Ion Exclusion from Multilamellar Lipid Vesicles

Abstract

We have shown previously that ions and zwitterionic pH buffers affect the interaction of neutral lipid membranes by a mechanism that involves both a reduction of van der Waals attraction and a creation of electrostatic repulsion. The electrostatic repulsion occurs due to binding of ions and zwitterions to phosphatidylcholine lipid headgroups. To properly analyze lipid interactions, we need to know how salt ions and buffer molecules partition between lipid bilayers and open solution. To address this issue, we use a sequence of solute concentrations to achieve neutral buoyancy (density matching) for suspensions of dilauroylphosphatidylcholine (DLPC) lipid membranes. We then calculate the ratio of solute concentrations outside and inside the multilamellar lipid vesicles from this density match point. We find that distinct series of monovalent salts, organic salts, and zwitterionic pH buffers are excluded from the interlamellar space, with more polarizable solutes being excluded less, in accord with measurements on membranes interactions. Our quantitative measurements are important for a proper analysis of ionic interactions in membrane systems with applications to membrane biology.