Biophysical studies of membrane proteins/peptides. Interaction with ...

signal for neighbouring cells to phagocytose the dead cell and digest it (Alberts et al.,
2002).
1.4. Lipid structure and curvature
Lipids in hydrated conditions exhibit polymorphic behaviour as they can assemble
in different structures. The structure adopted by a lipid aggregate can be influenced by
the molecular structure of the lipid itself and a myriad of environmental conditions,
such as water content, pH, ionic strength, temperature and pressure. Lipid molecules can
assemble in an aqueous environment either in a lamellar structure (as in a lipid bilayer)
or in non-lamellar phases (micelle, hexagonal, inverted hexagonal phases, and the cubic
phase (see Fig. I.4)). Because phase transitions between these different aggregates can
be activated by changes in water content, these phases are called lyotropic.
The different possibilities for organizations of lipids are the result of the intrinsic
shape of the lipid molecule. When the headgroup of a lipid occupies the same area than
the area occupied by its hydrophobic section, the leaflets or monolayers formed by
association of molecules of this lipid will present zero spontaneous curvature, forming a
planar membrane for which both leaflets (or monolayers) present null curvature. A lipid
bilayer composed of this lipid with the same number of molecules in each monolayer
should be flat. However, in order to eliminate the aqueous exposure of the hydrophobic
edges of the bilayer, the bilayer can unite the edges and form a curved vesicle. In this
aggregate the exterior monolayer must present a concave (or positive) curvature and the
interior monolayer must present a curvature with opposite direction. In this way,
spontaneous curvature of a lipid monolayer refers to the curvature observed in the
absence of edge conditions (Zimmerberg, 2000).
For monolayers composed of lipids presenting headgroups with a cross-section
different to that of the hydrophobic tails, a spontaneous curvature will be present, and
the packing of these lipids in a lipid bilayer with a lamellar structure will result in
curvature stress, that can be supported by the lamellar structure only up to a certain
extent. In case the lipid bilayer cannot sustain this curvature stress, the lamellar
structure (L) will be broken and non-lamellar phases will arise. The basic structural
phase of biological membranes is nevertheless a lamellar phospholipid bilayer matrix,
and deviations from a lamellar arrangement are generally not desirable in the plasma
membrane. The inclusion of lipids with propensity to non-lamellar structure leads to a
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