Submitted on September 8, 2005
Revised on November 1, 2005
Accepted on November 2, 2005
Swelling of phospholipids by monovalent salt
Horia I. Petrache, Stephanie Tristram-Nagle, Daniel Harries, Norbert Kucerka, John F. Nagle, and V. Adrian Parsegian
Laboratory of Physical and Structural Biology, National Institutes of Health, Bethesda, MD 20892-0924
Corresponding Author: petrachh{at}mail.nih.gov
Critical to biological processes such as membrane fusion and secretion, ion-lipid interactions at the membrane-water interface still raise many unresolved 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. While both attractive and repulsive forces could be modified by salt, we show experimentally that swelling is primarily driven by weakening of the van der Waals (vdW) attraction. To isolate the effect of salt on vdW interactions, we focus on high salt concentrations where 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 1M concentrations of KBr or KCl decrease the vdW strength by 50%. By weakening vdW attractions, salt increases energy barriers to membrane contact, possibly affecting cellular communication and biological signaling.
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