Biology Reference
In-Depth Information
One measure of this, a parameter known as the 'packing free volume' (f
v
), was first intro-
duced in a series of papers in the late 1980s by Straume and Litman to quantify bulk phos-
pholipid acyl chain packing
[3]
. This parameter was derived from time-resolved
anisotropy decay of the fluorescent lipid bilayer probe DPH (1,6-diphenyl-1,3,5-hexatriene,
see Chapter 9) which characterizes the volume available for probe re-orientational motion
in the anisotropic bilayer relative to that available in an unhindered, isotropic environment.
The DPH-containing membrane (where DPH displays partially hindered, anisotropic
motion) is placed in the fluorescence sample cuvette, while the reference cuvette contains
POPOP (1,4-bis(5-phenyloxazol-2-yl)benzene) in absolute ethanol. POPOP in ethanol
displays totally unhindered, isotropic motion. Totally hindered motion has an f
v
of 0.0, while
the f
v
of totally isotropic motion is 1.0. The measured f
v
for a membrane will fall between
0 and 1. The f
v
is particularly adept at measuring small changes in lipid packing that can
profoundly affect integral membrane protein conformation.
Mitchell and Litman
[4
e
6]
have extensively used the packing free volume parameter to
explore the effect of membrane lipid unsaturation on the visual receptor metarhodopsin I
4
metarhodopsin II equilibrium.Metarhodopsin II is the active formof the receptor and occupies
a larger volume than does metarhodopsin I.
Table 11.1
presents f
v
s for several PCs commonly
found in the rod outer segment membrane
[7,8]
. At near physiological temperature (40
C), the
values increase from liquid crystalline 14:0,14:0 PC with no double bonds ( f
v
¼
0.101) to
22:6,22:6 PC with 12 double bonds (f
v
¼
0.201). This demonstrates an increase in packing
free volume with increasing phospholipid unsaturation. Also, f
v
decreases for every phospho-
lipid at lower temperature (10
C) and increasing cholesterol content. Lower temperature
decreases the number of gauche kinks in the acyl chains, thus decreasing the packing free
volume. And, as discussed below, cholesterol 'condenses' fluid state phospholipids, also
decreasing the packing free volume. In addition, Mitchell and Litman measured the
TABLE 11.1
Values of the Packing Free Volume Parameter, f
v
, in Phosphatidylcholine
Bilayers.
f
v
40
C
2
, 30 mol%
cholesterol
Acyl Chain
Composition
40
C
1
10
C
1
di 22:6n3
0.201
0.013
0.133
0.01
0.122
0.009
di 20:4n6
0.278
0.016
0.205
0.01
0.11
0.01
di 18:1n9
0.147
0.005
0.119
0.005
0.062
0.002
16:0, 22:6n3
0.154
0.005
0.096
0.005
0.074
0.003
16:0,20:4n6
0.155
0.003
0.124
0.004
0.067
0.002
16:0,18:1n9
0.130
0.005
0.073
0.003
0.056
0.002
di 14:0
0.101
0.006
-
0.011
0.002
1
Values from Mitchell and Litman. 1998. Biophysical Journal 74, 879
e
891.
[7]
2
Values from Mitchell and Litman. 1998. Biophysical Journal 75, 896
e
908.
[8]
Values kindly provided by Drake Mitchell, Department of Physics, Portland State University.
