Biomedical Engineering Reference
In-Depth Information
tween the support and at least the proximal leaflet of the lipid bi-
layer may induce undesirable asymmetries in compositional, struc-
tural, mechanical and dynamic properties of the lipid bilayer.
131
The lubrification effect of the water layer imparts a significant
long-range lateral mobility to the lipid bilayer. However, signifi-
cant frictional coupling between the bilayer and the underlying
substrate slows down lateral diffusion, which may be accompanied
by a breakdown of the two-dimensional fluid nature of the mem-
brane.
The above biomembrane models are not suitable for studying
the function of integral proteins. In fact, these proteins have hy-
drophilic domains protruding outside the lipid bilayer. To avoid
their denaturation and to promote their function, the incorporation
of integral proteins into biomembrane models must ensure that
their protruding hydrophilic domains are accommodated in a hy-
drophilic medium on both sides of the lipid bilayer. Not surprising-
ly, embedded membrane-spanning proteins usually show no lateral
diffusion, because of their interaction with the substrate
132
, even
though some of them maintain their function if their active site is
far from the solid substrate.
The majority of sBLMs are formed on nonconducting sup-
ports such as silica, glass or mica.
18,32,37
Consequently, they are not
amenable to electrochemical measurements. Thus, for instance,
sBLMs on silica were recently formed from mixtures of native
Escherichia coli
bacterial inner membrane (IM) vesicles, which
contain membrane proteins, diluted with egg-PC vesicles.
132
These
sBLMs were investigated by QCM, AFM, attenuated total inter-
nal-reflection Fourier-transform infrared spectroscopy (ATR-
FTIR) and FRAP. Samples with less than 40% IM were found to
form sBLMs by vesicle fusion. They were fluid, as shown by
FRAP, and those with 30% IM contained the protein, as confirmed
by ATR-FTIR. Conversely, the samples with more than 40% IM
were dominated by vesicle adsorption. By forming a sBLM on
silica, side by side with an alkanethiol/lipid hybrid bilayer on gold,
it was found by FRAP that the lipid molecules of the distal mono-
layer of the sBLM could move laterally, mixing freely with those
of the hybrid bilayer.
133
Moreover, the distal and proximal mono-
layers of the sBLM on silica could also mix by flip-flop.
In what follows, a few examples of sBLMs investigated by
electrochemical techniques will be examined. Phospholipid bi-
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