Adhesion between Phosphatidylethanolamine Bilayers

Abstract

The goal of this study is to provide additional information on the short-range interactions that determine the adhesion energy between bilayer surfaces. The specific problem concerns the hydration properties of bilayers of the membrane lipid phosphatidylethanolamine (PE), which imbibe much less water than bilayers composed of the other common ziwtterionic membrane lipid, phosphatidylcholine (PC). The osmotic stress/X-ray diffraction method was used to measure pressure−distance relations for PE and PC bilayers containing known mole fractions of the charged lipid phosphatidic acid (PA). The addition of 20 mol % PA to either PC or PE bilayers swelled the bilayers by an amount predictable from electrostatic double-layer theory. However, whereas the addition of 5 mol % PA disjoined PC bilayers, it did not change the fluid space between PE bilayers. By calculating the magnitude of the electrostatic pressure necessary to disjoin the bilayers, we estimate the adhesion energy for gel phase PE bilayers to be about −0.7 erg/cm2, a value considerably larger than previously measured values for gel phase PC bilayers. The magnitude of the adhesion energy indicates that, in addition to the attractive van der Waals pressure, there is another attractive pressure between adjacent PE layers that prevents them from swelling to the extent of PC bilayers. We argue that a small fraction of direct electrostatic interbilayer interactions or indirect hydrogen-bonded water interactions between the N+H3 group in one bilayer and the PO4- group in the apposing bilayers could account for the additional attractive interactions in PE bilayers.