Biology Reference
In-Depth Information
3.3
CH
3
1.8
1.1
(0.6)
0.2
0.1
2.3
0.1
-
CH
2
-
CH
2
-
(CH
2
)
10
-
CH
2
-
CH
2
-
CO
-
-
CH
2
O
O
HC
O
C
O
-
-
0.1
0.7 CH
3
T
1
s of DPPC
N
+
2
HC
-
OO
-- -
P
CH
2
-
CH
2
-
-
CH
3
0.1
0.3
0.3
0.7
O
-
0.7 CH
3
FIGURE 9.33
13
C-NMR-derived T
1
s for DPPC. A high T
1
means the carbon exhibits a lot of motion. A lower T
1
indicates less motion. 'Micro-viscosity' of the carbons of DPPC varies 70 fold from the most restricted carbons near
the polar head group to the least restricted carbon at the omega terminus of the acyl chain.
proteins, and is sensitive to membrane physical state. The transition temperature, T
m
,of
DPPC as determined by FP (
Figure 9.30
) agrees with those reported by DSC (
Figure 9.22
)
and FTIR (
Figure 9.26
).
G. MEMBRANE 'FLUIDITY'
The term 'fluidity' is one of the most used, yet one of the most poorly understood in the life
sciences
[53]
. While everyone has a general feel for what 'fluidity' means, the devil is in the
detail. A dictionary definition of a 'fluid' is something that is 'capable of flowing, not a solid'.
In biology, fluidity refers to the viscosity of the lipid bilayer component of a cell membrane.
For water, fluidity is defined as the inverse of viscosity. Historically, viscosity was easily esti-
mated by simply measuring how fast a spherical marble falls through a solution. The concept
of viscosity was therefore developed around isotropic motion with equal forces on all sides of
a perfect sphere. But how much of this applies to a complex, highly anisotropic biological
membrane?
In a biological membrane there are three complex types of motion to consider. First there
is non-homogeneous lateral movement in the plane of the lipid bilayer. This is complicated
by lipid domains of various compositions and properties, crowding by multiple proteins
and protein complexes and interactions with the sub-membrane cytoskeleton. Next there
is rotational movement around the longitudinal axis of the molecule. Finally there is
trans-membrane lipid movement or flip-flop. In addition, regardless of how membrane
'fluidity' is defined, there will be a fluidity gradient from the aqueous interface through
to the bilayer interior. The extreme heterogeneity of biological membranes makes analysis
of 'fluidity' far more complicated than simply dropping a marble through a solution.
Usually with membranes, molecular motion is monitored with either spin probes or fluo-
rescent probes
[54]
. However, unlike marbles, the probes are not perfect spheres and so
do not behave isotropically. Instead, they exhibit preferred orientations (they are aniso-
tropic) in the bilayer and may preferentially partition into proteins or different membrane
domains. As a result, membrane probes are sensitive to both rates of motion and to
constraints on motion, and so report on both dynamics and order. Therefore, in biological
