LATERAL ELECTRICAL-CONDUCTIVITY OF MICA-SUPPORTED LIPID BILAYER-MEMBRANES MEASURED BY SCANNING-TUNNELING-MICROSCOPY

Lateral electric conductivity of mica-supported lipid monolayers and o f the corresponding lipid bilayers has been studied by means of scanni ng tunneling microscopy (STM). The surface of freshly cleaved mica its elf was found to be conductive when exposed to humid air. Lipid monola yers were transferred onto such a surface by means of the Langmuir-Blo dgett technique, which makes the mica surface hydrophobic and suppress es the electric current along the surface in the experimentally access ible humidity (5-80%) and applied voltage (0-10 V) range. This is true for dipalmitoylphosphatidylethanolamine (DPPE) as well as dipalmitoyl phosphatidylcholine (DPPC) monolayers. Repeated deposition of DPPC lay ers by means of the Langmuir-Blodgett LB technique does not lead to th e formation of a stable surface-supported bilayer because of the high hydrophilicity of the phosphatidylcholine headgroups that causes DPPC/ DPPC bilayers to peel off the supporting surface during the sample pre paration. In contrast to this, a DPPE or a DPPC monolayer on top of a DPPE monolayer gives rise to a rather stable mica-supported bilayer th at can be studied by STM. Electric currents between 10 and 100 fA, dep ending on the ambient humidity, flow along the DPPE bilayer surface, i n the humidity range between 35 and 60%. The DPPC surface, which is mo re hydrophilic, is up to 100 times more conductive under comparable co nditions. Anomalous high lateral conductivity thus depends on, and pro bably proceeds via, the surface-adsorbed water layers. The prominence of ambient humidity and surface hydrophilicity on the measured lateral currents suggests this. The combination of our STM data and previousl y published water adsorption isotherms as a function of the relative h umidity indicate that one layer or less of adsorbed water suffices for mediating the measurable lateral currents. The fact that similar obse rvations are also made for other hydrophilic substrates supports the c onclusion that lateral conductivity via surface-adsorbed water is a ra ther general phenomenon.