Lecture 17:
Lipid Vesicles and Membranes
What did we cover in the last
lecture?
Amphiphilic molecules contain a
hydrophobic head group and
hydrophobic tail group.
When added to water in they form
micelles above a critical concentration.
Their shape is determined by the
volume and length of the tail and the
1
v
1
optimum area of the molecular


3 l c ao 2
headgroups.
A geometric packing parameter can be
used to identify whether spherical,
cylindrical or bilayer structures will
form.
Hydrophilic
head groups
Hydrophobic
tail groups
v
1

l c ao 3
1
v

1
2 lc ao
In this lecture…
1) Lipids
2) Bilayers revisited
3) Lipid Vesicles
4) Curvature of lipid membranes
Further Reading
Intermolecular and Surface Forces
J. Israelachvili, p378 - 385
Amphiphile shapes
In the last lecture we saw that the shape of
micelles formed by amphiphiles is determined
by a geometric packing parameter
v
H
l c ao
Area ao
1
H
3
1
1
H
3
2
1
 H 1
2
Small v
Large lc and ao
Volume
v
Large v
Small lc and ao
lc
Lipid bi-layers
Lipids are naturally occurring
amphiphiles. They have two short,
bulky hydrocarbon chains
This means that lc is small, but v is
quite large. They therefore have a
tendency to form bi-layer
structures.
Phosphatidylcholine (PC)
is a phospholipid that is
one of the main
constituents of all animal
cell membranes
Vesicle formation
When lipid bi-layers are
formed in solution, there
is an excess energy
associated with the
exposed hydrophobic tail
groups at the edges of
the structure
vesicle
The bi-layers can
offset this energy by
folding around to
close themselves off
and form an isolated
shell or vesicle
Applications of vesicles
http://www.youtube.com/watch?v
Vesicles are closed structures
=04SP8Tw3htE
and can be used to encapsulate
materials in their interior for use
in drug delivery
If the outside of the vesicle is
decorated with specific chemical
receptors which cause the
vesicle to rupture when it
reaches its target it can act like
a ‘magic bullet’
They are also an excellent
model of mammal cells for use
in biophysics experiments
Curvature of membranes
Curvature of membranes introduces
constraints into the way in which
amphiphiles can pack in a
membrane.
It also varies the headgroup
separation of lipids and thus has an
energy penalty associated with it
(just like bending a sheet of
material). Smaller radii of curvature
Rc
require more bending energy.
These constraints result in a
minimum radius of curvature below
which vesicles cannot form. (See
OHP)

lc 3  34 H  1

31  H 
v
H
ao l c

How does R vary with l?
As the length of the hydrocarbon chain increases, the
radius decreases
R takes its
minimum


12
v
value when
3 

3

ao l
the


Rl
hydrocarbon

v 

31 
chain is fully
ao l 

l=lc
stretched.
i.e. when
l=lc
Geometric packing parameters
(again)
Our expression for the minimum radius of curvature of a
vesicle forces us to reinterpret the range of values of H for
which bilayers and vesicles will form
It is clear that any bi-layer will tend to try to curve up on
itself to eliminate unfavourable hydrophobic/water contacts

lc 3  34 H  1
Rc 
31  H 

When H=1, Rc = ∞, so perfectly flat bi-layers will form
When
1
 H  1 Rc is finite and vesicles will form
2
Problem
Calculate the minimum radius of curvature of vesicles
made from egg phosphatidylcholine which has measured
headgroup area and hydrocarbon volume of 0.717 nm2
and 1.063 nm3 respectively, if the critical chain length for
this lipid is 1.75nm
Calculate an estimate of the number of lipid molecules in
the vesicle wall
Summary of key concepts
Lipid/amphiphile bilayers will tend
to close up to form vesicles to
eliminate unfavourable
hydrophobic contacts
Geometrical constraints and the
energy penalty associated with
bending the membranes places a
lower limit on the radius of
vesicles
Vesicles are excellent models of
cell membranes and can be used
for targeted delivery of drugs and
other chemicals

lc 3  34 H  1
Rc 
31  H 
