Forces between bilayers

Abstract

Direct measurements are reported of the full interbilayer force laws (force vs. distance) between bilayers of zwitterionic phosphatidylcholines (PC) and phosphatidylethanolamines (PE), uncharged galactolipids, anionic phosphatidylglycerols (PG), and cationic dioctadecyldimethylammonium surfactants (DOA) in aqueous electrolyte solutions. Bilayers were in each case deposited on molecularly smooth mica surfaces and the inter bilayer forces then measured at a distance resolution of 1-2 A. Three types of forces were identified: attractive van der Waals forces, repulsive electrostatic double-layer forces, and (at short range) repulsive steric-hydration forces. Accurate measurements have been made of the van der Waals forces between uncharged bilayers. In high salt, the van der Waals force between galactolipid bilayers is screened to about half its strength in pure water, which agrees with the theoretical prediction. On the other hand, the van der Waals force between uncharged PE or PC bilayers is already quite weak in pure water. It is proposed that the high concentration of ionic groups between the bilayer surfaces may already screen the van der Waals force in pure water. Double-layer forces between bilayers arise when the bilayers carry a net surface charge. This occurs when the arnphiphile headgroups are ionized or, in case they are zwitterionic, when ion binding takes place. From the measured double-layer forces as a function of the bilayer separation and electrolyte concentration, the binding of various cations to PB, PC and PG, and the binding of various anions to DOA surfactants is investigated. Excellent agreement of the measured double-layer forces with theory is obtained. Slight deviations only occur at surface separations less than 25 A, which might be related to discrete charge effects and/or ion-ion correlation effects. A short-range steric-hydration repulsion was observed only between uncharged galactolipid bilayers and zwitterionic phospholipid bilayers. The short-range repulsion, which balances the van der Waals force at separations of 10-30 A, is apparently due to a cmnbination of hydration and steric repulsion, the latter arising from thcrrnal motions of headgroups and thickness fluctuations of fluid bilayers. No repulsive hydration forces were measured between the ionic PG and DOA bilayers. It is shown that for two bilayers in "contact" at. their equilibrium separation, their adhesion energies vary on addition of salt due to changes in the repulsive double-layer and hydration forces rather than to a change in the attractive van der Waals force. Also, it is concluded that bilayer fusion is not simply related to the interbilayer force-law, but must be related to a structural instability of the membrane (see also Horn (1984)). An idea about the relative intrabilayer interactions and the equilibrium headgroup area of the amphiphiles in bilayers is obtained by comparing the monolayer compression isotherms for different electrolyte solutions. It emerged that a correlation between interbilayer and intrabilayer interactions can only be drawn to a limited extend, which is mainly due to the complexity of the short-range forces and counterion specificities involved in the intrabilayer interactions. Show more