Self-assembly of phospholipid molecules at a Au(111) electrode surface

We described the first scanning tunneling microscopy study of spreading unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) at a Au(111) electrode surface. At the initial stage of the film formation, the molecular resolution images revealed that DMPC molecules are adsorbed flat with the acyl chains oriented parallel to the surface. The molecules assemble into double rows by aligning the acyl chains in the nearest neighbor direction of the reconstructed Au(111) surface and assuming a 90 +/- 10 degrees angle with respect to line of the molecular row. After approximately 30 min, this film is transformed into a hemimicellar state with long rows characteristic for the formation of hemicylindrical surface micelles. At hydrophilic surfaces such as glass, spreading of vesicles involves adsorption, rupture, and sliding of a single bilayer on a lubricating film of the solvent. We have provided the first evidence that a different mechanism is involved in spreading the vesicles at gold. The molecules released by rupture of vesicles self-assemble into an ordered film, and the assembly is controlled by the chain-substrate interaction.