Localization of lipid domains in supported lipid bilayers induced by graphene oxide

抄録

The lipid bilayer is a self-assembled structure of amphiphilic lipid molecules, such as phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Chol), in aqueous solution, and is the fundamental structure of biomembranes represented by cell membranes. Lateral organization of lipids and proteins in cell membranes are key factors for the control and efficiency of the transportation substances and signals into and out of cells. To understand the processes of these cell membrane reactions, artificial lipid bilayer systems have been applied. The supported lipid bilayer (SLB) is one of the artificial bilayer systems existing at solid-liquid interfaces. Physical and chemical properties of the solid substrate surface affect the structure and characteristics of SLB [1]. Therefore, chemical modification and microfabrication on the substrate surfaces are applied to patterning of SLB itself and also inner domains in SLB. We have reported formation and physical properties of SLB on graphene oxide (GO) that was deposited on a thermally oxidized Si wafer (SiO2/Si) [2]. GO is a chemical derivative of graphene modified with hydrophilic functional groups such as hydroxy, carboxy, and carbonyl groups. In this talk, localization of specific lipid domains in multicomponent lipid bilayers on GO and its mechanism are described [3].

GO was prepared by the chemical exfoliation method and casted on a SiO2/Si substrate. Binary SLBs of dioleoyl-PC (DOPC) and dipalmitoyl-PC (DPPC), or ternary SLBs of egg-derived PC, egg-derived SM and Chol were prepared on GO/SiO2/Si by the vesicle fusion method and observed with an atomic force microscope (AFM) and a fluorescence microscope.

The transition temperature between the liquid crystal (Lα) and gel phases of DOPC and DPPC are -17 °C and 41 °C, respectively, and thus the phase separation occurs in DOPC+DPPC-SLB at 25 °C. The AFM topography of DOPC+DPPC-SLB on GO/SiO2/Si (Figure 1) shows that two regions with different height existed on GO, while its height was uniform on the bare SiO2 surface. Observation of fluorescence microscopy fluorescence recovery after photobleaching showed that the DOPC+DPPC-SLB on the bare SiO2 region was in the fluid Lα phase, and thus indicates the gel-phase domains were condensed on GO. We attribute the domain localization to the preferential nucleation on GO at the initial step of the domain growth, based on the dependence of the localization efficiency on the cooling rate. This mechanism was supported by the results in PC+SM+Chol-SLB: domains of the less fluid liquid ordered phase were localized on GO, leaving more fluid liquid disordered bilayers on the bare SiO2 surface.

[1] R. Tero, Materials. 5, 2658 (2012).

[2] R. Tero, Vac. Surf. Sci. (in Japanese) 66, 343 (2023).

[3] R. Tero, Y. Hagiwara and S. Saito, Int. J. Mol. Sci. 24, 7999 (2023).

Figure 1. AFM topography of DOPC+DPPC-SLB on GO/SiO2/Si. Scale bar: 500 nm.