Cross-linking polymerization in two-dimensional assemblies: Effect of the reactive group site

The cross-linking polymerization of hydrated amphiphiles in monolayers, bilayers, and nonlamellar phases, i.e., bicontinuous cubic and the inverted hexagonal phases, is an effective method to modify their properties. Polymerization of monomeric amphiphiles in an assembly proceeds in a linear or cross-linking manner depending on the number and location of polymerizable groups per monomer. Polymerization of hydrated lipids with reactive groups in each hydrophobic tail yields cross-linked polymers. Sisson et al. (1996) examined the cross-linking of bilayers as a function of the mole fraction of bis-substituted lipids (bis-SorbPC), where the reactive groups were located at the end of the lipid tails. The onset of cross-linking was determined by changes in lipid lateral diffusion, bilayer vesicle stability, and polymer solubility. These data indicated that a substantial mole fraction (0.30 +/- 0.05) of the bis-substituted lipid was necessary for bilayer cross-linking. Analysis of the cross-linking and competing reactions suggested that the location of the reactive group, i.e., reaction site, in the amphiphile and therefore within the bilayer assembly influences the cross-linking efficiency. To assess this possibility, the cross-linking of dienoyl-substituted phospholipids, (E,E)-DenPC, where the reactive diene is located near the glycerol backbone of the lipid, was compared with SorbPC. The cross-linking of (E,E)-DenPC was found to be substantially more efficient than that of SorbPC. It is proposed that this effect is partly a consequence of the relative probability of macrocyclization and cross-linking reactions.