Formation and Characterization of Hybrid Bilayers and Diffusion of Cations Across Liposomal Membranes: Studies Based on Polymerizable Lipids

Abstract

Cellular energy transduction processes are often driven by transmembrane ion gradients, and a number of artificial membrane systems have been developed that allow for chemically or light-induced transport of ions across lipid bilayers. These liposomal architectures, however, are not readily interfaced to a solid-state transducer. A significant step toward this goal is described here by assessing the possibility of coupling a lipid bilayer directly to a transducer to form a stable uniform film using hybrid bilayer membranes (HBMs).Although the surface attachment of self-assembled monolayer increases the robustness of the lipid assembly, HBMs cannot maintain film uniformity under harsher conditions due to the absence of strong lipid-lipid interactions. Therefore, HBMs were prepared and characterized using a cross-linking polymerizable lipid, bis-SorbPC. Several parameters relating to lipid deposition and film stabilization through polymerization were examined. Film characterization strongly suggests that polymerization of bis-SorbPC stabilizes the HBM such that its structure is largely preserved even after the dehydration process. This work suggests that network formation in the upper monolayer is not enough to prevent oligomer desorption, intermonolayer covalent linking is also a prerequisite in making uniform, defect-free planar supported lipid assemblies.Some of the challenges associated with the application of lipids involve the creation of supported bilayers that are stable to chemical and physical disruptions, yet retain their ion barrier properties, and allow transmembrane ion transport by lipid-soluble shuttles. Polymerized lipid films provide the stability required for these structures, but permeability properties of cations across poly(lipid) membranes are not known. Therefore, convenient liposome-based proton and calcium permeability assays were developed. These assays were applied to various poly(lipid) compositions.In addition, three novel sorbyl-substituted head group polymerizable lipids, which have been synthesized based on a strategy that head group polymerization would minimally perturb the characteristic ion impermeability of the membrane, were evaluated for their lipid characteristics and ability to form polymers. None of these compounds forms vesicles by itself. Therefore, attempts were made to form mixed vesicles with other fluid lipids. The miscibility of the mixed monolayers was assessed using Langmuir isotherms.