Biology Reference
In-Depth Information
a particular phase is just a reflection of lipid molecular shape that manifests itself by
imposing molecular strain on adjacent proteins. This in turn would affect the protein's
conformation and thus activity. This is further discussed below. Perhaps non-lamellar phases
are just a curiosity observed only in well-defined lipid bilayers.
C. LIPID PHASE DIAGRAMS
Membrane lipids come in an overwhelming variety of chemical structures ('lipid diver-
sity') and each molecular species, when isolated in pure form, can aggregate into a large
number of long-range structures or phases ('lipid polymorphism'). Environmental condi-
tions (e.g. temperature, lateral pressure, extent of hydration, aqueous solution ionic strength,
presence of polyvalent metals, pH etc.) can further affect the lipid phase. The situation
becomes far more complicated with multi-component lipid mixtures, as the lipids interact
with one another resulting in phase separation into discrete lipid microdomains. And all
of this is before proteins are even incorporated into the membrane!
For years lipid biophysicists have attempted to pictorially describe various lipid inter-
actions and phases through what are known as 'phase diagrams'
[33]
.Phasediagramscan
take a wide variety of forms that report lipid behavior from different perspectives. As the
number of lipid components increases, the phase diagrams become ever more complex.
Anyone who is not confused by a three component phase diagram just does not under-
stand membrane structure. There are now a countless number of phase diagrams that
have been compiled
[34]
. Below are five examples of phase diagrams, starting with simple
one-component DPPC phase diagrams and progressing through to a complex, three-lipid
phase diagram of a lipid raft model.
Single-Component Lipid Phase Diagrams
We have already encountered one single-lipid phase diagram in the form of a DSC scan
of DPPC. This is depicted in Figure 9.22. The DSC scan follows the phase transitions of
DPPC from low temperature gel phase (L
'
) to high temperature liquid crystalline phase
b
(L
). With pure DPPC, there are two transitions, a lower temperature pre-transition (at
~35.6
C) and a higher temperature main transition, T
m
(at ~41.3
C). In the region between
the pre-transition and main transition the bilayer exists in a non-flat ripple structure (P
a
'
).
b
Therefore the DSC-derived phase diagram shows the following transitions:
L
P
L
b
0
/
b
0
/
a
A second single-component phase diagram looks at the DPPC transitions from a different
perspective.
Figure 10.12
shows the effect of hydration on DPPC lipid phase
[35]
. The lipid
solution becomes more dilute from left to right. The dotted line indicates maximum adsorp-
tion of water at the membrane surface. At any point to the right of the dotted line DPPC is
saturated with water. Normal biological water levels are
99.9%. Note that at almost all
hydration levels DPPC undergoes the same transitions reported in the DSC scan shown in
Figure 9.22 (L
>
). However at very low water content and high temperature,
additional non-lamellar phases, including cubic and H
II
appear. H
II
phase is not normally
P
L
/
/
b
'
b
'
a
