M. Heim et al., LATERAL ELECTRICAL-CONDUCTIVITY OF MICA-SUPPORTED LIPID BILAYER-MEMBRANES MEASURED BY SCANNING-TUNNELING-MICROSCOPY, Biophysical journal, 69(2), 1995, pp. 489-497
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.