Biology Reference
In-Depth Information
head group
O
OCCH
2
O
O C
16:0,16:0 PC
(DPPC)
FIGURE 10.25
Symmetric acyl chain 16:0,16:0 PC (DPPC) demonstrating that the sn-1 chain protrudes directly
into the bilayer interior while the first two carbons of the sn-2 chain run parallel with the membrane surface before
also bending into the bilayer interior.
interdigitated bilayers with 2 acyl chains subtended under the head group (
Figure 10.24
,A).
So, even in the melted, liquid crystalline state, lipids with large asymmetries in acyl chain
length may still retain significant interdigitation.
The maximum possible chain asymmetry exists for lyso lipids or lipids where the sn-2
chain consists of either a proton or a very short acetyl ester. An example of a biologically
important interdigitated lipid is C18,C2 PC, the platelet-activating factor. These lipids are
fully interdigitated in the gel state where the short sn-2 chain lies parallel to the plane of
the membrane, but upon undergoing a thermotropic phase transition develops membrane
disrupting micellar structure.
Interdigitated Structure is Inherent in Symmetrical Chain Phospholipids
Surprisingly, even phospholipids with identical sn-1 and sn-2 chains exhibit chain inequi-
valence. While the sn-1 chain drops straight down into the bilayer interior, the first two
carbons of the sn-2 chain run parallel to the membrane plane, before bending into the bilayer
