Selective Isotopic Labeling Resolves the Gel-to-Fluid Phase Transitions of the Individual Leaflets of a Planar-Supported Phospholipid Bilayer

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity

The assembly of biomimetic, planar supported lipid bilayers (SLBs) by the popular vesicle fusion method, which relies on the spontaneous adsorption and rupture of small unilamellar vesicles from aqueous solution on a solid surface, typically works with a limited range of support materials and lipid systems. We previously reported a conceptual advance in the formation of SLBs from vesicles in the gel or fluid phase using the interfacial ion-pairing association of charged phospholipid headgroups with electrochemically generated cationic ferroceniums bound to a self-assembled monolayer (SAM) chemisorbed to gold. This redox-driven approach lays down a single bilayer membrane on the SAM-modified gold surface at room temperature within minutes and is compatible with both anionic and zwitterionic phospholipids. The present work explores the effects of the surface ferrocene concentration and hydrophobicity/hydrophilicity on the formation of continuous SLBs of dialkyl phosphatidylserine, dialkyl phosphatidylglycerol, and dialkyl phosphatidylcholine using binary SAMs of ferrocenylundecanethiolate (FcC₁₁S) and dodecanethiolate (CH₃C₁₁S) or hydroxylundecanethiolate (HOC₁₁S) comprising different surface mole fractions of ferrocene (χ_Fc^surf). An increase in the surface hydrophilicity and surface free energy of the FcC₁₁S/HOC₁₁S SAM mitigates the decrease in the attractive ion-pairing interactions resulting from a reduced χ_Fc^surf. SLBs of ≳80% area coverage form on the FcC₁₁S/HOC₁₁S SAM for all the phospholipid types down to χ_Fc^surf of at least 0.2, composition yielding a water contact angle (θ_W) of 44 ± 4°. By contrast, a greater number of ion-pairing interactions is required on the hydrophobic FcC₁₁S/CH₃C₁₁S surface to drive the vesicle fusion process; bilayers or bilayer patches form at χ_Fc^surf ≳ 0.6 (θ_W = 97 ± 3°). These findings will aid in tailoring the surface chemistry of redox-active modified surfaces to widen the conditions that yield supported lipid membranes.

Control of Lipid Bilayer Phases of Cell-Sized Liposomes by Surface-Engineered Plasmonic Nanoparticles

Liquid-ordered (Lo)-phase domains, a cholesterol-rich area on lipid bilayers, have attracted significant attention recently because of their relevance to lipid rafts, the formation/collapse of which is associated with various kinds of information exchange through the plasma membrane. Here, we demonstrate that the formation/collapse of Lo-phase domains in cell-sized liposomes, that is, giant unilamellar vesicles (GUVs), can be controlled with bioactive plasmonic nanoparticles and light. The nanoparticles were prepared by surface modification of gold nanorods (AuNRs) using a cationized mutant of high-density lipoprotein (HDL), which is a natural cholesterol transporter. Upon the addition of surface-engineered AuNRs to GUVs with the mixed domains of Lo and liquid-disorder (Ld) phases, the Lo domains collapsed and solid-ordered (So)-phase domains were formed. The reverse phase transition was achieved photothermally, with the AuNRs loaded with cholesterol. During these transitions, the AuNRs appeared to be selectively localized on the less fluidic domain (Lo or So) in the phase-mixed GUVs. These results indicate that the phase transitions occur through the membrane binding of the AuNRs followed by spontaneous/photothermal transfer of cholesterol between the AuNRs and GUVs. Our strategy to develop bioactive AuNRs potentially enables spatiotemporal control of the formation/collapse of lipid rafts in living cells.