Biology Reference
In-Depth Information
FIGURE 10.10
James Rothman. Courtesy of Yale University.
patch. Like H
II
, the cubic phase has also been proposed to be involved in fusion. However, the
role of the cubic phase in fusion is at best uncertain, as this phase has been far less extensively
investigated thanH
II
. The cubic phase is a lipid bilayer that is convoluted into a complex bicon-
tinuous surface. Although the role of the cubic phase in biological membranes is unknown, it
has found a home in a totally different arena. Q
224
(normally composed of mono-olein) has
been successfully employed to nucleate and grow membrane protein crystals for X-ray crys-
tallography
[32]
. The protein partitions into the cubic membrane phase while the detergent
used in the protein's isolation (discussed in Chapter 13) partitions into the aqueous phase.
There is great hope that this method may provide a new paradigm for crystallizing difficult
membrane proteins.
31
-P-NMR
Perhaps the most employed and easiest to interpret technique that can distinguish
membrane phase is
31
P-NMR. Although X-ray diffraction is the classic technique used to
define the parameters of model membranes, data interpretation is difficult and its application
to biologicalmembranes limited.
31
P-NMR is sensitive to phospholipidheadgrouporientation
and its spectra can clearly distinguish lamellar, H
II,
and micellar phases (
Figure 10.11
)
[19]
.
Lamellar phase spectra are distinguished by a broad (40
50 PPM), low-field shoulder and
a high-field peak. In total contrast, H
II
phase spectra are narrow (~20 PPM, half the width of
the lamellar phase spectra) and feature a low-field peak and high-field shoulder. A third
type of spectra is a narrow featureless peak that may indicate the presence of micellar struc-
ture, small rapidly tumbling vesicles or even cubic or rhombic phase. Unlike X-ray diffraction,
31
P-NMR can be used to follow lamellar
e
H
II
transitions. In one example, variousmixtures of
egg PC and soy PE were made into large unilamellar vesicles (methods discussed in Chapter
13). The vesicles were then subjected to
31
P-NMR analysis. The pure egg PC vesicles produced
a spectra indicating lamellar phase only (i.e. it had a low-field shoulder and a high-field peak).
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