Biology Reference
In-Depth Information
H
-
Q
-
P
Q
-
+ L
a
?
β
120
Homogeneous
Liposome Dispersion
80
L
a
+ L
β
'
60
L
a
40
P
β
'
L
β
'
20
10
20
30
C
-
(% H
2
O)
40
50
99.9
FIGURE 10.12
Phase diagram of DPPC showing the effect of water content (hydration) on lipid phase. At high
(biological) levels of water, DPPC bilayers go from L
b
'
/
P
b
'
/
L
a
. Other unusual phases exist under conditions of
extreme dehydration
[35]
.
reported for any PC, but can form under extreme dehydration. Importantly, dehydration may
occur at localized places on a membrane surface in the presence of chaotropic agents or diva-
lent metals including Ca
2
þ
. These agents are known to replace waters bound to a phospho-
lipid head group, thus creating a localized area of extreme dehydration.
Two-Component Lipid Phase Diagrams
A phase diagram for bilayers composed of mixtures of DMPC (14:0,14:0 PC) and DPPC
(16:0,16:0 PC) is shown in
Figure 10.13
, top panel (a). The T
m
of DMPC is 23.6
C and DPPC
41.3
C, as depicted on the Y-axis. Since the T
m
s are so close, mixtures produce a broad transi-
tion between the two single phospholipid T
m
extremes. The dotted line represents the theoret-
ical T
m
for various DMPC/DPPCmixtures. The solid lines on either side of the dotted line are
the experimentally derivedmelting values. The thick arrow follows a 1:1 (mol:mol) mixture of
DMPC/DPPC as it is heated from a temperature where the lipid mixture is entirely in the gel
(L
)
state. In the region between temperatures T
1
and T
2
the membrane consists of co-existing
gel and liquid crystalline states. The corresponding DSC scan is shown in the bottom
'
) state through the melting transition T
m
until the mixture is in the liquid crystalline (L
b
a
