Lateral organization and diffusion of lipids and membrane proteins are crucial factors of cell membrane reactions, such as the transportation of small molecules, ions, and signals into and out of cells. We are investigating the dynamics and assembly of lipids and proteins from single molecular level to mesoscale using supported lipid bilayers (SLBs), which are artificial lipid bilayer membranes at solid-liquid interfaces, as plasma membrane models.

Single Molecule Tracking in Supported Lipid Bilayers

On Opaque or Highly-Refractive Substrates

Single-molecule tracking (SMT) is a powerful method to investigate the molecular behavior in the SPBs, but generally the substrate material is restricted to glass or quartz because the SPB is illuminated by the evanescent light induced in the total internal reflection condition. We introduce the excitation light diagonaly to the sample substrate which is set upside-down 500 nm above the glass slip, for the observation of single fluorescence molecule without the restriction from the transparency or refractive index of substrates.
Single lipid diffusion in SLB was observed in-situ on step-and-terrace TiO2(100) at the time resolution of Δt=996 us - 33 ms revealed the crossover from anomalous diffusion to random diffusion around 10 ms[45]. (See also SLBs on atomic-step-and-terrace substrate )

Formation and Structures of Supported Lipid Bilayers

Effect of Substrate Surface Properties

The lipid molecules in SLB do not directly adsorb on solid substrates, but "float" 1-2 nm above the substrate surface. Even through this 1-2 nm thick water later, the surface properties of substrates affect the formation process, structure and characteristics of SLBs. Understanding the substrate effect is important to use SLBs as a plasma membrane model, and to exploit the substrate surface for the control of SLB structures and characteristics.

Hydrophilicity of substrates

The surface hydrophilicity of substrates is one of crucial factors in the SLB formation, because lipid bilayers themseflves are selfassembled structure of amphiphilic molecules. We controlled the surface hydrophilicity of SiO2/Si surfaces by annealing a chemically oxidized SiO2/Si under nitrogen. Moderate reduction in the surface hydrophilicity accelerates the SLB formation rate by the vesicle fusion method [23][17].
If the SiO2/Si surface is covered with highly-hydrophobic alkyl-SAM, lipid monolayer forms on SAM/SiO2 through zip-out of two monolayer leaflets of a bilayer.

SLBs on atomic-step-and-terrace substrate

We prepared a SLB of phosphatidylcholine on TiO2(100) single crystal surface consisting of single atomic steps (h = 0.23 nm) and flat terraces[32]. The SLB exhibits step-and-terrace morphology, precisely following the substrate structure on atomic scale. We evaluated the interaction energy between a SLB and the variety of oxide substrates considering DLVO and non-DLVO forces, and concluded that strong van der Waals attraction on TiO2 conquers the thermal undulation of lipid bilayers [32][36].
The atomic scale distortion in the SLB on the steps on the TiO2(100) surface works as the diffusion barrier against the lateral diffusion of lipids, and results in the anomalous diffusion [45]. (See also Single Molecule Tracking in SLB. )

Protein Assembly on Lipid Bilayers

Coming soon.

Lipid Membranes on Graphene

Coming soon.

Lipid Membranes on Sensing Devices

Coming soon.

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