Cell membranes are the reaction fields for recognition of outer environment and for transportation of information. Modified lipids with hydrophilic molecules on their head groups, as represented by glycolipids, play important roles in these biological reactions. Their functions and efficiency are influenced by the aggregation state, but localization of these lipids is less understood compared with that induced by the interaction in the hydrophobic core of lipid bilayers, probably because of competition between attraction via hydrogen bond and hydration fluctuation repulsion. These interactions also make the observation with atomic force microscope (AFM) difficult, inducing artificial forces between a sample and a cantilever tip. In this study, we aim to establish a methodology to measure the aggregation state and physical properties of the modified lipids. We used poly-ethylene glycol (PEG)-modified lipid (PEG-DSPE) as a model modified lipid, and formed supported lipid bilayers (SLBs) containing PEG-DSPE on a thermally oxidized SiO2/Si substrate by the vesicle fusion method. SLB is an artificial lipid bilayer system existing at solid-liquid interfaces, and used for the investigation of the fundamental physicochemical properties of lipid bilayers. We prepared PEG-SLBs with various PEG-DSPE concentration (CPEG) and investigated their structures and fluidity with AFM, epifluorescence microscopy (epi-FM) and the fluorescence recovery after photobleaching (FRAP) method. AFM topographies (Figure 1) show that protrusions with 1-2 nm height existed in flat and uniform PEG-SLBs at CPEG≦~6 %. Depression regions with 2-3 nm depth emerged in PEG-SLB at CPEG≧~5 % and increased with CPEG. Epi-FM observation showed that the depression regions are PEG-DSPE-rich domains. We may attribute these morphologies to two types of PEG chain state, known as mushroom and brush regime states. Resutls of FRAP showed that diffusion coefficient and mobile fractions decreased with the increase in CPEG. We concluded that PEG-DSPE formed domains via the interaction between PEG chains, and the domains reduced the fluidity of SLBs. We will describe the detailed physical properties of PEG-SLBs and defined relationship of CPEG and microdomains on PEG-SLBs.

Structural basis of hydrophilic-polymer-modified lipid domains in supported lipid bilayer and their physical properties

255th National Meeting (2018)

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