Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 149

port. Requirement (iv) is needed to make the biomembrane model
sufficiently blocking as to characterize ion channel or ion pump
activity by electrochemical means without the disturbing presence
of stray currents due to defects.
Apart from lipid molecules, the molecules that are most com-
monly employed for the fabrication of biomimetic membranes are
hydrophilic spacers and thiolipids . Hydrophilic spacers consist of
a hydrophilic chain (e.g., a polyethyleneoxy or oligopeptide chain)
terminated at one end with an anchor group for tethering to a sup-
port and, at the other end, with a hydrophilic functional group
(e.g., a hydroxyl group). Sulfhydryl or disulfide groups are em-
ployed as anchor groups for tethering to metals such as gold, silver
or mercury (see Fig. 1B ); methyl-, methyloxy- or chloride-
substituted silane groups are used for tethering to glass, quartz,
silica or mica. The latter supports are nonconducting and cannot be
investigated by electrochemical techniques. Hydrophilic spacers
serve to separate the lipid bilayer from a solid support, to compen-
sate for surface roughness effects, to prevent any incorporated pep-
tides or proteins from touching the support surface (thus avoiding
loss of their functionality due to denaturation), and to provide an
ionic reservoir underneath the lipid bilayer.
Thiolipids differ from hydrophilic spacers in that the hydro-
philic chain is covalently linked to one or, more frequently, two
alkyl chains at the opposite end with respect to the anchor group,
as shown in Fig. 1A . The alkyl chains simulate the hydrocarbon
tails of a lipid molecule and provide one half of the lipid bilayer to
the biomimetic membrane. When tethered to a support, hydro-
philic spacers expose to the bulk aqueous phase a hydrophilic sur-
face, while thiolipids expose a hydrophobic surface. Clearly, lipid
bilayers formed on top of hydrophilic spacers are noncovalently
linked to them and can be regarded as freely suspended . Converse-
ly, lipid monolayers assembled on top of thiolipid monolayers
form lipid bilayers that are tethered to the support.
Before discussing advantages and disadvantages of these sys-
tems, we will describe in detail the electrochemical technique that
is commonly employed for their investigation, namely electro-
chemical impedance spectroscopy (EIS), as well as some fabrica-
tion methodologies. In fact, our attention will be mainly focused
on those biomimetic membranes that are amenable to investigation
by electrochemical methods. Biomimetic membranes that are in-