SHORT-RANGE PRESSURES BETWEEN LIPID BILAYER-MEMBRANES

For many years a large, short-range repulsive interaction has been obs erved between a variety of hydrated surfaces. The physical origin of t his ubiquitous interaction has been controversial. In the case of lipi d bilayers, proposed mechanisms include a hydration pressure, due to w ater polarization and/or hydrogen-bond reorientation by the bilayer su rface, and several types of entropic (steric) pressures, due to motion of individual lipid molecules or undulations of the entire bilayer. T his review focuses on a number of recent osmotic stress/X-ray diffract ion experiments performed with phosphatidylcholine bilayers designed t o determine the distance range where each of these pressures dominates . At very short interbilayer separations (less than about 4 Angstrom), the pressure-distance curve depends on the volume fraction of head gr oups at the interface, indicating the presence of a large steric barri er arising from direct interactions between head groups from opposing bilayers. The range of this steric pressure can be increased by the ad dition of lipids with larger head groups, such as glycolipids or lipid s with covalently attached polymers (polyethylene glycol lipids). For intermediate interbilayer separations (about 4-10 Angstrom), the press ure-distance curves are similar for liquid-crystalline and crystalline phosphatidylcholine bilayers, the pressure-fluid spacing relationship is nearly independent of temperature, and the magnitude of the pressu re depends on the dipole potential. In this range of fluid spacings we argue that the pressure can be best accounted for by a hydration pres sure. For interbilayer spacing greater than about 10 Angstrom, the mag nitude and range of the observed pressure depend on temperature and on the bending modulus of the bilayer. These observations provide compel ling evidence for the presence of a longer-range undulation pressure, that markedly influences the hydration properties of phospholipid bila yers.