Biology Reference
In-Depth Information
TABLE 11.2
Packing Free Volume (f
v
) and Metarhodopsin I
Metarhodopsin II
Equilibrium (K
eq
) for Various PCs Commonly Found in the Rod Outer
Segment.
4
meta I
Packing free
volume (
4
meta II (
Phosphatidylcholine
K
eq
)
f
v
)
22:6,22:6 PC
6
0.201
20:4,20:4 PC
4.7
0.278
16:0,22:6 PC
3.3
0.154
16:0,20:4 PC
2.7
0.155
14:0,14:0 PC
1.7
0.101
metarhodopsin I
metarhodopsin II equilibrium, expressed as K
eq
, in bilayer membranes
made from five of the same lipids reported in
Table 11.1 [7,8]
.
Table 11.2
shows a general rela-
tionship between formation of the more voluminous, active metarhodopsin II (expressed as
K
eq
) and the packing free volume (expressed as f
v
). A larger f
v
drives the metarhodopsin I
4
4
metarhodopsin II equilibrium to the right. These investigators also reported a reduction in
K
eq
with both lower temperature and cholesterol. From their observations,Mitchell andLitman
proposed a molecular reason for the naturally high levels of docosahexaenoic acid in the rod
outer segment and the requirement of this fatty acid for vision
[9]
.
Lipid Monolayers (Langmuir Film Balance)
The first reported membrane physical properties were determined on lipid monolayers
using a Langmuir film balance or trough. This device was named after Irving Langmuir,
an early pioneer in the field (see Chapter 2). A schematic of a Langmuir film balance is
depicted in
Figure 11.1
. The trough is normally made of an inert, non-wettable material
such as Teflon or polytetrafluoroethylene (PTFE). The monolayer is formed by adding a small
volume of a volatile organic solution containing the dissolved lipid to the surface of an
Fixed
Barrier
Mobile
Barrier
Balance
Li
p
id
Mo
no
l
a
y
er
Subphase
(water)
clean
surface
FIGURE 11.1
Diagram of a Langmuir Film Balance. A lipid monolayer is deposited on a clean aqueous surface
between two barriers. Movement of the mobile barrier from right to left compresses the monolayer as the lateral
pressure is continuously monitored via a sensitive balance attached to a Wilhelmy plate inserted through the
monolayer.
