Preparation and Structural Investigation of Lipid Nanoparticles with Nonlamellar Phases

Abstract

Aqueous dispersions of lyotropic liquid crystalline phases (cubosomes and hexosomes) were prepared by highpressure emulsification using lipid mixtures, monoolein (MO) and oleic acid (OA), and an emulsifier Pluronic F127 with changing their composition. Small-angle X-ray scattering experiments confirmed the presence of the liquid crystalline phases in the particles. The lipid ratios and solvent pH strongly affected the morphology of the internal structure of the particles. By increasing the weight fraction of OA in the lipid mixtures, the internal structure transformed in the order of bicontinuous cubic-inverted hexagonal-inverted cubic. In addition, transformation from the cubosome to the hexosome was observed by decreasing the pH, suggesting that the interior of the nanoparticles is responsive to the outer environment. 13C NMR experiments with paramagnetic shift reagent showed that the signal of carbonyl carbon of oleic-1-13C acid mixed in the particles readily shifted to a lower magnetic field by addition of europium ion (Eu3+), indicating high accessibility of the ion into the water channel inside the particles. Phase behavior and the changes in the local orientation order of lipids in a binary system consisting of phosphatidylcholine and MO were also investigated. Time-resolved fluorescence anisotropy measurements using cubosomes and liposomes revealed that the order parameter of the probe in the lamellar phase increases with increasing MO, and that it decreases during the transition to the bicontinuous cubic phase. This observation suggests that the lateral pressure in the acyl chain regions increased by the addition of the nonlamellar-forming lipid is released by the phase transition.