Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 182

which is then followed by a further sigmoidal step, with a plateau
at ~ 7.5 PF cm -2 . These C versus c A-I steps correspond to two con-
secutive penetration steps into the hydrocarbon tail region, proba-
bly associated with two different conformational changes of apoA-
I. 10 mol % cholesterol in the DOPC monolayer suppresses the
second penetration step, while 25 mol % suppresses both steps.
This may be explained by the cholesterol molecules rigidifying the
lipid monolayer, thereby preventing it from associating with some
penetrating domains of apoA-I.
Small lipophilic ions such as tetraphenylphosphonium
(TPhP + ) and tetraphenylborate (TPhB - ) prefer to be located in the
polar head region ( x = E) and in direct contact with the electrode
surface ( x = 0) rather than in the much less polarizable intermedi-
ate hydrocarbon tail region. We can therefore envisage two poten-
tial energy wells for these ions at x = 0 and x = E , with a potential
energy barrier in between. 62 The charge involved in a potential
step from a value negative (positive) enough for the anion (the
cation) to be entirely located at x = E to a value positive (negative)
enough to induce a complete translocation of the ion to x = 0, once
decreased by the charge involved in the same potential step in the
absence of the incorporated ion, yields directly the opposite of the
charge density of the ion in the lipid film. This procedure allowed
the determination of the adsorption isotherms of the TPhP + and
TPhB - ions in DOPC and DOPS monolayers. 62 In interpreting the-
se isotherms, discreteness-of-charge effects were considered.
6-Ketocholestanol (KC), a steroid that differs from cholesterol
mainly by the presence of a carbonyl group, was reported to form
pores inside a mercury-supported DOPC monolayer by a mecha-
nism of nucleation and growth similar to that of a number of chan-
nel-forming peptides. 63 The potential steps responsible for pore
formation by KC molecules give rise to the potentiostatic charge
vs. time curves shown in Fig. 12 , whose sigmoidal shape reveals
two consecutive two-dimensional phase transitions. The first phase
transition is ascribed to an increase in the alignment of the KC
dipoles along the direction of the interfacial electric field, with the
oxygen of the carbonyl group more fully turned toward the aque-
ous solution. The increase in differential capacitance from 1.2 to
6.5 PF cm -2 observed over the potential range between the two
phase transitions is ascribed to the first phase transition giving rise