The interaction forces and fusion mechanisms of mixed zwitterionic-anionic phospholipid bilayers were measured with the surface forces apparatus. The bilayers were 3:1 mixtures of either dimyris-toylphosphatidylcholine and dimyristoylphosphatidylglycerol (DMPC/DMPG) or dilauroylphosphatidyl-choline and dilauroylphosphatidylglycerol (DLPC/DLPG), and experiments were carried out in NaCl solutions with and without CaCl2. In NaCl solutions, the forces between either mixed bilayer system were consistent with the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory of repulsive electrostatic and attractive van der Waals forces, and fusion did not occur. At high pH (>6) and in high (20 mM) NaCl concentrations, a short-range hydration force extending about 13 Å was evident, indicative of Na+ binding to the surfaces. In the presence of this large hydration repulsion, the interbilayer adhesion was abolished. When CaCl2 was added to the bathing solutions in the presence or absence of NaCl, the bilayers phase separate into small domains, coinciding with the occurrence of a large, long-range attractive force. Fusion occurred readily between the more fluid domains. The phase separations and fusion events could be directly visualized by observing the shapes of the optical fringes used to measure the surface separation and the change in surface profiles with time. The ease of fusion between mixed bilayers in the presence of calcium correlated closely with the strength of the long-range attractive force. This force is attributed to the additional hydrophobic force between domains or domain boundaries due to the exposure of excess hydrophobic groups resulting from the Ca2+-induced condensation of the PG- headgroups. Fusion is, therefore, attributed to the enhanced hydrophobicity of phase-separated domains and only indirectly to ion binding, to an enhanced adhesion, or to dehydration effects. These results provide the first direct evidence of a molecular relationship between calcium-induced phase separation and bilayer fusion.
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