Observation of Intermediates in Lamellar to Cubic Phase Transformations of Lipid Nanoparticles

Abstract

Self-assembled lipid systems have recently come to prominence in medical applications as potential carriers for a range of bioactive agents (1); these include medical imaging, therapeutic compounds. One of their primary advantages is versatility as they can be adapted to different agents and target sites.During the preparation of cubic phase lipid nanoparticulate dispersions, we observed the presence of new intermediates during the transition from the fluid lamellar lyotropic phase to the cubic phase. This phase transition is regarded as a model for membrane-fusion processes(2). Many organelles demonstrate highly ordered cubic membrane structures. Determining the mechanistic origins of such lipid organelle complexity has been elusive.We increased the lifetime of very short-lived non-equilibrium intermediate structures by the use of steric stabilizer in the dispersions. These structures were characterized using synchrotron small-angle X-ray scattering and cryo-transmission electron microscopy. We visualised the intermediate initial bilayer contacts and stalk formation, followed by pore development, pore evolution into 2D hexagonally packed lattices, and finally creation of 3D bicontinuous cubic structures (3).In a biological context, the experimental corroboration of transitional lipid self-assembly structures furthers the understanding of organelle morphogenesis and maturation.The ability to manipulate intermediate structures in nanoparticulate dispersions of self-assembled structures may provide a unique system for encapsulation and controlled release of bioactives. The capability to control intermediate transformations may also permit the development of flexible growth media for applications such as in-cubo integral membrane protein crystallization or liquid crystal templating of nanostructured materials.1.Drummond, C.J.; et al., Current Opinion in Colloid & Interface Science 1999, 4, (6), 449-456.2. Conn, C.E.; et al., Physical Review Letters, 2006, 96, (10), 1081023. Mulet X., et al., ACS Nano, 2009, 3 (9), 2789-2797