Physical force considerations in model and biological membranes

We have emphasized the general lessons learned by measuring the forces between and within many kinds of phospholipid bilayers. Interbilayer forces within about 30 A separation (1 A = 0.1 nm) are dominated by a strong hydration repulsion that we believe exists for all interacting hydrophilic surfaces. This hydration can be characterized by the polarization properties of water. Its decay with distance is common to all hydrophilic surfaces, while its strength is characteristic of the specific surface groups. Interactions at distances greater than 30 A between surfaces charged by adsorbed ions or by dissociation of surface groups qualitatively follow expectations for electrostatic double-layer forces, but quantitatively reveal an influence by the long-range hydration on electrostatic double layers. Bilayer lateral pressures and compressibilities, measured at thermodynamic equilibrium, are nonlinear and highly dependent on lipid species. Comparison with monolayers is not possible, since the latter are usually metastable states. In applying these results to contact and fusion of phospholipid vesicles, we define weak and strong regimes of interbilayer force, the former being too weak to overcome the hydration barrier. The special case of divalent cation binding with acidic phospholipids provides sufficient attraction to compete with hydration repulsion. However, such attraction will distort vesicles to rupture. Any observed fusion in such systems may be incidental to this destruction. This suggests that the control of biological membrane fusion, divalent cation antagonism, and postfusion stability is lacking in these systems. Biological fusion more likely involves focal biochemical change simultaneous with reduction of the hydration forces.