| AVS 59th Annual International Symposium and Exhibition | |
| Biomaterial Interfaces | Wednesday Sessions |
| Session BI+SS+NS-WeM |
| Session: | Bio/Nano Interfaces with Applications in Biomedicine and Energy |
| Presenter: | H. Shimizu, University of Tokyo, Japan |
| Authors: | H. Shimizu, University of Tokyo, Japan S. Matsunaga, University of Tokyo, Japan T. Yamada, RIKEN, Japan T. Kobayashi, RIKEN, Japan M. Kawai, University of Tokyo, Japan |
| Correspondent: | Click to Email |
We obtained molecular-scale images of phospholipid layers spread on a modified Au(111) immersed in a buffer solution, by means of in situ electrochemical scanning tunneling microscopy (EC-STM). Real cell membranes consist of a bilayer of phospholipids which continually gather and interact. There are various kinds of phospholipids in the real cell membranes. To understand the action of these molecules, a dynamic molecular-scale method of observation is necessary. Lipkowski [1] first visualized static monolayers of phospholipid on Au(111) by in situ EC-STM. Matsunaga et al. [2] revealed dynamic, microscopic motion of phospholipid monolayer on alkanethiol-modified Au(111) immersed in a buffer solution. We intended to compose a bilayer of phospholipid on a hydrophilic substrate in order to mimic the real cell membrane more truly. We used a hydrophilically modified Au(111), anticipating that the first lipid monolayer with the hydrophilic head group down to the surface, and the second lipid monolayer with the hydrophobic alkyl chains down, all spontaneously in aqueous buffer solution.
For this purpose, we used 3-mercaptopropionic acid (MPA) self-assembled monolayer (SAM) on Au(111), in which the COOH groups are expected to be exposed out of the surface. We first observed a (√3 x √3) type adlattice of MPA SAM by STM. Then the sample was immersed in 50 mM phosphate buffer containing minimal lipid particles of 200 μM 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with or without 50 μM cholesterol.
We uniquely observed a 2-dimensional adlattice with a parallelogram unit cell of 1.0 nm x 1.9 nm. Along the short segment, blight spots are aligned. The adlattice did not change with or without cholesterol, indicating that it was composed exclusively of pure POPC. The interval of 1.9 nm is apparently shorter than the full length of POPC molecule (≈2.5 nm). To interpret the adlattice structure, we considered a model structure composed of tilted POPC, with the head group attached to the MPA SAM. This model involves a strong affinity between the hydrophilic groups.
Although this frozen adlattice does not completely match our target structure of mobile lipid bilayer, we consider we could partly utilize the hydrophobicity/hydrophilicity of the phospholipid molecules to compose a uni-directional membrane. We will further develop this kind of methods by choosing the proper modifier on Au(111), aiming the bilayer structure. By this we expect to go closer to the nanometer-scale reality of cell membranes containing functional proteins.
[1] J. Lipkowski, Phys. Chem. Chem. Phys. 12, 13874 (2010).
[2] S. Matsunaga et al., Electrochem. Commun. 9, 645 (2007).