Design and characterization of synthetic bilayer vesicles with a polymerized inner bilayer leaflet

Four new phospholipid derivatives containing a beta-nitrostyrene unit linked to the phosphate headgroup have been synthesized and converted into unilamellar vesicles. The vesicles were characterized by freeze-fracture 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 beta-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 beta-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 beta-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 effusion of bilayers formed from synthetic amphiphiles.