Why seek rafts?
Animal cell plasma membranes include areas where
different species of lipid congregate, these areas
are known as rafts. They support specific proteins
vital to cell function.
Polymersomes are currently used in medicine as
microencapsulation to deliver therapeutic substances
such as drugs, DNA and proteins.
Engineering rafts in artificial membranes would open
up new potential applications for polymersomes and
make a step towards producing artificial cells.
Making polymersomes
1. Dissolve amphiphilic plastic in a good
solvent such as tetrahydrofuran (THF)
2. Drip pure water into the solution while
stirring: the plastic forms bilayers to keep
its hydrophobic chains away from the
water. In 80% water, the lowest energy
configuration of a bilayer is a closed shell:
a polymersome.
Result 1: a weird structure!
Seeking rafts and finding holes Richard Wilson
A polymersome is an artificial vesicle with a bilayer membrane composed of amphiphilic plastic polymers.
The term derives from “liposome” which denotes a biological vesicle – a hollow ball formed from a
phospholipid bilayer membrane.
Copolymers
The Mini-project
Polymersomes (hollow plastic balls) were made
from two types of plastic differing in length and
composition. The intention was to make rafts in
the membrane similar to lipid rafts found in
biological cell membranes.
Result 3: perforated membrane
The polymersomes produced were hundreds of
nanometres in diameter so electron microscopy
was used to observe them.
Negative staining showed puzzling structures
which cryogenic TEM seemed to show were
aggregations of smaller vesicles. However, the
scanning electron microscope revealed holed
membranes.
Result 2: an aggregation of vesicles?
scale bar 200 nm
scale bar 50 nm
Negative stain TEM
Acknowledgements
Supervisor: Dr ir Stefan A F Bon
Mentor: Christopher C J Parmenter
TEM facilities: Dr Svetla S McPhie
Support: MOAC
Funding: EPSRC
Hitchhiker’s Guide pic: BBC
The plastics used in these experiments
were PEO45-PMMA180 (polyethylene oxidepolymethyl methacrylate) and Brij-97
(polyoxyethylene-10-oleyl-ether). Both are
copolymers - formed by covalently bonding
two different polymers – and amphiphilic one polymer is hydrophilic, the other
hydrophobic.
SEM scale bar 100 nm
Conclusion
Mixing two different polymers has resulted in unusual
structures that invite further study and will probably
be useful in future plastics technology.
However, to produce rafts would seem to require a
different combination of polymers. Perhaps this is
because Brij-97 by itself doesn’t form bilayers: a third
species that does could be added or substituted.
References
Cryo-TEM scale bar 100 nm
Dennis E Discher and Adi Eisenberg, Polymer Vesicles, Science, 2002,
297:967
Wolfgang H Binder, Veronique Barragan and Frederic M Menger, Domains
and rafts in Lipid Membranes, Agew. Chem. Int. Ed., 2003, 42, 5802