Polymerizable Bio-Membranes: Synthesis, Properties and Application

"Molecular self-assembly of phospholipids and other amphipathic molecules offers a versatile approach to form self-organized microstructures in various morphologies. Stabilization of microstructures in specific morphologies by polymerization renders them useful for practical applications in areas ranging from controlled release technology to template mediated synthesis of metals. Our efforts are focussed on the development of polymerizable biomembranes derived from phospholipids, their properties and utility e.g., as stable templates for studying chemistry at the lipid interface. Polymerized vesicles formed from mixtures of 1,2-bis(tricosa-1 O,12-diynoyl)-sn-glycero-3-phosphocholine, 1, with corresponding lipids having phosphohydroxyethanol,2, or -hydroxypropanol,3, headgroups have been used in electroless metal plating of vesicle surface with gold utilizing selective binding of Pd(NH3)4 2 + to 2 or 3. The surface bound palladium ion served as catalyst for metalization. Metalization of polymerized vesicles constructed from charge neutral diacetylenic phosphocholine,1, did not occur using this process. The headgroup structure of the negatively charged phospholipids and the degree of vesicle polymerization had an effect on the stability of the vesicles after metalization. Vesicles formed from a mixture of 1 with polymerizable diacetylenic phosphatidic acid, nonpolymerizable 1,2- dipalmitoyl phosphohydroxyethanol or 1,2-diacyl phosphohydroxyethanol (from soy lecithin) were not stable to metalization process. The metal coating of the vesicles was examined by electron diffraction and revealed that the gold metal on vesicles was polycrystalline and face centered cubic. The results demonstrate the utility of polymerized synthetic membranes as template for easy and efficient synthesis of nanoparticles. IntroductionPhospholipids, synthetic or natural, provide a simple and straight forward route to construct membranes in a variety of morphologies due to their inherent property of selforganization in the presence of water[1]. Vesicles are closed spherical structures comprised of concentric walls made from lipid bilayers. Vesicles are morphologically simple structures with enormous technological potential. Polymerizable phospholipids prepared by incorporating a polymerizable group in the molecules demonstrate physical properties comparable to their non-polymerizable counterparts and have the added virtue of providing enhanced physical and chemical stability to the membranes[2,3]. The size, shape and properties of the organized assemblies are dependent on the lipid structure and the nature of the aqueous environment[4,5]. The headgroup geometry, surface charge density, and spatial arrangement of the phospholipids are known to affect the surface behavior of the lipid membranes, which is evident through their interaction with other molecular (hormones, antigens, antibodies etc) or lomc species. Since phospholipid headgroup structure and charge can be modified either synthetically or by chemical perturbation of the aqueous medium (e.g., pH or ionic strength), phospholipid membrane surfaces offer a potential for developing novel strategies in those areas which involve the use of molecular reactive sites. One such strategy which could be envisioned explores the use of molecularly engineered phospholipid assemblies to design and prepare template structures to regulate formation of ordered materials [6]."