Biophysical studies of membrane proteins/peptides. Interaction with ...

1.7. Lateral heterogeneity in lipid bilayer
As already pointed out, the lipid composition of biomembranes is highly complex.
In the case of multicomponent lipid bilayers, no longer a single T m applies to the whole
population of lipid molecules. Now the transition takes place over a range of
temperatures (between the T m of the lipid with the lower transition temperature and the
T m of the lipid with the higher transition temperature), in which the lipids phase separate
in gel and fluid phases (Mouritsen, 2005). This phase separation is a direct consequence
of the highly cooperative character of lipid phase. Immiscibility between lipids can
result from differences in acyl-chain length, unsaturation or headgroup composition
(Shimshick and McConnell,1973; Arnold et al., 1981; Lentz et al., 1976).
Phase diagrams provide an efficient tool for the description of phase behaviour of
multicomponent lipid mixtures. In the phase diagrams, the lines correspond to the
temperatures at which a transition starts or is finished. Three phase diagrams for three
different binary lipid mixtures are presented in Figure I.7. It is clear from Figure I.7 that
as the divergence in acyl-chain length increases, the size of the region of gel-fluid phase
coexistence also increases due to a more accentuated immiscibility between the two
lipids.
Figure I.7 – Phase diagrams of three different binary mixtures of saturated PC lipids presenting different
acyl-chain lengths – dilauroyl-sn-glycerol-3-phosphatidylcholine-DLPC (12 carbons); DMPC (14
carbons); DPPC (16 carbons) and distearoyl-sn-glycerol-3- phosphatidylcholine -DSPC (18 carbons). The
fluid phase is represented by an f and the gel phase by g (taken from Mouritsen, 2005).
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