Biology Reference
In-Depth Information
(a)
T
2
41.3
TºC
TºC
23.6
T
1
0
0.5
MOL FRACTION
DPPC
1.0
(b)
T
2
T
1
Δ
H
TºC
FIGURE 10.13
(a) Phase diagram for mixtures of DMPC (14:0,14:0 PC) and DPPC (16:0,16:0 PC). The T
m
of
DMPC is 23.6
C and DPPC 41.3
C. LUVs made from a 1:1 mixture of DMPC/DPPC are subjected to increasing
temperature from low, where both lipids are in the gel state, to high temperature, where both lipids are in the liquid
crystalline state (thick arrow). (b) Corresponding DSC scan for the DMPC/DPPC mixed membrane.
panel (b). As the gel state mixture is heated, the membrane begins to melt at temperature T
1
.
The first component tomelt is enriched in the lowermelting lipid, DMPC. In theDSC scan, T
1
is
the temperature where the membrane starts to melt and the scan leaves the baseline. As the
melting continues, the mixture becomes more enriched in the higher melting DPPC. At T
m
the mixture is half gel and half liquid crystal. At temperature T
2
the entire mixture melts as
the membrane enters the liquid crystalline (L
a
) phase where the DSC scan returns to the base-
line. The last component to melt is highly enriched in DPPC, and is depleted of DMPC.
A second two-component phase diagram is shown in
Figure 10.14
for mixtures of DPPC
and cholesterol
[36]
. Clearly this phase diagram is far more complex than that of DMPC/
DPPC. At 0 mol% cholesterol the lipid melts at 41.3
C, confirming it is DPPC. At low choles-
terol concentrations (
5 mol%), DPPC can simultaneously exist in either a liquid crystalline
(l
d
) or gel (S
o
) state, depending on the temperature. At high cholesterol levels (
<
30 mol%),
DPPC only exists in the liquid ordered (l
o
) state. Liquid ordered (l
o
) is a state mid-way
between liquid crystalline (l
o
or L
>
'
) states, having properties of both. Liquid
ordered state is characterized by having tightly packed lipids (like in gel) that nevertheless
support rapid lateral diffusion (like in liquid crystalline). Between 5
) and gel (S
o
or P
a
b
e
30 mol% cholesterol,
DPPC exists in mixed states, l
d
e
l
o
or S
o
e
l
o
.
A Three-Component Lipid Phase Diagram
Three-component phase diagrams are normally quite complex and not easy to inter-
pret
[37]
.
Figure 10.15
shows an example of a lipid raft model phase diagram by de
Almeida et al. (2003)
[38]
. Membranes were composed of cholesterol/POPC/PSM where
