Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 179

direction up to film desorption, the fluorescence intensity main-
tains a constant low level along the flat capacitance minimum; a
large increase in fluorescence is observed at far negative poten-
tials, indicating a separation of the lipid monolayer from the elec-
trode surface. By scanning back the potential, the fluorescence
decreases slowly down to the positive potential limit; however,
neither the fluorescence nor the capacitance recovers the pristine
value, denoting a defective reformed monolayer. In contrast to a
similar system on gold 47 , the fluorescent particles or aggregates on
Hg are freely mobile, preventing an image analysis yielding their
number density and size.
As a rule, incorporation of neutral hydrophobic organic com-
pounds, such as polynuclear aromatic hydrocarbons, 48 polychlorin-
ated biphenyls and phenothiazine, 49 into DOPC monolayers on Hg
causes a negative shift and a depression of peaks 1 and 2; this is
often accompanied by a slight decrease in the differential capaci-
tance minimum. This effect becomes more pronounced with an
increase in the aromaticity and hydrophobicity of the compound.
Thus, while the effect of benzene and naphthalene is negligible, it
becomes appreciable with hydrocarbons with three, four or five
aromatic rings. 50 Hydrophobicity alone, as measured, say, by the
octanol/water partition coefficient, is not sufficient to explain this
effect; thus, undecane and dodecane have a high partition coeffi-
cient but no observable effect on lipid monolayers. The slight de-
crease in the flat capacitance minimum which is often observed
with an increase in the bulk concentration of these compounds is
probably to be ascribed to a thickening of the film following their
incorporation in the lipid monolayer; this should more than com-
pensate for the expected increase in capacitance stemming from
the higher dielectric constant of aromatic compounds compared to
that, # 2, of the lipid tails. The concomitant decrease in the height
of the reorientation peaks 1 and 2 and their broadening is due to a
decrease in the cooperativity of the reorientation of the lipid mole-
cules, caused by the intercalation of the foreign molecules.
Phospholipid-coated mercury electrodes have been used to
measure the intrinsic protonation constants of DOPC, di-
oleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-
serine (DOPS) 51 and dioleoylphosphatidic acid (DOPA), 52 the sur-
face dipole potential of DOPC, DOPS and DOPA, 53 as well as the
change in the dipole potential of DOPS and DOPA with a change