Characterization of Polymeric Liposomes

Several investigations dealing with the characterization of polymeric membrane systems have been reported lately. A. Kusumi 99 studied phase transitions, fluidity and polarity properties of polymerized and nonpolymerized methacryloyl-derivatized phosphatidylcholine (57) vesicles by DSC and electron spin resonance (ESR). 

Miscibility of a natural lipid (DMPC) and the monomeric and polymeric lecithin analogue (26) was studied in large unilamellar vesicles using freeze-fracture electron microscopy and photobleaching by H. Gaub 100 . Before polymerization the two lipids appear miscible at all compositions in the fluid state and at DMPC concentrations at or below 50 mol/o in the solid state. After polymerization a two-dimensional solution of the polymer in DMPC is obtained at T T (T phase transition temperature of polymeric 26) while lateral phase segregation into DMPC-rich domains and patches of the polymer is observed T T. The diameter of the polymerized lipid domains was found to average 400 A. 

Phase-separated monolayers and liposomes were characterized by R. Elbert1011 who synthesized saturated and polymerizable fluorocarbon amphiphiles (59, 60, 61) and investigated their mixing behavior with CH2-analogues and natural lipids. In these systems the fluorocarbon compounds are incompatible with hydrocarbon lipids in a wide range of compositions and tend to form domains of pure fluorocarbon and hydrocarbon amphiphiles. The domains can be visualized by freeze-fracture electron microscopy. 

The physical properties of diacetylene-containing phospholipid liposomes (25) have been investigated by D. Chapman 102 103 104 by means of UV/VIS and 13C-NMR spectroscopy. They found that short linear segments of polymer are interconnected through the glycerol backbone of the lipid in identical-chain phosphatidyl cholines, but the molecular weights of the polymers are still unknown. 

Dorn 105 polymerized dialkylammonium lipids with the polymerizable methacryloyl moiety either in the head group (29) or at the end of one of the hydrophobic chains (5). GPC revealed Mw 1.9 x 106, Mn 3.5 x 105, Mw/Mn 5.4 for (29) and Mw 1.9 xl06,Mn 3.9 x 105, Mw/Mn 2.4 for (5). It was also found that Mw varies inversely with the time of sonication, i.e. in smaller liposomes lower-molecular-weight polymers are formed. In a following paper, K. Dorn 108 present data for the permeability of monomeric and polymeric vesicles from (29). 

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