Biology Reference
In-Depth Information
40
30
ab
c
20
10
0
40
60
80
100
120
Area (Å
2
/molecule)
FIGURE 11.3
P
-A curves for lipids of different compressibilities. Curve a is the highly incompressible 16:0,16:0
PC which remains in the solid-like phase throughout the compression; curve b is compressible 22:6,22:6 PC; and
curve c is mixed chain egg PC which remains in a liquid-like phase throughout the transition
[12]
.
measurements involves the melting (gel
liquid crystal) behavior of a di-saturated PC. In
the gel state DSPC (18:0,18:0 PC) has a cross-sectional area of ~48
˚
2
. Upon melting to the
liquid crystalline state the area increases to ~70
˚
2
. Interestingly, DOPC (18:1,18:1 PC)
occupies almost the same cross-sectional area (~72
˚
2
) as liquid crystalline DSPC. Therefore
it can be concluded that the ~30
kink imparted by a cis double bond in an acyl chain
increases the molecular area of a phospholipid by approximately the same amount as that
which arises upon increasing the gauche kinks upon chain melting.
/
Lipid 'Condensation'
'Condensation' is an expression of how closely two lipids behave from ideal with respect
to their areas per molecule. If two molecules behave as non-compressible (ideal) spheres,
when mixed their combined measured areas should equal the sum of the area/molecule
determined independently for each component
[13]
.
A
2
Þ
p
Where
c
1
is the mol fraction of component 1 and (A
1
)
p
and (A
2
)
p
are the mean molecular
areas of components 1 and 2 at surface pressure
p
. Negative deviations of the experimentally
measured value (A
exp
) from A
ideal
represent attraction ('condensation') while positive devi-
ations represent repulsion. If A
ideal
¼
A
ideal
¼ c
1
ð
A
1
Þ
p
þð
1
c
1
Þð
the experimentally measured value, the lipids behave
ideally. The extent of non-ideal behavior is expressed as '% condensation':
A
ideal
100
where A
ideal
is the mean molecular area calculated assuming ideal additivity and A
exp
is that
observed experimentally.
% condensation
¼½ð
A
ideal
A
exp
Þ=
