Creating Asymmetric Phospholipid Vesicles via Exchange with Lipid-Coated Silica Nanoparticles

Abstract

Lipids, together with embedded proteins, are essential components of cellular membranes. In vivo, these membranes not only have unique lipid and protein compositions, but also have a strict asymmetric distribution of lipids between the two leaflets of the membrane. What is the energetic cost of maintaining lipid asymmetry? The study of the passive movement of lipids within model membranes can provide insight into this energetic toll. To do so, recent effort has been placed into fabricating model, free-standing asymmetric lipid membranes (vesicles). Here we report on the use of lipid coated silica nanoparticles to exchange lipids with initially symmetric vesicles to generate composition controlled asymmetric vesicles. In contrast to the use of methods that can potentially interfere with the membrane structure, such as hemifusion events of membranes or lipid exchange via a cargo method, like cyclodextrin, our method relies on the simple and natural exchange of lipids between membranes through an aqueous medium. Through temperature, time, and ratio of lipid coated silica nanoparticles to vesicles, a desired composition is reached at which point the silica nanoparticles are removed leaving vesicles with an asymmetric composition of lipids. In the present work we report on the production of asymmetric vesicles composed of isotopically distinct, dipalmitoylphosphatidylcholine vesicles. Lipid asymmetry is detected by both small angle neutron scattering (SANS) as well as proton nuclear magnetic resonance (1H NMR), and the rate at which the membrane homogenizes at 75°C is assessed.