CROSS-LINKING POLYMERIZATIONS IN 2-DIMENSIONAL ASSEMBLIES

Polymerization of monomeric lipids in an assembly proceeds in a linear or cross-linking manner depending on the number of polymerizable grou ps per monomeric lipid. Lipids that contain a single reactive moiety i n either of the hydrophobic tails or associated with the hydrophillic head group yield linear polymers. Polymerization of lipids with reacti ve groups in each hydrophobic tail generally yield cross-linked polyme ric networks. This report describes three approaches to the characteri zation of the gel point for polymerizations constrained by the two-dim ensional nature of lipid bilayers. The gel point for two-dimensional l ipid assemblies was determined by correlation of the onset of signific ant changes in the physical properties of the polymerized bilayers wit h the bilayer composition. The properties examined in this study were the lateral diffusion of a small molecule probe of the bilayer, the st ability of polymerized bilayer vesicles in the presence of surfactants , and the solubility of lipid polymers isolated from the bilayers afte r removal of water, Each of the three methods used indicated that a su bstantial mole fraction (0.25-0.35) of the bis-substituted lipid was n ecessary to cause cross-linking of the bilayer. The general agreement between the methods provides confidence that these results accurately indicate the relative inefficiency of the cross-linking process in bil ayers composed of lipids having a reactive group at the hydrophobic te rminus of the lipid tail(s). The possible explanations for the ineffic ient nature of the lipid bilayer cross-linking are discussed with rega rd to the preferred conformation of monomeric lipids in the bilayer, t he motions of the lipid tails, and competing side reactions. These stu dies provide a new insight into the behavior of polymerizations in org anized assemblies, which will aid in the design of new materials based on bilayers or other types of assemblies, e.g. inverted hexagonal or bicontinuous cubic phases.