Biomedical Engineering Reference
In-Depth Information
then cause the insertion of the membrane proteins in the resulting
lipid bilayer.
4.
Rapid Solvent Exchange
A further procedure for depositing a lipid monolayer is the
rapid
solvent exchange
. This method is usually employed to self-
assemble a lipid monolayer onto a hydrophobic surface exposing
alkyl chains to the bulk aqueous phase. The method involves plac-
ing a small amount of lipid dissolved in a water miscible solvent,
such as ethanol, onto the hydrophobic substrate and incubating it
for a few minutes.
34,35
The ethanol solution is, then, vigorously
displaced in a few seconds by a large excess of an aqueous buffer
solution, taking care to avoid the formation of air bubbles at the
surface. During the lipid addition and the subsequent rinsing with
the aqueous solution, the lipid self-assembles to form a lipid
monolayer and the lipid in excess is rinsed away.
5.
Fluidity in Biomimetic Membranes
A fundamental property of biological membranes is the long-range
lateral mobility of the lipid molecules. The fluidity of the plasma
membrane, interposed between the cytosol and the extracellular
space, should be preserved in supported biomimetic membranes.
Free movement of lipid molecules enables the biomimetic mem-
brane to react to the presence of proteins, charges and physical
forces in a dynamic and responsive manner. A satisfactory fluidity
allows biomimetic membranes to reorganize upon interaction with
external perturbations, mimicking the functionality of living cell
membranes. In particular, lateral mobility enables a biomimetic
membrane to incorporate large membrane proteins from their de-
tergent solutions by making space for them; it also determines the
spontaneous separation of the components of a lipid mixture (de-
mixing), giving rise to the formation of important lipid microdo-
mains, called lipid rafts.
The fluidity and lateral mobility of biomimetic membranes
can be characterized quantitatively by AFM
23
and fluorescence
microscopy. Fluorescence recovery after photobleaching (FRAP)
is one of the most popular ways of measuring molecular diffusion
in membranes.
36
It relies on introducing a small amount of fluores-
Search WWH ::
Custom Search