Self-Assembled Fluid Phase Floating Membranes with Tunable Water Interlayers

Abstract

We present a reliable method for the fabrication of fluid phase, unsaturated lipid bilayers by self-assembly onto charged Self-Assembled Monolayer (SAM) surfaces with tunable membrane to surface aqueous interlayers. Initially, the formation of water interlayers between membranes and charged surfaces was characterized using a comparative series of bilayers deposited onto charged, self-assembled monolayers by sequential layer deposition. Using neutron reflectometry, a bilayer to surface water interlayer of ∼8 Å was found between the zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane and an anionic carboxyl terminated grafted SAM with the formation of this layer attributed to bilayer repulsion by hydration water on the SAM surface. Furthermore, we found we could significantly reduce the technical complexity of sample fabrication through self-assembly of planar membranes onto the SAM coated surfaces. Vesicle fusion onto carboxyl-terminated monolayers yielded high coverage (>95%) bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) which floated on a 7–11 Å solution interlayer between the membrane and the surface. The surface to membrane distance was then tuned via the addition of 200 mM NaCl to the bulk solution immersing a POPC floating membrane, which caused the water interlayer to swell reversibly to ∼33 Å. This study reveals that biomimetic membrane models can be readily self-assembled from solution onto functionalized surfaces without the use of polymer supports or tethers. Once assembled, surface to membrane distance can be tailored to the experimental requirements using physiological concentrations of electrolytes. These planar bilayers only very weakly interact with the substrate and are ideally suited for use as biomimetic models for accurate in vitro biochemical and biophysical studies, as well as for technological applications, such as biosensors.

Large Scale Membrane Movement Induced by a Cation Switch

“…The formation of self-assembled floating bilayers by vesicle rupture onto COOH-OEG-SAMs has been described by us previously [5] . Briefly, the concentration of the SUV suspension was reduced to 0.2mg/mL and injected into the solid/liquid flow cells using pre-washed disposable syringes at room temperature (~20°C).…”

“…These interfaces encoded high amplitude fringes in the experimental data and therefore allowed for unambiguous resolution of the interfacial architecture upon data analysis. Indeed, previously we have been able to show that idealised labelling strategy of a hydrogenous SAM, deuterated water interlayer, hydrogenous membrane, deuterated bulk solution allowed for the solution interlayers between the SAMs and floating membranes to be adequately resolved [5] .…”

Design and use of model membranes to study biomolecular interactions using complementary surface-sensitive techniques

“…These methods preclude the inclusion of integral membrane proteins in the model and can present difficulties with the deposition of airsensitive lipid species. However, recently a methodology was developed which allows for floating bilayers to be self-assembled onto surfaces by vesicle rupture, which opens up new possibilities for studies with this sample type [107].…”

Large Scale Membrane Movement Induced by a Cation Switch

“…These interfaces encoded high amplitude fringes in the experimental data and therefore allowed for unambiguous resolution of the interfacial architecture upon data analysis. Indeed, previously we have been able to show that idealised labelling strategy of a hydrogenous SAM, deuterated water interlayer, hydrogenous membrane, deuterated bulk solution allowed for the solution interlayers between the SAMs and floating membranes to be adequately resolved [5] . Therefore, here, in every sample condition the surface assemblage was measured in D2O and 75% D2O (gold matched water, AuMW) buffer solutions, with both solution contrast conditions being highly sensitive to the overall membrane structure and SAM structure as well as the water inter-layer between them.…”