Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 181

capacitance, C c , of the film of the compound will be in series
with C m , causing a decrease in the overall capacitance
C # ( C m 1 + C c -1 ) -1 . In general, this decrease is low, because the
polarizability of the compound (and, hence, its C c value) is higher
than that of the lipid tails. While remaining merely adsorbed in the
polar head region, the compound may also determine a condensa-
tion and an increase in the alignment of the lipid monolayer, re-
sulting in its thickening and in a decrease in capacitance. On the
other hand, if the compound penetrates the hydrocarbon tail re-
gion, the fraction T of the electrode surface covered by the com-
pound will have a capacitance C c higher than that, C m , of the pure
lipid domains. As a rough approximation and disregarding any
edge effects at the boundary between the two different domains,
the overall capacitance is given by C = T C c + (1 - T) C m and is,
therefore, higher than that of the pure lipid film. The cationic phe-
nothiazine derivatives cause a moderate increase in the capacitance
minimum. 49,58 The anionic diuretic furosemide causes an initial
slight decrease in the capacitance minimum at low bulk concentra-
tions 59 and then an increase at higher concentrations. 58,59 This may
indicate that these anions start interacting with the polar heads at
low concentrations, before penetrating the tail region at higher
concentrations. All these ions depress and broaden peaks 1 and 2,
just as neutral compounds. The cationic antibiotic polymyxin nar-
rows the minimum capacitance region on the negative side and
decreases the capacitance minimum by about 0.35 PF cm -2 . 60 This
indicates that this peptide does not penetrate the hydrocarbon tail
region.
Lipoproteins are molecular aggregates that transport water-
insoluble lipids in the blood plasma: they contain a core of neutral
lipids, coated with a monolayer of phospholipids in which special
proteins (apolipoproteins) and cholesterol are embedded. The in-
teraction of apolipoprotein A-I with DOPC-coated mercury pro-
ceeds in steps when increasing progressively its bulk concentra-
tion, c A-I . 61 For c A-I 4 Pg cm -3 the differential capacitance mini-
mum C is not affected, but the concomitant decrease in the orienta-
tion peaks of DOPC points to an interaction of apoA-I with the
polar heads of the lipid, possibly electrostatic, through its hydro-
philic amino acids. With a further increase in c A-I the plot of C ver-
sus c A-I shows a first sigmoidal step, with a plateau at ~ 4 PF cm -2 ,