surface tension measurements (Langmuir method) of phosphatidylcholine monolayers as a function of Mg2+ concentration were carried out at 22°C using a Teflon trough and a Nima 9000 tensiometer. Interactions between phosphatidylcholine and Mg2+ result in significant deviations from additivity. An equilibrium theory was developed to obtain the stability constants and areas occupied by one molecule of LMg+ and L2Mg. The stability constants were K 1 = 9.95×102 m2 mol−1 and K 2 = 3.87×104 m2 mol−1. The area occupied by LMg+ is 77 Å2 molecule−1, while that occupied by L2Mg is 109 Å2 molecule−1. The Gibbs free energies of complexation for LMg+ and L2Mg are −16.91 ± 0.51 and −25.88 ± 0.76 kJ mol−1. Graphical abstract

A quantitative analytical method to test for salt effects on giant unilamellar vesicles

Today, free-standing membranes, i.e. liposomes and vesicles, are used in a multitude of applications, e.g. as drug delivery devices and artificial cell models. Because current laboratory techniques do not allow handling of large sample sizes, systematic and quantitative studies on the impact of different effectors, e.g. electrolytes, are limited. In this work, we evaluated the Hofmeister effects of ten alkali metal halides on giant unilamellar vesicles made of palmitoyloleoylphosphatidylcholine for a large sample size by combining the highly parallel water-in-oil emulsion transfer vesicle preparation method with automatic haemocytometry. We found that this new quantitative screening method is highly reliable and consistent with previously reported results. Thus, this method may provide a significant methodological advance in analysis of effects on free-standing model membranes.

Effects of monovalent anions of the hofmeister series on DPPC lipid bilayers Part I: swelling and in-plane equations of state

Aiming to improve understanding of the mechanisms behind specific anion effects in biological systems we have studied the effects of sodium salts of simple monovalent anions belonging to the Hofmeister series on the bilayers of the zwitterionic lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine using small-angle x-ray scattering and the osmotic stress technique. NaCl, NaBr, NaNO(3), NaI, and NaSCN were used in this investigation. The electrolytes were found to swell the bilayers and to increase the area per lipid headgroup at each value of the osmotic pressure, suggesting the association of anions with the bilayer-lipid interfaces. The effects follow the Hofmeister series with SCN(-) inducing the most pronounced changes. "Ion competition" experiments with mixed NaI/NaCl solutions at total salinity 0.1 and 0.5 M revealed that the effect of ions on the lipid equation-of-state is roughly linear at low concentrations, but strongly nonlinear at high concentrations. The experimental results are fitted in a companion article to provide "binding" or "partitioning" constants of anions in the lipid bilayers.

Effects of monovalent anions of the hofmeister series on DPPC lipid bilayers Part II: modeling the perpendicular and lateral equation-of-state

The effects of Hofmeister anions on the perpendicular and lateral equation-of-state (EOS) of the dipalmitoylphosphatidylcholine lamellar phase discussed in the companion article are here examined using appropriate free energy models for the intra- and interbilayer interactions. Minimizing the free energy with respect to the two basic geometrical parameters of the lamellar phase, which are the interbilayer water thickness, d(w), and the lipid headgroup area, a(L), provides the perpendicular (osmotic pressure balance) and lateral EOS. Standard models were used for the hydration, undulation, and Van der Waals attractive force between the bilayers in the presence of electrolytes whereas two alternative treatments of electrostatic interactions were used to obtain "binding" or "partitioning" constants of anions to the lipid bilayers both in the absence and in the presence of sodium binding. The computed binding constants depend on anion type and follow the Hofmeister series, but were found to increase with electrolyte concentration, implying that the local binding approximation cannot fit bilayer repulsion data. The partitioning model was also found inadequate at high electrolyte concentrations. The fitting attempts revealed two additional features worthy of future investigation. First, at maximum swelling in the presence of electrolytes the osmotic pressure of the bilayer system cannot be set equal to zero. Second, at high salt concentrations an additional repulsion appears to come into effect in the presence of strongly adsorbing anions such as I(-) or SCN(-). Both these phenomena may reflect an inconsistent treatment of the ion-surface interactions, which have an impact on the osmotic pressure. Alternatively, they may arise from bulk solution nonidealities that cannot be handled by the classical Poisson-Boltzmann formalism. The inability of current models to explain the "lateral" EOS by fitting the area per lipid headgroup as a function of salt type and concentration shows that current understanding of phospholipid-ion interactions is still very incomplete.

Cell hybridization by electrofusion on filters

Electric field pulses induce permeabilization and associated fusogenicity in cell membranes. Electrofusion of cells is usually performed in two steps: the first is the creation of close intercellular contacts; the second is an application of electric pulses that induces membrane fusion. Very large cell contacts can be obtained by a filter aspiration method. A cell monolayer is created by controlled suction on biocompatible filter. No spontaneous fusion results. Just after filtration, electrofusion is obtained by field pulses applied parallel to the filter. Cell viability is not strongly affected and cells recover their spherical shape in the minute time range after filtration. The electrical parameters, the cell density, and the flow rate control fusion. Fusion is obtained with cells of different origins with very different adhesion properties. Hybrid cells are easily formed. This approach appears to be a very efficient method for cell hybridization with an easy-to-use protocol.

Fluorescence characteristics of pyrene and phosphatidylethanolamine-bound pyrene incorporated into lipid vesicles solubilized in media of differing NaCl concentrations

We used the excimer/monomer ratio of pyrene (PY) and N-(1-pyrenesulfonyl)dipalmitoyl-L-alpha-phosphatidylethanolamine (DPPE-PY) fluorescence intensities (IE/IM), and the polarity ratio I/III to investigate the state of the polar head group region of small, unilamellar phosphatidylcholine vesicles (SUV-PC) solubilized in media of differing NaCl concentrations. PY or DPPE-PY excimer formation resulting from vesicles' collisions is not affected by the presence of monovalent ions. In addition, the ionic strength does not alter the dielectric environment in the neighborhood of PY incorporated into SUV-PC. Since IE/IM of both PY and DPPE-PY is insensitive to variations in the ionic strength, we conclude that the probes' diffusion in SUV-PC, and consequently the membrane fluidity, are independent of NaCl concentration at least up to 0.5 M. The vesicles' concentration in the aqueous solution was the only factor which induced a rise of IE/IM. To explain the results in the context of the transient-fusion model developed previously (G.P. L'Heureux and M. Fragata, Biophys. Chem. 30 (1988) 293) and the hypothesis of repulsive hydration forces, we postulate a heterogeneous distribution of dehydrated domains, or contact areas, on the outer surfaces of colliding vesicles.

Ionic-strength modulation of electrically induced permeabilization and associated fusion of mammalian cells

Application of a high electric field to cells in culture has been shown to make them both permeable and fusogenic. The molecular events involved in the phenomenon are still poorly understood. In this study we investigated the effects of the ionic strength of the pulsing buffer on the electropermeabilization and electrofusion of Chinese hamster ovary cells. Increasing the ionic strength of the pulsing medium results in an increase in sieving of transient permeant structures, but decreases the fusion index. Treatment of cells with trypsin or pronase before application of the pulses abolishes the ionic modulation of both electropermeabilization and electrofusion. A similar rate of expansion of permeabilization is obtained whatever the ionic content of the pulsing buffer, and cells fuse even at high ionic strength. This observation lends support to our hypothesis that membrane proteins play a role in electrofusion.

Interaction of 1-anilinonaphthalene 8-sulfonic acid with interfaces containing cerebrosides, sulfatides and gangliosides

The fluorescence lifetime, quantum yield and emission spectra of 1-anilinonaphthalene 8-sulfonic acid (ANS) associated with interfaces of pure dipalmitoylphosphatidylcholine or its mixtures with phosphatidylserine, galactosylceramide, sulfatide or gangliosides GM1 and GD1a were studied at low and high ionic strength. Modification of the molecular organization of the lipid interfaces in the presence of the probe was also studied with mixed lipid monolayers. ANS has little affect on the intermolecular packing of the lipids but influences their surface potential, consistent with a location of ANS in the polar head group region of the interface. ANS senses a more polar microenvironment when associated with interfaces containing anionic glycosphingolipids at low ionic strength but, except for interfaces containing phosphatidylserine, it detects approximately the same polarity for neutral or anionic interfaces in 0.25 M NaCl.

Monovalent ion-phosphatidylcholine interactions: an electron paramagnetic resonance study

The apparent Mn2+ binding constant for L-alpha-dipalmitoylphosphatidylcholine (DPPC) bilayers dispersed in monovalent salt and MnCl2 dispersions was determined as a function temperature using electron paramagnetic resonance (ERP). Reproducibility in the data sets requires the use of a standard salt solution and dual cavity techniques. Changes in the binding constant at different phase states and temperatures were observed and correlated to the influence of monovalent salts on the thermal properties of DPPC. The turning points (i.e. changes in slope) in the curves of the apparent Mn2+ binding constant versus temperature can be understood in terms of differences in ion binding to headgroups with different bilayer surface areas. The influence of Li+ and SCN- on Mn2+ binding is viewed as a function of their presence in the ionic media in contact with the bilayer rather than as a competitive event. Other monovalent ions studied appear to have little effect on the measured apparent Mn2+ binding constants for DPPC headgroups.

Thiocyanate and bromide ions influence the bilayer structural parameters of phosphatidylcholine bilayers

The influence of monovalent cations and anions on the structural parameters of dipalmitoylphosphatidylcholine (DPPC) bilayers was examined at 25 degrees C using X-ray diffraction. It was shown that monovalent salts, in general, have little effect on lipid packing within the bilayer. However, fully hydrated DPPC bilayers in 1 M KSCN pack in an interdigitated acyl chain phase. This is the first observation of an ion-induced interdigitated bilayer phase in a zwitterionic lipid. In addition, gel state DPPC bilayers in 1 M KBr imbibe approx. 10 A more solvent than bilayers in water. The influence of these same salts on the phase transitions of DPPC bilayers was also examined using high-resolution differential scanning calorimetry. These results are discussed in terms of ion-induced changes in solvent and solvent/bilayer structure.

The influence of ion species on phosphatidylcholine bilayer structure and packing

The effects of various monovalent cations and anions on the bilayer packing and structure of dipalmitoylphosphatidylcholine were studied using X-ray diffraction and differential scanning calorimetry. It was observed from the X-ray diffraction studies that monovalent salts, in general, have no effect on bilayer packing. The results of DSC studies on metal chloride systems are consistent with the interpretation that cations in general and Li+ in particular bind to DPPC bilayers. The effect of potassium salts on pre- and main-transition temperatures suggest that anions, such as Acetate-, also significantly bind to DPPC head groups.