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

Abstract

Knowledge of the thermotropic phase behavior of solid-supported bilayer lipid assemblies is essential for mimicking the molecular organization and lateral fluidity of cell membranes. The gelto-fluid phase transitions in a homologous series of single phospholipid bilayers supported on planar silicon substrates were investigated by temperature-controlled atomic force microscopy (AFM) and attenuated total reflection infrared (ATR-IR) spectroscopy to obtain complementary information at the mesoscopic and molecular scales. Symmetric bilayers of dipalmitoylphosphatidylcholine (DPPC) and vertically asymmetric bilayers comprised of a leaflet of DPPC and another of acyl-chain-deuterated DPPC (DPPC-d62) were prepared by the Langmuir-Blodgett technique. The selective deuteration of one of the bilayer leaflets enabled the simultaneous monitoring by IR spectroscopy of the acyl chain melting in each leaflet via the spectrally isolated CH2 and CD2 stretching vibrations. Two gel-to-fluid transitions were discerned for both the symmetric and asymmetric bilayers in ultrapure water. The deuterium isotope effect observed in free-standing membranes was maintained for the supported bilayers. IR spectroscopy revealed that the melting of one leaflet promotes the disordering of the acyl chains in the adjacent one. The findings suggest that the two leaflet phase transitions do not evolve in isolation. This work sheds insight into the nature of leaflet-leaflet interactions and the thermodynamic properties of surface-confined phospholipid bilayers.

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

“…In the case of DMPS and DPPC, the SLB thickness is consistent with the gel phase based on the thicknesses of monolayer films of DMPS on water determined by X-ray reflectometry of 2.0 ± 0.3 nm for the fluid/liquid-expanded phase and 2.8 ± 0.2 nm for the gel/condensed phase and the thicknesses measured by AFM-based indentation for SLBs of DPPC of 5.8 nm at 22 °C ( T < T m ) and 4.0 nm at 65 °C ( T > T m ) . Bilayers in the fluid phase are thinner than those in the gel state due to a higher proportion of gauche conformational defects of the alkyl chains compared to the predominantly trans conformation in the gel phase …”

“…Planar lipid bilayers on solid supports are one type of model system employed to investigate the biophysical chemistry of cell membranes and simulate biochemical processes occurring at their surface for applications including biosensing, drug discovery, cell mechanobiology and cell culture, and energy capture and storage. − When immobilized on metal surfaces, supported lipid bilayers (SLBs) constitute an attractive platform due to their compatibility with a wide range of surface-sensitive characterization techniques, such as surface plasmon resonance, quartz crystal microbalance, electrochemistry, infrared reflection–absorption spectroscopy, and atomic force microscopy (AFM). − The spontaneous adsorption and rupture of small unilamellar vesicles (SUVs) suspended in aqueous solution onto a solid surface to form a lipid layer, referred to as vesicle fusion, is the most widely used approach for the preparation of SLBs to date . The adsorbed vesicles rupture from the stress or deformation applied on them by vesicle–substrate and vesicle–vesicle interactions .…”

“…64 Bilayers in the fluid phase are thinner than those in the gel state due to a higher proportion of gauche conformational defects of the alkyl chains compared to the predominantly trans conformation in the gel phase. 7 Fc/CH 3 Versus Fc/OH SAM. The phospholipid surface coverages as a function of χ Fc surf track the trends of ΔΘ min versus χ Fc surf (Figures 6, 7, and S11).…”

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

“…21 According to a recent study on a model system, the phase transition of each leaflet in the bilayer can also be controlled separately. 22 Therefore, if the action mechanism of lipid phases is established, artificial phase-controlling methods can potentially be utilized to identify their currently unknown roles.…”

“…Artificial control is possible using synthetic phospholipids for bilayer preparation, cyclodextrin derivatives as the cholesterol carrier, or membrane-adhesive nanoparticles . According to a recent study on a model system, the phase transition of each leaflet in the bilayer can also be controlled separately . Therefore, if the action mechanism of lipid phases is established, artificial phase-controlling methods can potentially be utilized to identify their currently unknown roles.…”