Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 175

cent probe molecules, usually covalently bound to lipids, into the
membrane. A short burst of intense excitation light is projected
onto the membrane, destroying the fluorescence of the fluorophore
molecules in a well defined spot, a photochemical process called
photobleaching. The gradual fluorescence recovery within the giv-
en spot is followed as a function of time, thus permitting an esti-
mate of the lipid diffusion coefficient. In supported membranes,
this coefficient typically ranges from 1 to 10 Pm 2 s -1 . When ap-
plied to lipid bilayers self-assembled on smooth supports such as
silica, glass, quartz, mica or indium tin oxide (ITO), FRAP usually
confirms a satisfactory lateral mobility of lipid molecules. 18,37
Conversely, biomimetic membranes consisting of a thiolipid mon-
olayer tethered to a gold electrode, with a self-assembled lipid
monolayer on top of it, does not exhibit lateral mobility. This is
also true for the distal lipid monolayer noncovalently linked to the
thiolipid monolayer, no matter if obtained by vesicle fusion or by
Langmuir-Schaefer transfer. 6,30 A biomimetic membrane consist-
ing of a hydrophilic spacer tethered to gold, with a lipid bilayer
formed on top of it, was reported to exhibit fluorescence recovery
after photobleaching, if the noncovalently bound lipid bilayer was
formed by Langmuir-Blodgett and Langmuir-Scheafer transfers. 6
Fluorescence recovery in the gold-supported biomimetic mem-
brane could only be observed for a short period of time, due to the
gradual energy transfer from the fluorophore molecules to gold
(quenching). No fluorescence recovery could be observed by form-
ing the lipid bilayer on top of the hydrophilic spacer by vesicle
fusion. Evidently, the unavoidable presence of adsorbed and hemi-
fused vesicles prevents lateral mobility of the lipid molecules of
the distal monolayer. The strict connection between the presence
of adsorbed vesicles and the lack of lateral mobility allows conclu-
sions on lateral mobility to be drawn by monitoring the presence of
adsorbed vesicles by a QCM-D.
Electrostatic interactions between vesicles and support play an
important role in vesicle fusion. Thus, variously charged gold-
supported mixed monolayers of alkanethiols Z-functionalized with
the neutral -OH group, the positive -NH 3 + group or the negative -
COO - group induce the fusion of oppositely charged vesicles, as
monitored by FRAP. 38 Conversely, the fusion of vesicles made of
neutral, zwitterionic lipids takes place only if the charge of the
mixed functionalized alkanethiols exceeds a minimum value; this