Two‐Dimensional polymerization of supramolecular assemblies: Synthesis and bilayer polymerization of lipids containing α‐methylene‐substituted acyl chains

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Abstract
Polymerization of lipid assemblies may be usefully employed to alter the properties of the assemblies. The possible locations of the reactive group in the lipids include (1) the chain terminus, (2) the head group, and (3) near the lipid backbone. The third strategy yields polymerized assemblies which retain their head group functionality and lipid chain motion. We have designed and synthesized new members of this later category by the use of 2‐methylene‐substituted acyl chains. The main transition temperature (Tm) from gel to liquid crystalline phase of hydrated bilayers of 1‐palmitoyl‐2‐(2‐methylene)palmitoyl‐sn‐glycero‐3‐phosphocholine (1) and the disubstituted 1,2‐bis(2‐methylenepalmitoyl)‐sn‐glycero‐3‐phosphocholine (2) were 33.6 and 25.3°C, respectively. The Tm of the mono‐substituted 1‐oleoyl‐2‐(2‐methylene)palmitoyl‐sn‐glycero‐3‐phosphocholine (3) bilayers was detected in a range from −15 to −10°C by x‐ray diffraction. Hydrated bilayers of each individual lipid were successfully polymerized with a water‐soluble initiator, azobis(2‐amidinopropane) dihydrochloride (AAPD). These results indicate the lipid 2‐methylene groups are accessible to the water interface. Thermal polymerization of the mono‐substituted lipids in aqueous suspensions with AAPD, yielded oligomers. However the bis‐2‐methylene PC (2) was successfully polymerized to yield stabilized crosslinked bilayers. © 1994 John Wiley & Sons, Inc.