Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Weiche Materie Poster: Do., 13:00–15:30 D-P350
Molecular structure and mobility of spider dragline silk and adsorbed water
Daniel Sapede 1,2 , Tilo Seydel 1 , V.Trevor Forsyth 1 , M.Marek Koza 1 , Ralf
Schweins 1 , Martin Müller 3 , Fritz Vollrath 4 , Christian Riekel 2
1 Institut Laue-Langevin, B.P.156, F-38042 Grenoble – 2 European Synchrotron Radiation
Facility, B.P.220, F-38043 Grenoble – 3 IEAP, Universität Kiel, D-24098 Kiel –
4 Dept.of Zoology, University of Oxford, U.K.
Spider dragline silk is known for its outstanding mechanical properties which it obtains
despite being spun at close to ambient temperature and pressure using water as a
solvent. Our study using neutron diffraction, neutron backscattering and time-of-flight
spectroscopy aims at elucidating the crucial role of adsorbed water to spider silk. As
spider silk is currently only available in limited quantities from living spiders, a major
challenge has been met in establishing the feasibility of neutron scattering experiments.
Spider dragline silk is a semicrystalline biopolymer made out of an aminoacid sequence
with dominating alanine and glycine molecules. From diffraction studies, a model of
a fibrillar structure made out of small crystalline blocks in a matrix containing both
partly oriented and unoriented amorphous material has been proposed. The crystalline
fraction is composed basically out of poly(alanine) chains while the oriented amorphous
fraction is glycine rich. This oriented material seems to be connected by a further fraction
of random polymer chains. Although the aminoacid sequence in the polymer chains
is being unravelled, our understanding of the link between primary structure and function
remains limited. The mechanical properties of spider silk are strongly influenced
by its water content and water-induced mobility at a molecular level. Thus in order
to obtain its superior mechanical properties, the extruded fibre - which is spun from
aqueous solution - has to be rapidly dried. X-ray small- and wide-angle scattering shows
that the water content does not influence the fibrillar system. Apparently the water is
absorbed by the random polymer chains whereas the crystallites are not accessible to
the guest molecules. Since spider silk is exposed in nature to different humidity levels,
it is interesting to note that the absorption of water is reversible. It seems that the
polymer matrix mechanically behaves like a rubber when swollen with water.
We present recent results from neutron small-angle scattering, neutron diffraction and
neutron spectroscopic experiments on oriented spider dragline silk fibres as a function
of temperature and humidity [1,2], as well as complementary synchrotron microbeam
experiments.
[1] D.Sapede, T.Seydel, V.T.Forsyth, M.M.Koza, R.Schweins, F. Vollrath, C. Riekel;
Macromolecules 38, 8447 (2005).
[2] D.Sapede; PhD Thesis, University Joseph Fourier, Grenoble (2006).
(http://tel.ccsd.cnrs.fr/tel-00011700?langue=en)