Biomedical Engineering Reference
In-Depth Information
and reproducibility to the monolayer. This self-assembly proce-
dure exploits the fact that mercury is the most hydrophobic metal.
This by no means implies that mercury has no affinity for the oxy-
gen of water; in fact, at the potential of zero charge, the water mol-
ecules adsorbed on bare mercury are slightly oriented with the
oxygen turned toward mercury, although less than on all other
metals.
42
However, as the adsorbing lipid molecules have to decide
where to turn their hydrophilic polar head to attain a minimum in
their adsorption free energy, they choose to turn it toward the
aqueous phase, where the polar head can form hydrogen bonds
with the adjacent water molecules.
Thanks to the liquid state of the mercury support, this simple
biomembrane model is fluid and allows lateral mobility of the lipid
molecules. However, it has no hydrophilic ionic reservoir on the
metal side of the lipid film, and consists of a single lipid monolay-
er. Consequently, it is not suitable for the study of the function of
integral proteins. It can be used to investigate the behavior of the
polar heads of the lipid monolayer with varying pH, the behavior
of small lipophilic biomolecules incorporated in the lipid film and
that of peripheral redox proteins adsorbed on the film surface.
Over the potential region of minimum capacitance, which
ranges from -0.15 to -0.75 V/SCE, the film is impermeable to
inorganic metal ions, whereas it becomes permeable outside this
region. The differential capacitance
C
of a lipid monolayer on
mercury over this region is about1.7-1.8 PF cm
-2
, namely twice
the value for a solvent-free black lipid membrane (BLM). At posi-
tive potentials the region of minimum capacitance is delimited by
a capacitance increase that precedes mercury oxidation; at negative
potentials it is delimited by two sharp peaks that lie at about 0.9
and -1.0 V/SCE (peaks 1 and 2 in
Fig. 11
) and by a third peak at
about -1.35 V/SCE in the case of DOPC. Peaks 1 and 2 of DOPC
are capacitive in nature and are due to field induced two-
dimensional phase transitions. The third peak in
Fig. 11
exhibits
hysteresis in the reverse potential scan
41,43
and is due to partial
desorption of the lipid. Complete desorption takes place at poten-
tials negative enough to cause a merging of the curve of the differ-
ential capacitance
C
against the applied potential
E
recorded on
lipid-coated mercury with that obtained on bare mercury, under
otherwise identical conditions.
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