Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 180

in the charge density of the polar heads of these phospholipids 54
and upon adsorption of certain anti-tumor drugs. 55 These investiga-
tions were conducted by measuring the differential capacitance C
and the charge density V M of lipid-coated mercury at a special
homemade hanging mercury drop electrode. 56 In particular, V M
was measured by contracting a lipid-coated mercury drop while
keeping its neck in contact with the lipid reservoir spread at the
water/argon interface, so as to allow a free exchange of lipid mate-
rial between the lipid monolayer that coats the mercury drop and
the lipid film spread on the aqueous solution. 57 This procedure
ensures that the thickness and all other properties of the lipid mon-
olayer remain unaltered during the contraction. The charge follow-
ing the contraction divided by the decrease in drop area yields di-
rectly the charge density V M on the mercury surface. The experi-
mental data were analyzed on the basis of a model of the mem-
brane/solution interphase that accounts for the presence of charged
ionizable groups either exposed to the aqueous phase or buried
well inside the polar head region. 51 The behavior of DOPS was
found to be particularly involved, with its phosphate group buried
somewhere inside the polar head region, and the carboxyl and
ammonium groups exposed to the aqueous phase. It was concluded
that the overall charge density of a DOPS monolayer varies from
slightly negative to slightly positive values as the bulk pH of the
bathing solution is varied from 7 to 4. 54 Generalizing this model to
account for the expansion of a lipid-coated Hg drop completely
immersed in an aqueous solution and for the resulting tilt in the
self-assembled phospholipid molecules, the dipole potentials of
DOPS and DOPC monolayers were estimated at +140 - +150 mV,
positive toward the interior of the film. The fact that these mono-
layers have very similar dipole potentials indicates that this dipole
potential is not to be ascribed to the serine or choline group of their
polar heads, but rather to a group common to these two lipids and
buried deeper inside the polar head region. This can be reasonably
identified with the glycerol backbone.
The effect of lipophilic ions on the capacitance minimum of
lipid-coated mercury may be summarized as follows. If the com-
pound is simply adsorbed on top of the lipid monolayer or interca-
lated between the polar heads, it does not affect the tail region,
whose capacitance C t is much less than that of the head region and
close to that, C m , of the whole lipid monolayer. In this case the