In the spinning passage the threads are oriented in parallel and form so-called micro-crystallites. The free N terminal domains (red) are layed together into to dimers and from a very stable material as a result of the cross-linked micro-crystallites. Source: TUM |
It
has five times the tensile strength of steel and is stronger then even
the best currently available synthetic fibers: Spider thread is a
fascinating material. Yet, to date no one has managed to produce the
material on an industrial scale. Scientists of the TU Muenchen (TUM) and
the Universitaet Bayreuth (UBT) have now succeeded in unveiling a
further secret of silk proteins and the mechanism that imparts spider
silk with its strength. They have published the results of their work in
the professional journal Angewandte Chemie.
“The
strength of spider dragline silk exceeds that of any material produced
in laboratories, by far. All attempts to manufacture threads of similar
strength have failed thus far,” explains Professor Horst Kessler, Carl
von Linde Professor at the Institute for Advanced Study at the TU
Muenchen (TUM-IAS). In collaboration with the workgroup of Prof. Thomas
Scheibel, who was a researcher at the TU Muenchen until 2007 and who now
holds a chair of the Institute of Biomaterials at the Universitaet
Bayreuth, Professor Kessler’s team has been researching for years to
unveil the secret of spider silk.
How
do spiders manage to first store the silk proteins in the silk gland
and to then assemble them in the spinning passage in a split second to
form threads with these extraordinary characteristics? And what exactly
gives the threads their tremendous tensile strength? Scientists have now
come one step closer to answering these key questions for the
production of artificial spider silk.
Spider
threads consist of long chains of thousands of repeating sequences of
protein molecules. These silk proteins are stored in the silk gland in a
highly concentrated form until they are needed. The long chains with
their repeating sequences of protein molecules are initially unordered
and must not get too close to each other as they would immediately clump
up. Only in the spinning passage, just before being used, are the
threads oriented parallel to each other and form so-called micro
crystallites that are, in turn, assembled to stable threads with cross
links.
During
the last year, the scientists in Kessler’s and Scheibel’s team
investigated the common garden spider (“cross spider”) to discover the
mechanism behind the transition from individual spider silk molecules to
connected treads: The individual spider silk proteins are first stored
in the silk gland in small drops called micelles.
The
scientists identified the regulating element that is responsible for
assembling a strong thread from the individual parts. It is the
so-called C terminal domain of the silk protein. It prevents the
formation of threads in the silk duct with its strong salt
concentrations. In the spinning passage, however, where the salt
concentration is low and sheer forces are abundant, this domain becomes
instable and “sticky.” This causes the chains to overlap and a strong
spider silk thread is formed. The discovery of the significance of this
relatively small C terminal domain, when compared to the overall length
of the protein thread, was a sensation at the time and was published in
the renowned scientific journal Nature.
Now
the same group of researchers has put in place a further piece in the
spider silk puzzle. They showed that the other end of the long thread,
the so-called N terminal domain, plays an important role in the design
of strong threads with great tensile strength. This time, the scientists
investigated the head ends of the spider silk proteins of the “black
widow” (Latrodectus hesperus). The result: The N terminal head ends
exist in the silk duct as single strands (momomers). Only in the sinning
passage are the head to tail pairs (dimers) formed.
The
process of laying together is regulated via the change in pH values and
salt concentrations between the silk duct and the spinning canal. In
the silk duct, a neutral pH value of 7.2 and a high salt concentration
prevent the N terminal head ends from combining. In the spinning
passage, however, the environment becomes acidic (pH value around 6.2)
the salt is removed. Now the ends can come together. In this process,
the N terminal ends connect to the respective other ends–a practically
endless chain of linked up spider silk proteins is formed.
“In
our work we were able to show, in addition to our previous research,
that both the pH value and the salt concentration influence the
monomer-dimer balance,” says Franz Hagen, corresponding author of the
study, in summing up the results. “Both factors influence the formation
of dimers and thus the efficient cross-linking to very long silk
proteins.”
Ultimately,
this cross-linking is what gives the spider silk threads their enormous
tensile strength. The small crystallites first formed in the parallel
cross-linking of the protein chains following the controlled unfolding
of the C terminal domain are connected to each other via the N terminal
domains of the spider silk protein to form a very long chain.
“This
is the effect that eventually explains the enormous tensile strength of
the spider silk thread,” says Kessler. To date, this ingenious form of
cross-linking–called “multivalence”–has not been implemented in
artificial polymers. “Most polymer chemists focus on the length of the
thread. So far, no one has come up with the approach of cross-linking
the ends of the threads and thereby opening the door to virtually
unlimited lengths of polymer chains,” beleives Kessler. These new
findings may provide chemists with a model for manufacturing new
materials with improved characteristics.
The
scientists used the method of nuclear magnetic resonance (NMR) to
analyze the structure of spider silk. Segments of spider silk are
dissolved under conditions similar to those found in spider organs and
exposed to radio wave impulses in a very strong magnetic field. The
scientists can deduce the exact molecular structure from the “response”
of the molecules. Using this method, environmental influences (e.g. salt
concentration and pH value) can be studied accurately under simulated
natural conditions. The development and application of NMR methods to
biomolecules has been a longstanding focus of the Bavarian NMR Center in
Garching.
Original publication:
F.
Hagn, C. Thamm, T. Scheibel, H. Kessler; pH Dependent Dimerisation and
Salt Dependent Stabilisation of the N-terminal Domain of Spider Dragline
Silk – Implications for Fibre Formation, Angew.Chem. Int. Ed. 2011, 50,
310-313.
Previous publications on the spinning process:
F.
Hagn, L. Eisoldt, J. Hardy, C. Vendrely, M. Coles, T. Scheibel, H.
Kessler, A highly conserved spider silk domain acts as a molecular
switch that controls fibre assembly, Nature 2010, 465, 239-242.
