Biology Reference
In-Depth Information
100
˚
) suggests
that there is little inherent curvature in the lipid (i.e. the lipid prefers a cylindrical shape). A
lower absolute value (
from X-ray diffraction of lipid films. A large negative or positive value (
>
100
˚
) suggests that the lipid will add a curvature stress (negative
or positive) and thus has a cone or inverted cone shape.
Table 10.2
lists the R
o
s of a fewcommon
membrane lipids
[23]
. The cylindrical lipids, DOPS and DOPC, have R
o
s
<
100 and so form
a lamellar structure. Lyso PC has a small positive R
o
indicating it is an inverted cone and so
prefers the micellular phase. DOPE, cholesterol,
>
-tocopherol, and DOG (dioleoylglycerol)
all have low negative R
o
s indicating they are cone-shaped and so prefer a non-lamellar phase
like H
II
.
a
Inverted Hexagonal Phase (H
II
)
The most studied of the non-lamellar phases is the inverted hexagonal phase, referred to
as H
II
[18,20,26]
.H
II
phase (
Figure 10.8
) consists of 6 long, parallel cylindrical tubes of indef-
inite length that are distributed into a hexagonal pattern, thus giving rise to the designation
inverted hexagonal phase. Water fills the cylinders and hydrocarbon chains fill the voids
between the hexagonally packed cylinders, holding the entire H
II
structure together. The
molecular shape of H
II
-preferring lipids (wide base, narrow head) gives negative curvature
strain to adjacent lipids or proteins.
A full H
II
phase would require a substantial number of cone-shaped lipids to be recruited
to one location in a biological membrane. This seems highly unlikely and many unsuccessful
attempts have been made to clearly identify H
II
structure in complex biological membranes.
But this does not necessarily mean that the H
II
phase is totally absent. Perhaps characteristic
signals from the limited H
II
phase are simply lost due to the 'population weighted average
problem'. Even though it is unlikely that any significant level of H
II
actually exists in biolog-
ical membranes, this non-lamellar phase has received, and continues to receive, considerable
attention in the world of membrane biophysics. Primarily through model membrane studies,
H
II
phase-preferring lipids have been strongly linked to fusion and other membrane contact
phenomena
[27]
. It was proposed that during fusion, the lipids of two stable bilayers
must intermingle through some kind of unstable intermediate. This process is known as
FIGURE 10.8
The reverse hexagonal H
II
phase consists of an array of six cylinders. In each cylinder, water fills
the channel interior that is lined by the lipid polar head groups. The outward facing acyl chains interact with each
other, stabilizing the entire structure via the hydrophobic effect
[18]
.
