Scattering Studies of Hydrophobic Monomers in Liposomal Bilayers: An Expanding Shell Model of Monomer Distribution

Abstract

Hydrophobic monomers partially phase-separate from saturated lipids when loaded into lipid bilayers in amounts exceeding 1:1 monomer:lipid molar ratio. This conclusion is based on agreement between two independent methods of examining the structure of monomer-loaded bilayers. Complete phase separation of monomers from lipids would result in increase in bilayer thickness and slight increase in the diameter of liposomes. Homogeneous distribution of monomers within the bilayer would not change the bilayer thickness and would lead to the increase in the liposome diameter. The increase in bilayer thickness, measured by the combination of small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS), was approximately one half of what was predicted for complete phase separation. The increase in liposome diameter, measured by dynamic light scattering (DLS), was in the middle between values predicted for homogeneous distribution and complete phase separation. Combined SANS, SAXS, and DLS data suggest that at 1.2 monomer:lipid ratio, approximately one half of monomers are located in an interstitial layer sandwiched between lipid sheets. These results expand our understanding of using self-assembled bilayers as scaffolds for directed covalent assembly of organic nanomaterials. In particular, partial phase separation of monomers from lipids corroborates successful creation of nanometer-thin polymer materials with uniform imprinted nanopores. Pore-forming templates do not need to span the lipid bilayer to create a pore in the bilayer-templated films.

Bilayer‐Templated Two‐Dimensional RAFT Polymerization for Directed Assembly of Polymer Nanostructures

“…This interpretation of the NMR signals is in line with previously reported analysis of the NMR spectra of bilayer‐bound molecules . It is also supported by our previous study of the distribution of monomers in the bilayer based on small‐angle neutron and X‐ray scattering . Specific ratios of the differential locations of monomers appear to depend on the mobility and hydrophobicity of the monomers and their ability to form intra‐ and intermolecular complexes within the bilayer.…”

Facile Directed Assembly of Hollow Polymer Nanocapsules within Spontaneously Formed Catanionic Surfactant Vesicles

“…20 The same type of analysis was performed for the surfactant vesicles by analyzing data in the bilayer-thickness Guinier region. At these Q values (∼0.04− 0.08 Å −1 ), the bilayer appears as a flat sheet with thickness d. For QR g < 1, where R g is related to the thickness as d = R g (12) 1/2 , the intensity of scattering from a sheet goes as I ∝ exp(−Q 2 R g 2 )/Q 2 .…”

The Bio-SANS instrument at the High Flux Isotope Reactor of Oak Ridge National Laboratory

“…SANS is also an excellent tool for studying biomembranes, such as pure lipids (Kučerka et al, 2011;Pan et al, 2012) and their interactions with membrane-active peptides in both solid state (Lee et al, 2011) and vesicle solutions (Qian & Heller, 2011). Selective deuterium labeling made it possible to determine where small hydrocarbons reside in lipid bilayers (Richter et al, 2011). The Bio-SANS was also used to study differences between the thylakoid membranes inside of living wild-type and mutant cyanobacteria and to understand how their thylakoid membranes respond to changes in illumination (Liberton et al, 2013).…”