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Journal of Lipid Research, Vol. 47, 302-309, February 2006
Copyright © 2006 by American Society for Biochemistry and Molecular Biology

Swelling of phospholipids by monovalent salt

Horia I. Petrache^1,*, Stephanie Tristram-Nagle, Daniel Harries^*, Norbert Kuerka, John F. Nagle^, and V. Adrian Parsegian^*

^* Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-0924
Biological Physics Group, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213
Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213

Published, JLR Papers in Press, November 2, 2005.

¹ To whom correspondence should be addressed. e-mail: 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 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.

Supplementary key words bending rigidity • halides • hydration • ion binding • lipid head group • membrane interactions • solvation

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