Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 186

homogeneous chemical step mimicking the deactivation and re-
lease of a Tl + -gramicidin intermediate from the electrode surface,
in competition with the surmounting of the potential energy barrier
located within the channel.
Two lipophilic neutral molecules are commonly incorporated
into membranes with the aim of altering their dipole potential,
namely phloretin, which creates a dipole potential negative toward
the interior of the lipid film, and 6-ketocholestanol, which creates
a dipole potential positive toward the interior, due to the presence
of a carbonyl group. 74,75 As expected, incorporation of phloretin in
the lipid monolayer containing gramicidin has no effect on the
kinetics for thallous ion reduction. 72 In fact, the favorable dipole
potential created by phloretin, negative toward the interior of the
film, cannot accelerate the overcoming of the potential energy bar-
rier more than predicted by a Nernstian-type behavior. Conversely,
the unfavorable dipole potential, positive toward the interior of the
film, created by incorporating 23 mol % 6-ketocholestanol in the
DOPC monolayer, slows down this intermediate step by increasing
the height of the potential energy barrier within the ion channel.
This leads to a decrease in the rate constant for the surmounting of
the barrier, which has the typical Butler-Volmer dependence upon
the applied potential. 72

2. Alkanethiol-Lipid Hybrid Bilayers

Great interest has been focused on self-assembled films that are
attached to a solid support by formation of a covalent linkage be-
tween the self-assembled molecules and the solid support, yielding
structures of long-term, mechanical stability. Monolayers of al-
kanethiols on gold are probably the most widely used and best
characterized of all self-assembled films to date. Self-assembly
involves the anchoring of the thiol to the gold surface through the
sulfhydryl group, often accompanied by its deprotonation and elec-
tron transfer from the sulfur atom to the metal. The hydrocarbon
chains are, therefore, directed toward the aqueous solution. The
self-assembly is normally carried out by keeping the electrode
immersed in an ethanol solution of the thiol from 12 to 24 hr for
Au or Ag and from 5 to 20 min for Hg.
The deposition of a phospholipid monolayer on top of an al-
kanethiol monolayer tethered to gold can be accomplished by sev-