Abstract:
Phospholipids in water form lamellar phases made up of alternating
layers of water and bimolecular lipid leaflets. Three complementary
methods, osmotic, mechanical, and vapour pressures, were used to measure
the work of removing water from lamellar phases composed of frozen
dipalmitoylphosphatidylcholine ( DPPC ), melted DPPC, egg phosphatidylethanolamine
or equimolar mixtures of DPPC and cholesterol ( DPPC/CHOL ),
Concurrently the structural changes that resulted from this water removal
were measured using X-ray diffraction. The work was divided into that
which forces the bilayers together ( F ) and that which compresses the
molecules together within the bilayers ( F )#
A large repulsive force exists between bilayers composed of each
of the lipids studied and this force increases exponentially as bilayer
separation is decreased. F is affected by the nature of the head groups,
conformation of the acyl chains and heterogeneity of these chains. In
general all of the melted phosphatidylcholines ( melted DPPC, egg lecithin
and DPPC/CHOL ) have large equilibrium separations in excess water resulting
from large repulsive hydration forces between these bilayers. By comparison,
egg PE has an increased attractive force, and frozen DPPC has a decreased
hydration force; each results in smaller separations in water for these two
lipids. The chemical potentials of the water between the bilayers for
all these lipids lie on a continuum, indicating that interbilayer water
cannot be characterized by two discrete states, usually referred to as
"bound" or "non**bound".
For all lipids studied a maximum of 25 % of the total work done
on the system goes into deforming the bilayers. The method used here
viii
to separate repulsion from deformation, developed for us by v. A. Parsegian,
provides a unique method for the measurement of lateral pressure of a
bilayer and its modulus of deformability ( Y ). Lateral pressure is affected
by the nature of the head group, conformation and heterogeneity of the
acyl chains. For small changes in molecular surface area ( A ) near
equilibrium, both melted and frozen DPPC have similar values for the
deformability modulus. Thus in this regime it requires about the same
force to change the angle of tilt of frozen chains as it does to compress
the fluid bilayer. The introduction of cholesterol into bilayers of DPPC
reduces dramatically the lateral pressure of the bilayers over a large
range of molecular surface areas ( A ).
The variation in the magnitude of bilayer repulsion with different
phospholipids provides a basis for the mechanism of lipid segregation in
mixed lipid systems and suggests that interacting heterogeneous membranes
may influence or modulate the composition of the opposing membrane.
The measurements of deformabilities of bilayers provides a direct
comparison of them with the properties of monolayers.
