Cross-linking polymerization of hydrated lipid bilayers

The objective of this work was to investigate the cross-linking polymerization of lipids in supramolecular assemblies in order to stabilize the assembly structures.; One approach to obtain a cross-linked polymer is by addition of a cross-linking agent. Homobifunctional lipids with two polymerizable groups, one in each of the two hydrophobic chains, can serve as a cross-linking agent. The lipids chosen were dienoyl-substituted lipids, mono- and bis-DenPC, where the reactive diene group is located near the glycerol backbone of the lipids. MonoDenPC _16,18 and bisDenPC_18,18 were synthesized. Redox initiation was used to polymerize mixed mono/bisDenPC vesicles. To test the stability of the polymeric vesicles, Triton X-100 (TX-100) was added to the sample and the size determined by quasi-elastic light scattering (QELS). The stability of polymerized LUV to TX-100 depended on the initial composition of the LUV. It was found that 15 mol % bis-substituted lipids, bisDenPC, were needed for cross-linking polymerization. The capability of varying the cross-linking density in these poly(lipid) structures provides researchers with a means to further modify the physical properties of these polymers and membranes.; Another approach to achieve cross-linking utilizes heterobifunctional lipids, which have two different reactive groups in one of the lipid tails. These heterobifunctional lipids formed bilayer vesicles or lamellar structures. The properties of the vesicles could be varied by the polymerization of the reactive groups. The design of heterobifunctional monomers allowed either simultaneous or selective polymerization of these groups. The difference in reactivity as well as the difference in polarity of each group also made possible selective polymerization of these reactive groups. Simultaneous polymerization of both reactive groups was achieved by either redox polymerization or direct photoirradiation. UV polymerization showed the formation of oligomers, which dissolved in organic solvents and TX-100. On the other hand, redox polymerization gave cross-linked polymers, which were insoluble in most organic solvents and stable towards addition of TX-100. The cross-linking density could be varied by choosing different polymerization techniques. Electron micrographs of the cross-linked polymeric vesicles showed they could retain their size and shape after freeze-drying and re-dispersal in water. The surface of the cross-linked vesicles could be modified for many applications.