Biology Reference
In-Depth Information
(a)
DPPC
DPPC
tilted
DPPE
s
s
s
2
∑
2
∑
S = 2
2
∑
S > 2
∑
S = 2
∑
∑
(b)
DPPC
L
P
β'
β'
L
α
DPPE
L
L
β
α
FIGURE 9.23
(a) Gel to liquid crystal phase transitions for DPPC and DPPE. Chain tilt noted for DPPC in the gel
state (L
b
'
) is not seen with DPPE (L
b
). Neither DPPC nor DPPE exhibit chain tilt in the liquid crystal (L
) state. (b)
Another unusual feature of the DPPC DSC scan that is not observed for DPPE membranes is the existence of 'ripple
structure' (P
b
'
) between the pre-transition and the main transition. PEs exhibit neither a pre-transition nor ripple
structure.
a
membrane contains hundreds to thousands of different lipids, greatly diminishing the likeli-
hood of ripple phase. Ripple phase is also not observed for heteroacid molecular species that
are predominant in membranes. Heteroacid phospholipids have different acyl chains
attached to the sn-1 and sn-2 locations.
The DSC curve for DPPC in
Figure 9.22
shows a narrowmain transition. The narrower the
transition, the more lipids melt as a single unit. Almost any lipid contaminant or a fast
temperature (scan rate) will reduce and broaden the curve. In fact, an absolutely pure phos-
pholipid scanned at an infinitely slow temperature ramp would theoretically result in
a vertical line where all of the gel state lipid would melt at once. Lipid contaminants broaden
and reduce the main transition.
Figure 9.24
shows the effect of increasing amounts of choles-
terol on the DPPC main transition. Even 1 mol % cholesterol noticeably affects the transition,
