Phase Structures of a Hydrated Anionic Phospholipid Composition Containing Cationic Dendrimers and Pegylated Lipids

The effect of 4th generation poly(amidoamine) dendrimer (4G PAMAM) present in an anionic phospholipid composition, consisting of hydrogenated soyphosphatidylcholine (HSPC), cholesterol (CH), dicetyl phosphate (DCP), and poly(ethylene glycol) (M w ∼ 2000) derivatized phosphatidylethanolamine (PEG2000-PE), on the hydration and liquid crystalline structure formation was investigated. The optical and polarized light microscopies of the liposomal dispersion obtained from the hydrated lipid composition show two types of birefringent structures (mesophases): plastic, wormlike microstructures and conventional, over-elongated lamellae. Differential scanning calorimetry (DSC) shows an increase in the liquid crystalline phase transition (T g) of the lipid composition from 60 to 94 °C with increasing 4G PAMAM concentrations from 0 to 0.011 mM, respectively. The T g values of the two microstructures were 68 and 84 °C, respectively, indicating that the plastic microstructures were 4G PAMAM/DCP-complexes-rich (α mesophases) and the conventional and elongated lamellae were dendrimer-doped HSPC/CH-rich microstructures (β mesophases). Optical microscopy shows that the α mesophases convert into various other types of vesicular structures such as giant unilamellar vesicles and biliquid foams, upon heating above the phase transition temperature of the lipid composition (∼60−65 °C). The microstructure transformation is a result of an osmotic influx of water and the detergent action of PEG2000-PE present in the lipid composition. The transmission electron microscopy (TEM) images of the liposomal dispersion show particles embedding circular transparent domains that exactly correlate to the theoretical 4G PAMAM/DCP complex sizes, thus, providing evidence of 4G PAMAM interspersed within the two mesophases. Small-angle X-ray scattering (SAXS) measurements indicate that the α mesophases are a dendrimer-interlinked, symmetrically undulated lamellar phase and the β mesophases are dendrimer-doped, occasionally kinked lamellae. An increase in dendrimer concentration in the lipid composition was found to decrease interlamellar spacing. On the basis of optical microscopy, DSC, TEM, and SAXS data, a model of dendrimer-doped mesophase structure and lamellae fusion is proposed. This investigation provides new self-assembled materials for drug/gene delivery and supplements the understanding of mechanisms involved in various biological processes such as membrane fusion, transmembrane permeatio