Biology Reference
In-Depth Information
destructive interference (
Figure 9.4
). Eventually a black spot, indicative of total destructive
interference, appeared. The black spot rapidly expanded to occupy the entire hole. Using
Bragg's law, it was a simple calculation to determine that the BLM was ~40
80
˚
thick.
e
This experiment proved that the lipid bilayer was indeed stable!
X-Ray Diffraction
X-Ray diffraction can generate limited high-resolution structural information, but only for
membranes that have highly ordered crystalline repeating units. A few biological
membranes (e.g. myelin sheath and the rod outer segment) are naturally stacked and so
are ideally suited for X-ray diffraction studies. In fact, as early as 1935 X-ray studies on
myelin were consistent with the presence of a lipid bilayer. Also, mitochondria and erythro-
cyte membrane vesicle preparations and phospholipid vesicles (liposomes) can be collapsed
by centrifugation, also making them suitable for X-ray diffraction analysis. The electron
density profiles of all membranes look very similar by X-ray diffraction. They consist of
a low electron density hydrocarbon interior flanked by a high electron density polar group
on either side.
Figure 9.5
shows an electron density profile of DMPC and DMPC/cholesterol
liposomes
[4]
. This bilayer has a 43
˚
separation between polar head groups. The addition of
more lipids or even proteins only slightly modifies the profile. Details of membrane proteins,
1
0
-1
0:1 C:PL
0.6:1 C:PL
-2
-30
-20
-10
0
Angstroms
10
20
30
FIGURE 9.5
Electron density profile obtained by X-ray diffraction for lipid vesicles (liposomes) made from DMPC
(solid line) or DMPC/cholesterol (0.6:1 mol:mol, dotted line). The most electron-dense part of the spectrum corresponds
to the polar head group regions. The least electron-dense region is the bilayer center found between the two leaflets
[4]
.
