Lipids in Membranes - Membrane Biophysics, Biological and Medical Physics, Biomedical Engineering - page 39

Fig. 3.5 Phase diagram showing different lipid phases and the change of the phase transition tem-
perature (T, ◦ C) as a function of the change of hydrocarbon chain length (L, Å) of lipid 1,2-dialkyl-
3-O-( -D-glucopyranosyl)- rac -glycerols. This figure is redrawn in light of the results published in

Ref. [ 18 ]. The slope of each curve determining the boundary between two different phases and the
intersection points between different curves are expected to vary substantially for different lipids

β

thickness. The phase diagram in Fig. 3.5 [ 18 ] shows how the various lipid phases
depend on the structural component.
The transition temperatures between lamellar and non-lamellar, and between
different substrates within both phases vary with the lipid hydrocarbon chain lengths.
The whole picture needs to be replaced with a totally different phase diagram for a
different type of lipid in the phospholipid composition. It is possible to maintain the
same intrinsic lipid curvature as we extend the hydrocarbon acyl chain length. This
means that the length of the hydrocarbon chain also plays a crucial role in deter-
mining the lipid phase or the phase transition temperature as the lipid organization
undergoes a phase transition. The hydrocarbon-packing free energy, as mentioned
earlier, may vary with the change of hydrocarbon chain length and as a result may
also contribute differently into the overall free energy profile. The latter quantity
includes the monolayer elastic energy and other contributions which arise from the
consideration of lipid charges, etc. A thorough theoretical understanding of the exper-
imentally determined phase diagram presented here may be possible, but only after
we understand all the energy contributions due to the above-mentioned lipid phases.
For more experimental observations on how the phase diagram varies for various
lipids, we refer the reader to the relevant literature [e.g., 8 , 20 - 22 ] dealing with the
homologous series of n -saturated diacyl and dialkyl glycosyl glycerolipids. These
studies suggest that the effects of hydrocarbon chain lengths on their non-lamellar
phase behavior are similar to those observed with the PEs.