Biomedical Engineering Reference
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1.8 PF cm -2 , in good agreement with values obtained both at sol-
vent-free black lipid membranes (BLM) and at an octadecanethi-
ol/DOPC bilayer self-assembled on a hanging mercury drop elec-
trode. 79 The latter hybrid bilayer is readily prepared by first im-
mersing the mercury drop for one or two minutes in a thiol solu-
tion in ethanol; the phospholipid monolayer on top of the al-
kanethiol monolayer is then obtained by immersing the thiol-
coated mercury drop into an aqueous solution on whose surface a
phospholipid film has been previously spread.
Solid-supported alkanethiol/phospholipid bilayers are unsuita-
ble for incorporation of channel-forming peptides and membrane
proteins, because they do not fulfill the requirements listed in Sec-
tion II. Thus, no hydrophilic layer is interposed between the hybrid
bilayer and the electrode surface, thus excluding the space and
water required for the proper folding of the extramembrane do-
mains of integral proteins. Moreover, the flexibility and fluidity of
the chemisorbed alkanethiol monolayer in direct contact with the
electrode is much less than that of BLMs; this lack of flexibility
and fluidity makes these mixed bilayers practically impermeable to
lipophilic molecules of biological importance, such as ubiquinone-
10 and vitamin K 1 , and to lipophilic ions. 79 The kinetics of inor-
ganic redox couples exhibiting a Nernstian behavior on bare gold
is almost completely suppressed by hybrid alkanethiol/lipid bi-
layers for alkanethiol chains with more than eight carbon atoms,
pointing to an assembly that provides a substantial barrier to elec-
tron transfer and to hydrophilic ions. 79,80 Electron tunneling across
these films takes place only at very high overpotentials.
The polypeptide melittin is the major component of the venom
of the honey bee. This peptide carries five positive charges. From
its aqueous solutions, melittin binds spontaneously to biomem-
branes and BLMs. At low concentrations, it induces voltage-gated
channels in BLMs, while at concentrations higher than 1.2 Pg/mL
it causes their disruption. 81-84 When bound to lipid bilayers, melit-
tin adopts a highly D-helical conformation, with most hydrophobic
residues on one side and most hydrophilic residues on the opposite
side of the helix long axis. At zero transmembrane potential, these
amphipathic helices accumulate on the surface of the membrane,
parallel to the plane of the bilayer. As the transmembrane potential
is made negative on the opposite side of the membrane with re-
spect to melittin, the helices form membrane-spanning aggregates
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