Biology Reference
In-Depth Information
non-cylindrical lipids. Indeed, only 20
50% of the total lipids need be cylindrical to maintain
e
the normal lamellar phase.
A general description of lipid shape is not entirely satisfying as lipids actually exist at all
locations on the continuum, from truncated cone, through cylinder, to inverted cone
[24]
.
Therefore attempts have been made to quantify lipid shape. Two of these approaches are dis-
cussed below.
The first approach, proposed by Pieter Cullis in 1979
[19]
defines lipid shape by a dimen-
sionless parameter S:
¼
v
=
a
o
l
c
where a
o
is the optimum cross-sectional area per molecule at the lipid
S
water interface, v is
the volume per molecule and I
c
is the length of the fully extended acyl chain. Since these
parameters are not easy to obtain, a simplified version was suggested, where a
o
is the
cross-sectional area of the head group at the aqueous interface and a
h
is the cross-sectional
area at the bottom of the acyl chains. Therefore if:
e
a
o
=
a
h
>
1
then S
>
1/
inverted cone
a
o
=
a
h
¼
1
then S
¼
1/
cylindrical
cone
Unfortunately, S is not a fixed parameter of the lipid's shape, as it may vary with the
membrane's environment (i.e. bathing solution pH, ionic strength, atmospheric pressure,
lateral pressure, temperature etc.).
A second approach, the equilibrium curvature (R
o
) concept, was later proposed by Sol
Gruner
[25]
. R
o
is a quantitative measure of the propensity of a lipid to form a non-lamellar
structure. It is an estimate of the tendency of a monolayer made from a particular lipid to
curl, thus resulting in hydrocarbon packing strain. R
o
is normally derived experimentally
a
o
=
a
h
<
1
then S
<
1/
TABLE 10.2
The Spontaneous Radius of
Curvature (R
o
) for Several
Important Membrane Lipids.
Spontaneous radius of
curvature
R
o
(
˚
)
Lipid
DOPS
þ
150
Lyso PC
þ
38 to
þ
68
DOPE
28.5
DOPC
80 to
200
Cholesterol
22.8
a
-Tocopherol
13.7
DOG
11.5
The table was adapted from
[23]
.
