DIRECT INTERACTION FORCE MEASUREMENTS BETWEEN BILAYERS

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The stability and transformation of phospholipid membranes, surfactant microstructures and other macromolecules are controlled by the balance between attractive and repulsive forces when these colloidal materials are separated by very small distances (5 to 100 angstroms). A fundamental understanding of many important biological processes including membrane cell adhesion and fusion, vesicle formation and antibody-receptor recognition requires a knowledge of the nature of these short-range colloidal forces, how they depend on distances and how they can be modified by changes in surface morphology. The surface force apparatus permits these colloidal forces to be directly measured with a distance resolution of + or - 1 angstrom from contact to large separation (10,000 angstroms). The molecularly smooth mica used in the force measurements will be coated with a monolayer of polymerized surfactant to generate a flat molecularly smooth hydrocarbon-water interface. The first series of experiments will involve force vs. distance measurements for the two hydrocarbon surfaces as they are moved together in water and other polar solvents. This will establish the nature of hydrophobic forces which are responsible for the aggregation of surfactants and lipids to form microstructures (micelles, bilayers, membranes, vesicles, microtubules, etc.). In a second series of measurements, phospholipid monolayers will be adsorbed onto the hydrocarbon substrate and the interaction forces for various lipid head groups will be determined. The approach of the two phospholipid monolayers is equivalent to bringing together two planar bilayer membranes. The goal is to obtain evidence for a change from very strong repulsive hydration forces which stabilize membranes to equally strong attractive hydration forces which lead to membrane fusion. In a third series of experiments, myelin basic protein and blood factor V will be absorbed onto the lecithin monolayer and the adhesion of bilayers resulting from lipid-protein(-protein)-lipid interaction directly measured. Such interactions are important in stabilizing the multilayered membrane structure that protects the nerve axon. The information provided by the surface force apparatus is unique, the only such apparatus in the U.S. is located at the University of Minnesota.