Biomedical Engineering Reference
In-Depth Information
was observed in the absence of Ca
2+
ions, nor even in their pres-
ence, if the POPC-POPS mixture was replaced by pure POPC.
This indicates that annexin A1 requires the simultaneous presence
of a negatively charged membrane and Ca
2+
ions to interact with
membranes. The adsorption isotherm of annexin A1 to
POPC:POPS (4:1) was found to satisfy the Langmuir isotherm and
its association constant was determined at different Ca
2+
ion con-
centrations.
3. Bilayer Lipid Membranes (BLMs)
An important advance in biomembrane methodology was estab-
lished when Mueller et al.
92
reported a method for forming planar
bilayer lipid membranes separating two aqueous phases. Accord-
ing to this method, a lipid solution in a nonpolar solvent such as
decane is spread beneath an aqueous phase across an aperture sev-
eral millimeters in diameter drilled through a partition (septum) of
some hydrophobic material such as Teflon. This procedure for
forming the lipid bilayer is called the
painting method
. The spon-
taneous organization of the amphiphilic lipid molecules into a bi-
layer is driven by the strong self-association of water molecules,
which inhibits the mixing of water and amphiphile. Strong lateral
intermolecular forces between the long hydrocarbon tails of the
lipid molecules also contribute to this self-assembly. The layer of
the lipid solution in the nonpolar solvent becomes gradually thin-
ner, with rainbow interference colors appearing on it, followed by
black spots; finally, the whole layer becomes completely black.
The blackening marks the transition of the lipid layer from a mul-
timolecular to a bimolecular film, called bilayer (or black) lipid
membrane (BLM), which is a non-reflecting optically black film.
The resulting system consists of a lipid bilayer in quasi equilibri-
um with an annulus of lipid solution in the nonpolar solvent, called
torus or Plateau-Gibbs border, which forms a transition zone be-
tween the bilayer and the septum. This system has been studied
extensively as a model for the lipid bilayer of cell membranes.
Nonetheless, its usefulness has been questioned, because the thin
lipid film contains solvent molecules dispersed within the bilayer
and in the form of microlenses floating in the bilayer.
93-97
To what
extent the presence of the organic solvent limits the usefulness of
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