Mixed vesicles of dimyristoylphosphatidylcholine (DMPC) and a polymerizable lipid containing one diene group per chain are studied by freeze fracture electron microscopy and by the photobleaching (fluorescence recovery after photobleaching) technique. Large thin-walled vesicles of some micron in diameter become more stable after photochemical polymerization. Before polymerization bilayers of the diene lipid exhibit a liquid crystal-to-gel transition at Tg = 31 degrees C. Upon polymerization the transition remains but shifts to a slightly higher temperature (Tg* = 34 degrees C). The transitions in both cases are accompanied by a freezing in of the lateral mobilities. The mixed vesicle exhibits lateral phase separation after polymerization. Before polymerization the two lipids appear miscible at all compositions in the fluid state and at DMPC concentrations at or below 50 mol % in the solid state. After polymerization a two-dimensional solution of the polymer in DMPC is obtained at T greater than Tg*, while lateral phase segregation into DMPC-rich domains and patches of the polymer is observed at T less than Tg*. The domain structure appears identical irrespective of whether polymerization is performed at T greater than Tg or at T less than Tg. A typical value of the diameter of the polymerized lipid domains (approximately 400 A) indicates a rather small aggregation number (N less than 100 monomers). The lateral diffusion coefficient in butadiene-lipid bilayers only decreases from D1 = 3.10(-7) cm2/s to D1 = 8.10(-8) cm2/s (that is by a factor of 4) upon polymerization. This is consistent with the freeze fracture finding of a small aggregation number. We point out the similarities of the mixed vesicles with plasma membranes coupled to the cytoskeleton.

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Pclyrnsr chemists as poachers in foreign grounds? Why not? Nscromolecular chemistry has become a mature science with all advantages and handicaps of a well-established scientific discipline: many heights have been conquerred and the harvest is r;b!jnGant, but adventure and the future might be else!vtiere. i n tiii!e of bottomed out industrial profits in common old?tics, future polymer chemistry cannot be limited t o repetitive improvement of already successful mass polymers instead o f material science as well as life science where "polvmer thinking" might help t o lead t o new solutions o f current rob1ems.The first hesitant steps on the bridge towards membrane tiologj nave been made.Can polymer chemists really contribute to the understanding o r even mimicking of cell membrane functions and cell-cell interactio!is? Fascinated by the specificity and efficiency of interactions such as the destruction of tumor cells Dy l y m o b ocytes ( 1 ) and having in mind what biochemical analyses tell us about membrane composition, w e may try t o "synthesize" polymeric membrane and cell models. The commonly used model systems ( 2 ) such as planar lipid monolayers at the gas-water interface, bimolecular lipid membranes and spherical IiDosomes are much less stable than natural membrane systems (Fig. 1). r'This contribiition is an extended abstr3ct of a paper presented at the FKG/USSR-Symposium in Taschkent 13P3. It will be publisned in details elsewhere (Aavances in Polymer Science, 1 9 8 4 1.

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