Biomedical Engineering Reference
In-Depth Information
The lamellar-inverse hexagonal-lamellar phase transition may also dramatically
depend on the concentration of water molecules associated with the membrane form-
ing lipids. The L
-to- H II phase transition is observed to take place with the removal
of water. A possible explanation is that the removal of water causes a reduction in
the area per lipid polar head group which accounts for an increase of the probability
of a high-curvature structure. For details see [ 25 ]. Somewhat surprisingly, a study
[ 6 ] demonstrated that the removal of water can also induce an H II -to- L
α
transition in
dioleoylphosphatidylethanolamine. Careful mapping of the above-mentioned lipid
phase diagrams by X-ray diffraction and nuclear magnetic resonance spectroscopy
has revealed that this lipid in fact undergoes H II -to- L α - H II transitions with progres-
sive hydration, which has been called a 'reentrant' transition [ 16 , 24 ]. An attempt
has been made [ 16 ] to explain the reentrant transition by using a model that takes
into account a balance between elastic and hydration energies in the L α and H II
phases of dioleoylphosphatidylethanolamine. Here, the chemical potentials of lipid
and water molecules in both phases have been calculated to find the condition of
phase equilibrium and the phase diagram of the system has been reconstructed.
Let us consider the chemical potentials are respectively
α
μ l (
)
μ l (
)
L
and
H
for lipid
μ w (
)
μ w (
)
in lamellar and inverse hexagonal phases, while
are chemical
potentials for water in lamellar and inverse hexagonal phases. In the case of non-
equal or imbalanced chemical potentials for lipid and water, an independent phase
(lamellar or hexagonal) exists. However, both lamellar and inverse hexagonal phases
may coexist if the following condition is satisfied:
L
and
H
μ l (
L
) = μ l (
H
)
and
μ w (
L
) = μ w (
H
)
(3.2)
Considering the osmotic pressure that determines the chemical potential of the
lipid [ 16 ] and the hydration, we can now draw the phase diagrams for a reentrant
transition such as those in Figs. 3.6 and 3.7 . There is no external reservoir of water
in this case, and the chemical potential of water cannot be externally controlled. The
system itself sets the chemical potentials of water and lipid to minimize the free
energy.
FollowingEq. 3.2 , the coexisting lamellar and inverse hexagonal phasesmay occur
near the curved boundary separating the independent lamellar and inverse hexagonal
lipid phases (see Fig. 3.6 ). Identical coexisting lamellar and inverse hexagonal phases
may occur near the independent lamellar and inverse hexagonal phase boundaries
(see Fig. 3.7 ). In Fig. 3.7 , the presence and absence of hydration in both lamellar and
inverse hexagonal lipid phases is especially worth pointing out.
The phase diagrams plotted in Figs. 3.5 , 3.6 and 3.7 exhibit various membrane and
lipid geometries, hydration, osmotic pressure profiles, etc. However, as mentioned
earlier, all of them ignored a very important lipid property, namely the charge profile
of the participating lipids. Once the charge profile is considered, the whole picture
of energetics in lipid membranes in different phases will require a major revision
and perhaps a new phase diagrammay also be necessary. In later chapters, the reader
will learn about the importance of including charge profiles of lipids in view of our
discovery of electrostatic lipid-lipid and lipid-membrane protein interactions. If we
 
Search WWH ::




Custom Search