Swelling of phospholipids by monovalent salt

Abstract

Critical to biological processes such as membrane fusion and secretion, ion-lipid interactions at the membranewater interface still raise many unanswered questions. Using reconstituted phosphatidylcholine membranes, we confirm here that multilamellar vesicles swell in salt solutions, a direct indication that salt modifies the interactions between neighboring membranes. By varying sample histories, and by comparing with data from ion carrier-containing bilayers, we eliminate the possibility that swelling is an equilibration artifact. Although both attractive and repulsive forces could be modified by salt, we show experimentally that swelling is driven primarily by weakening of the van der Waals attraction. To isolate the effect of salt on van der Waals interactions, we focus on high salt concentrations at which any possible electrostatic interactions are screened. By analysis of X-ray diffraction data, we show that salt does not alter membrane structure or bending rigidity, eliminating the possibility that repulsive fluctuation forces change with salt. By measuring changes in interbilayer separation with applied osmotic stress, we have determined, using the standard paradigm for bilayer interactions, that 1 M concentrations of KBr or KCl decrease the van der Waals strength by 50%. By weakening van der Waals attractions, salt increases energy barriers to membrane contact, possibly affecting cellular communication and biological signaling. From bacteria to mammals, biological processes take place in salt solutions. Maintaining a delicate balance of ions, biomembranes interact preferentially with different ionic species (1). Specific ionic effects have been shown to influence the growth rates of bacteria (2) and fungi (3,4) and to affect the function of antibiotic channels (5). This investigation will focus on the halide salts because of their presence in intracellular and intercellular fluids. At biomembrane surfaces, halides encounter not only charged lipid species, such as phosphatidylserine and phosphatidylinositol, but more often common neutral phosphatidylcholines (PCs). Interactions with both types of lipids require investigation: by affecting lipid interactions, salt solutions modulate biological processes such as fusion and secretion.Despite being electrically neutral, PC membranes attract one another as a result of mutually induced charge fluctuations (6). Because of different dielectric properties of membranes and the intervening solvent, transient spontaneous electromagnetic fields in one membrane induce correlated fields in the neighboring membrane and vice versa, resulting in an attractive force. This "charge fluctuation" (van der Waals) force is responsible for the spontaneous formation of stable multilamellar structures [e.g., myelin sheets in vivo and multilamellar vesicles (MLVs) in vitro]. When the attractive van der Waals force is exactly balanced by repulsive forces, an equilibrium spacing between lamellae is established (6-9). Typically, as measured by small-angle X-ray scattering, the...

Ion Dynamics in Cationic Lipid Bilayer Systems in Saline Solutions

“…Experiments have shown that anionic 4, [22][23][24][25] and cationic 26,27 membranes interact readily with their counterions (especially divalent ones), whereas the interactions of zwitterionic lipid bilayers with salt ions appear rather sensitive to the size and valency of ions. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] As for molecular-level computational studies, the increase in computing power in the past few years has made it possible to extend computer simulations beyond the relatively long relaxation times of tens to hundreds of nanoseconds required for equilibration of ions in lipid/water systems. Although most computational studies by far have focused on the effects of salt ions on zwitterionic (neutral) lipid bilayers, 33,[42][43][44][45][46][47][48][49][50][51][52][53] there is also an increasing number of studies on anionic [54][55][56][57][58][59] and cationic 60,61 lipid bilayers.…”

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

“…Treatment of the forces competing under saline conditions is no trivial exercise. As verified experimentally, salts can quickly equilibrate between external solution and the interlamellar space (18). However, because of competition between salt ions and lipid headgroups for interfacial water, incorporation of salt is only partial.…”

“…In addition to separability, the exact functional form of these forces, especially for hydration and fluctuation, is still under debate. Although the exact values of interaction parameters obviously will depend on the choice of functional forms, the observed modification by salt is robust: weakening of vdW by monovalent salt is computed at Ϸ50% either using Helfrich power-law form for the fluctuation force (this work) or using an empirically determined exponential form (18). Once nonspecific vdW screening by salt is taken into account, specific ion adsorption, modeled at the level of additive molecular forces between bilayers, can be reconciled by assuming an effective weak binding of polarizable ions as described in the Hofmeister series.…”

“…Samples were thermally equilibrated at room temperature before being x-rayed for 0.5-1 h with a fine-focus fixed Cu anode x-ray source. X-ray exposures were typically taken at 25°C, 15°C, 35°C, and again at 25°C with variations in equilibration times (18). Sharp, uniform scattering rings were obtained indicative of sample homogeneity upon equilibration.…”

“…As measured by small-angle x-ray scattering, multilamellar DLPC vesicles swell progressively with added salt. However, the swelling is highly ion-specific: significantly more swelling is measured for Br salts compared with Cl (18,20,21). To distinguish between concurrent modification of vdW and electrostatic forces, we employ a charge-regulation model that calculates self-consistently the partitioning of salt-ions between membrane binding sites and the diffuse double layer (22,23).…”