COMPLEXATION OF LECITHIN WITH CATIONIC POLYELECTROLYTES - PLASTIC MEMBRANES AS MODELS FOR THE STRUCTURE OF THE CELL-MEMBRANE

The complexation of bilayer-forming lipids, dihexadecyl phosphate (DHP ) and soja-lecithin, with a cationic polyelectrolyte (PDADMAC) results in stable, highly ordered mesomorphous materials. For the DHP complex , a close to perfect L(beta)' morphology is found with SAXS. The incre ase of the phase transition temperature from T-c = 66 degrees C for th e free DHP membranes toward T-c = 84.4 degrees C for the polyelectroly te-lipid complex reflects the increased membrane stability due to poly meric counterions. Due to an extremely high glass transition of the io nic interlayers, the DHP complex is a nonplastic and brittle solid. Th e lecithin complex with its mixture of head groups and tail lengths ex hibits, opposite to its clean synthetic counterpart, a quite unconvent ional phase structure where the stack of lamellar bilayers undulates w ith very high amplitudes. A similar undulated structure has recently b een postulated for the natural lecithin membrane(1-3) and is obviously necessary for some biophysical membrane functions. In addition, a rem arkably improved mechanical behavior of the lecithin-complex film para lleled by the depression of the glass transition of the ionic layers t o T-g = 10 degrees C is observed which allows large amplitude deformat ion, mechanical orientation as well as thermomechanical processing of the complex. Such systems can be understood as ''plastic membranes'', are comparably cheap, and might be interesting as materials themselves . The observation of undulations as well as the unexpected good mechan ical properties of the polymer-lecithin complex underlines that such t hree-dimensional materials in the bulk may act as model systems for th e special properties of lecithin mesophases which arise from an approp riate mixture of tails and head groups, optimized by evolution process es.