Recently we designed a model cell membrane, which consists of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) monolayer over 1-octanethiol self-assembled-monolayer (SAM) on Au(111), and successfully observed the fluidic DHPC monolayer in aqueous solution by using the in situ electrochemical scanning tunneling microscope (EC-STM)1). For better understanding of the real cell membrane, photopholipid multi-component layers should be studied. This time we chose phosphoethanolamines (PE) in addition to PC's. We used several kinds of PC's and PE's, as well as mixtures of them, to understand the 2-demensional phase separation and the "raft-like" structure commonly observed on real cell membranes. Pure DHPC formed a monolayer on SAM. In highly concentrated solutions of DHPC, vesicles were formed on the monolayer and these vesicles were observed to move along the monolayer. Pure DHPE more readily formed the monolayer on SAM and was more interactive with the scanning tip than DHPC. In concentrated solutions, DHPE also formed multilayers and vesicles on the DHPE monolayer, but the DHPE multilayer has different property from DHPC. DHPE layer was relatively more fragile than DHPC layer. It was observed that DHPE layer was scratched by the scanning tip. DHPE+DHPC mixture also formed vesicles and multilayers. Higher DHPE percentage in the solution resulted in an increase of the miltilayer height. These properties of DHPC and DHPE layers can be understood through the hydrophilicity of the head group undertaking hydrogen bond. We also examined an unsaturated 16:1PE +14:1PC monolayer on SAM. Creation of the open space is expected around each lipid molecule with the bulky unsaturated tail parts. Compared with DHPC+DHPE layer, images of the unsaturated 14:1PE+16:1PC layer were not clear. This unclearness seems to be induced by the lager fluidity of head group because of open spaces. The existence of open spaces between phospholipid molecules is considered closely related to binding of protein molecules on the cell membrane, and STM observation of such processes will be also discussed.
1) S. Matsunaga et al., Electrochem. Commn. 9(2007), 645
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