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