Polymerization in Liposomal Systems

Aqueous dispersions of polymerizable lipids and surfactants can be polymerized by UV irradiation (Fig. 18). In the case of diacetylenic lipids the transition from monomeric to polymeric bilayers can be observed visually and spectroscopically. This was first discussed by Hub, 9) and Chapman 20). As in monomolecular layers (3.2.2) short irradiation brings about the blue conformation of the poly(diacetylene) chain. In contrast, upon prolonged irradiation or upon heating blue vesicles above the phase transition temperature of the monomeric hydrated lipid the red form of the polymer is formed 23,120). The visible spectra of the red form in monolayers and liposomes are qualitatively identical (Fig. 19). 

In the case of butadienic lipids, the polyreaction was followed by a decrease of the strong monomer absorption (260 nni)25). Disappearance of vinyl protons in the 2H NMR spectrum proved polymer formation for vesicles made from vinylic, acryloylic, and methacryloylic surfactants l3,16). 

Electron microscopy provides direct evidence that the polymerized dispersions still contain spherical liposomes (Fig. 20). Laser light scattering measurements also demonstrate the presence of vesicles in the irradiated dispersions16). The size distribution of vesicles is not significantly altered by polymerization 13) (Fig. 21) as confirmed by gel filtration before and after irradiation of vesicles of (7). 

An even more striking effect is observed by addition of the surfactant sodiumdodecyl sulfate (SDS) to vesicles (Fig. 23). While monomeric vesicles of (19) and dipalmitoylle-cithin are destroyed by low SDS concentrations, the polymerized vesicles are stable up to 2 - 10 3 mol/1 SDS25). Polymerized vesicle dispersions can be diluted with ethanol without precipitation.231 Polymeric liposomes of (20) are stable in 80% ethanol for weeks. This could also be shown by Regen et al. for polymerized vesicles of the methacryloylic lipids (4) and (6)13141 (Fig. 24) by monitoring the turbidity (absor- 

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