SHORT- AND LONG-RANGE FORCES BETWEEN MEMBRANCE SURFACES

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The overall goal of this application is to measure and characterize three fundamental, non-specific interactions that operate between macromolecular assemblies in water. The magnitude and range of the hydration, steric, and undulation repulsive pressures will be measured between lipid bilayer membranes by x-ray diffraction analysis of osmotically stressed liposomes. Electron density profiles will be used to obtain a moderate resolution view of the bilayer at each applied osmotic pressure. Bilayer membranes have been chosen for this study for two reasons. First, the composition, structure, cohesive properties, and electrostatic potential of the membranes can be precisely controlled so that key physical parameters of each repulsive pressure can be investigated and models for their origin tested. For example, the hypothesis win be tested that the hydration pressure is caused by the polarization of interlamellar water by electric fields of the. bilayer. Second, the repulsive interactions between membrane surfaces are critical in many physiological processes. In particular, these pressures will be measured for bilayers containing various glycolipids or containing lipids with covalently attached polyethylene glycol (PEG). Since glycolipids are thought to mediate many interactions of cells with their surroundings, experiments are designed to determine the structure and interactive properties of glycolipids in membranes. PEG-lipids extend blood Circulation time of liposomes, thereby allowing drug delivery to solid tumors. Experiments are designed to test the hypothesis that this increased circulation time is due to steric stabilization caused by the PEG-lipid. Proposed experiments are also aimed at increasing the steric barrier by modifying the cohesive properties of the bilayer matrix. Other techniques that will be used to address these hypotheses and obtain as complete a picture as possible of the three intersurface forces include: (1) water adsorption isotherms to determine the water content of the membrane and the energy of hydration, (2) micropipette manipulation of single-walled vesicles to obtain the adhesion energy and the cohesive properties of the membrane, (3) osmotic stress experiments performed as a function of temperature to provide the enthalpic and entropic contributions of the component pressures, (4) surface and zeta potential measurements to obtain bilayer electric fields, (5) surface pressure-area monolayer isotherms to obtain the in-plane interactions between the PEG-lipids, and (6) differential scanning calorimetry to obtain the phase behavior of bilayers containing PEG-lipids.