Membrane fusion has been examined in a model system of small unilamellar vesicles of synthetic lipids that can be oligomerized through the lipid headgroups. The oligomerization can be induced either in both bilayer leaflets or in the inner leaflet exclusively. Oligomerization leads to denser lipid headgroup packing, with concomitant reduction of lipid lateral diffusion and membrane permeability. As evidenced by lipid mixing assays, electron microscopy, and light scattering, calcium-induced fusion of the bilayer vesicles is strongly retarded and inhibited by oligomerization. Remarkably, oligomerization of only the inner leaflet of the bilayer is already sufficient to affect fusion. The efficiency of inhibition and retardation of fusion critically depend on the relative amount of oligomeric lipid present, on the concentration of calcium ions, and on temperature. Implications for the mechanism of bilayer membrane fusion are discussed in terms of lipid lateral diffusion and membrane curvature effects.

Four new phospholipid derivatives containing a-nitrostyrene unit linked to the phosphate headgroup have been synthesized and converted into unilamellar vesicles. The vesicles were characterized by freezefracture transmission electron microscopy (FFEM), cryo-scanning electron microscopy (cryo-SEM), light scattering, and differential scanning calorimetry (DSC). At pH 11.5 and at T < T m, the-nitrostyrene unit can be cleaved specifically exo-vesicularly. The cleavage process was analyzed in terms of a model providing independent rates of hydrolysis and flip-flop. Furthermore, the-nitrostyrene units can be polymerized yielding a polystyrene derivative. Neither the exo-vesicular cleavage nor the polymerization influence the morphology of the vesicles (FFEM, cryo-SEM, and light scattering). Exo-vesicular cleavage followed by polymerization of the remaining endo-vesicular-nitrostyrene units results in the first examples of vesicles containing a polymer-immobilized inner bilayer leaflet and a "monomeric" outer bilayer leaflet. Such vesicles are of interest for studies of the mechanism of fusion of bilayers formed from synthetic amphiphiles.

No abstract

Polymerization of the lipid headgroups inhibits calcium-induced fusion of small unilamellar vesicles of the lipid di-n-dodecyloxypropyl beta-nitrostyryl phosphate but does not influence vesicle aggregation. Addition of a copolymer of lauryl methacrylate and acrylamide (LMPAM) provides the vesicles with a steric shield that prevents both fusion and aggregation. Accurate microcalorimetric determination of the enthalpies of vesicle aggregation and fusion was possible by comparison of titrations of vesicles into CaCl2 in the absence and presence of LMPAM, both before and after polymerization of the lipids in the vesicles. Whereas calcium-induced aggregation is associated with an enthalpy of +2.6 ± 0.1 kJ/mol of lipid, fusion occurs with a minimal endothermic heat effect. We contend that the driving force of membrane fusion is of entropic origin.

No abstract