A T-Shaped Amphiphilic Molecule Forms Closed Vesicles in Water and Bicelles in Mixtures with a Membrane Lipid

Abstract

The T-shaped amphiphilic molecule A6/6 forms a columnar hexagonal liquid-crystalline phase between the crystalline and the isotropic liquid when studied in bulk (Chen et al., 2005). Because of the hydrophilic and flexible oligo(oxyethylene) side chain terminated by a 1-acylamino-1-deoxy-d-sorbitol moiety attached to a rigid terphenyl core with terminal hexyloxy alkyl chains, it was expected that also formation of lyotropic phases could be possible. We therefore studied the behavior of A6/6 in water and also in mixtures with bilayer-forming phospholipids, such as dipalmitoyl-phosphatidylcholine (DPPC), using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and solid-state nuclear magnetic resonance (ssNMR). DSC showed for the pure A6/6 suspended in water a phase transition at ca. 23 °C. TEM and cryo-TEM showed vesicular as well as layered structures for pure A6/6 in water below and above this phase transition. By atomic force microscopy (AFM), the thickness of the layer was found to be 5-6 nm. This leads to a model for a bilayer formed by A6/6 with the laterally attached polar side chains shielding the hydrophobic layer built up by the terphenyl core with the terminal alkyl chains of the molecules. For DPPC:A6/6 mixtures (10:1), the DSC curves indicated a stabilization of the lamellar gel phase of DPPC. Negative staining TEM and cryo-TEM images showed planar bilayers with hexagonal morphology and diameters between 50 and 200 nm. The hydrodynamic radius of these aggregates in water, investigated by dynamic light scattering (DLS) as a function of time and temperature, did not change indicating a very stable aggregate structure. The findings lead to the proposition of a new bicellar structure formed by A6/6 with DPPC. In this model, the bilayer edges are covered by the T-shaped amphiphilic molecules preventing very effectively the aggregation to larger structures.