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Biological
membranes are heterogeneous and highly dynamical organizations of
lipids and proteins that define the outer boundary of a living cell.
Solid supported lipid bilayers have often been studied as model systems
to understand the structures and properties of such cellular membranes.
Such systems are constrained to a planar geometry and unable to mimic
natural fluctuations and curvature of biological membranes. Presently,
we are involved in the studies of two soft supported systems; one being
the polymer-cushioned bilayer (Fig. 1: Left) and the other a -floating
bilayer (Fig.1: Right) to allow the bilayers to have their structural
freedom to precisely reproduce the morphology of a cell membrane.
Electron density profiles obtained from such systems by reflectivity
studies using synchrotron x-rays provide unprecedented structural
details of these systems.
The
effects of ions, such as Ca2+, on the structure of these systems and
the distribution of these ions near the bilayer have been investigated.
These ions are known to reorganize the lipid molecules in cellular
membranes to facilitate the binding and insertion of various proteins
and are also predicted to enhance the membrane fusion process. Our
study shows that these ions are preferentially bound to the head group
regions of the bilayer. Again, they are observed to intensify the
flexibility of the bilayers which is exhibited by the increased
inter-facial roughness.
Fig.
1: 'Soft supported' lipid bilayers: (Left) Single bilayer on top of a
polymer cushion, (Right) A floating bilayer on top of a second bilayer.
These
soft supported systems are further being used to study the
lipid-protein interactions. The adsorption of peripheral membrane
protein spectrin is found to depend on the phospholipid headgroups,
exhibiting different types of binding to phosphatidylcholine (PC) and
phosphatidylethanolamie (PE). The interaction of OMP-A, an integral
membrane protein is further observed to be sensitive to the various
phases of the lipids molecules.
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