1. Fiber of spiders
Eman youssif

2. Researchers in civil and environmental engineering
reveal that the strength of a biological material like
spider silk lies in the geometric configuration of
structural proteins, and the small clusters of weak
hydrogen bonds that work cooperatively to resist force
and dissipate energy. This structure makes proteinbased materials as strong as steel, even though the
hydrogen bonds that hold them together are 100 to
1,000 times weaker than the metallic bonds in steel.

3. This figure shows the structure of a beta-sheet protein, Z1-Z2 telethonin
complex, in the giant muscle protein titin. The inset shows the orientation of
the protein backbone of three beta strands (in purple) with hydrogen bonds
(yellow) holding the assembly together. Buehler and Keten found that
hydrogen bonds in beta-sheet structures break in clusters of three or
four, even in the presence of many more bonds.

4. researchers in Civil and Environmental Engineering at MIT reveal
that the strength of a biological material like spider silk lies in the
specific geometric configuration of structural proteins, which
have small clusters of weak hydrogen bonds that work
cooperatively to resist force and dissipate energy.

5. This structure makes the lightweight natural material as strong
as steel, even though the “glue” of hydrogen bonds that hold
spider silk together at the molecular level is 100 to 1,000 times
weaker than the powerful glue of steel’s metallic bonds or even
Kevlar’s covalent bonds.
Based on theoretical modeling and large-scale atomistic
simulation implemented on supercomputers, this new
understanding of exactly how a protein’s configuration enhances
a material’s strength could help engineers create new materials
that mimic spider silk’s lightweight robustness. It could also
impact research on muscle tissue and amyloid fibers found in
brain tissue.

6. In a paper published in the Feb. 13 online issue of Nano
Letters, Buehler and graduate student Sinan Keten describe how
they used atomistic modeling to demonstrate that the clusters of
three or four hydrogen bonds that bind together stacks of short
beta strands in a structural protein rupture simultaneously
rather than sequentially when placed under mechanical stress.
This allows the protein to withstand more force than if its beta
strands had only one or two bonds. Oddly enough, the small
clusters also withstand more energy than longer beta strands
with many hydrogen bonds.

7. But a material that employs many short beta strands folded and
connected by three or four hydrogen bonds may exhibit strength
greater than steel. In metals, the energy would be stored directly in
much stronger bonds, called metallic bonds, until bonds rupture one
by one. In proteins, things are more complicated due to the entropic
elasticity of the noodle-like chains and the cooperative nature of the
hydrogen bonds.”
Structural proteins contain a preponderance of beta-sheets, sections
that fold in such a way that they look a bit like old-fashioned ribbon
candy; short waves or strands appear to be stacked on top of one
another, each just the right length to allow three or four hydrogen
bonds to connect it to the section above and beneath.
Beta sheets with short strand lengths connected by three or four
hydrogen bonds are the most common conformation among all betastructured proteins, including those comprising muscle
tissue, according to experimental proteomics data on protein
structures in the Protein Data Bank.

8. This correlation of a common geometric configuration among
beta sheets—which are one of the two most prevalent protein
structures in existence—suggests that a protein’s strength is an
important evolutionary driving force behind its physical design.
The researchers observed the same behavior in similar small
clusters in alpha-helical structural proteins, the other most
prevalent protein, but haven’t yet studied those assemblies in
detail.

9. Spider silk is a protein fiber spun by spiders. Spiders use their silk
to make webs or other structures, which function as nets to
catch other animals, or as nests or cocoons for protection for
their offspring. They can also suspend themselves using their
silk.

10. All spiders produce silks, and a single spider can produce up to
seven different types of silk for different uses.[6] This is in
contrast to insect silks, where most often only one type of silk is
produced by an individual.[7] Spider silks may be used for a
number of different ecological uses, each with properties to
match the function of the silk (see Properties section). The
evolution of spiders has led to more complex and diverse uses of
silk throughout its evolution, for example from primitive tube
webs 300–400 mya to complex orb webs 110 mya.

11. Mechanical properties[edit]
Each spider and each type of silk has a set of mechanical properties optimised
for their biological function.
Most silks, in particular dragline silk, have exceptional mechanical properties.
They exhibit a unique combination of high tensile strength and extensibility
(ductility). This enables a silk fiber to absorb a lot of energy before breaking
(toughness, the area under a stress-strain curve).
A frequent mistake made in the mainstream media is to confuse strength and
toughness when comparing silk to other materials. As shown below in
detail, weight for weight, silk is stronger than steel, but not as strong as
Kevlar. Silk is, however, tougher than both.

12. Density[edit]
Consisting of mainly protein, silks are about a sixth of the density
of steel (1.31 g/cm3). As a result, a strand long enough to circle
the Earth would weigh less than 500 grams (18 oz). (Spider
dragline silk has a tensile strength of roughly 1.3 GPa. The tensile
strength listed for steel might be slightly higher—e.g. 1.65
GPa,[14][15] but spider silk is a much less dense material, so that
a given weight of spider silk is five times as strong as the same
weight of steel.)

13. Energy Density[edit]
The energy density of dragline spider silk is 1.2x108J/m3.[16]
Extensibility[edit]
Silks are also extremely ductile, with some able to stretch up to five
times their relaxed length without breaking.
Toughness[edit]
The combination of strength and ductility gives dragline silks a very
high toughness (or work to fracture), which "equals that of commercial
polyaramid (aromatic nylon) filaments, which themselves are
benchmarks of modern polymer fiber technology".[17][18]
Temperature[edit]
Whilst unlikely to be relevant in nature, dragline silks can hold their
strength below −40°C (-40°F) and up to 220°C (428°F).

14. ilks, as well as many other biomaterials, have a hierarchical
structure (e.g., cellulose, hair). The primary structure is its amino
acid sequence, mainly consisting of highly repetitive glycine and
alanine blocks,[24][25] which is why silks are often referred to as
a block co-polymer. On a secondary structure level, the short
side chained alanine is mainly found in the crystalline domains
(beta sheets) of the nanofibril, glycine is mostly found in the socalled amorphous matrix consisting of helical and beta turn
structures.[25][26] It is the interplay between the hard
crystalline segments, and the strained elastic semi-amorphous
regions, that gives spider silk its extraordinary properties.

15. Various compounds other than protein are used to enhance the
fiber's properties. Pyrrolidine has hygroscopic properties and
helps to keep the thread moist. It occurs in especially high
concentration in glue threads. Potassium hydrogen phosphate
releases protons in aqueous solution, resulting in a pH of about
4, making the silk acidic and thus protecting it from fungi and
bacteria that would otherwise digest the protein. Potassium
nitrate is believed to prevent the protein from denaturing in the
acidic milieu.[29]

16. suggested crystallites embedded in an amorphous matrix
interlinked with hydrogen bonds. This model has refined over
the years: Semi-crystalline regions were found[25] as well as a
fibrillar skin core model suggested for spider silk,[31] later
visualized by AFM and TEM.[32] Sizes of the nanofibrillar
structure and the crystalline and semi-crystalline regions were
revealed by neutron scattering.[33]

17. Various compounds other than protein are found in spider
silks, such as sugars, lipids, ions, and pigments that might affect
the aggregation behaviour and act as a protection layer in the
final fiber

18. Did you know? Spider silk is five times as strong as steel!
Scientists have discovered why spider webs are able to withstand
huge forces without breaking. To find out how much force spider
webs can stand, scientists tested real spider webs and ran
computer simulations. They found that some spider webs can
withstand hurricane-force winds!

19. Super strong silk
Although silk is very strong, that's not the only important factor
in a web's strength. Spider webs have a very complex design. The
way the web is built means that if a single strand of web
breaks, the strength of the web actually increases. Pretty
impressive from a humble spider!

20. A spider web becoming stronger when a thread breaks is an
incredibly clever material property. Imagine if we built objects
that, when a bit broke off, the objects got stronger! Scientists
hope that this finding could be used to help design new super
strong materials.

21. Scientists also found that spider silk can react differently to
different types of forces. If a light wind blows on the web, the
silk softens and becomes more flexible. The spider web can blow
in the breeze without breaking. If a larger force is applied to one
part of the web, the silk in that part of the web becomes stiff
and one or two threads break. The rest of the web stays intact.

22. Why do spiders need strong sillk?
These unique properties of silk are very important for spiders. It
takes a lot of energy to build a web. If only a couple of threads
break, the spider doesn't have to start building a whole web
from scratch. Also, spiders need their webs to catch food. If the
web broke every time an insect flew into it, it wouldn't be a very
good trap! Instead, the web is flexible enough to stretch when an
insect lands in it, strong enough not to break and sticky enough
to trap the insect.

23. Silks, as well as many other biomaterials, have a hierarchical
structure (e.g., cellulose, hair). The primary structure is its amino
acid sequence, mainly consisting of highly repetitive glycine and
alanine blocks,[24][25] which is why silks are often referred to as
a block co-polymer. On a secondary structure level, the short
side chained alanine is mainly found in the crystalline domains
(beta sheets) of the nanofibril, glycine is mostly found in the socalled amorphous matrix consisting of helical and beta turn
structures.

24. It is the interplay between the hard crystalline segments, and the
strained elastic semi-amorphous regions, that gives spider silk its
extraordinary properties.Various compounds other than protein
are used to enhance the fiber's properties. Pyrrolidine has
hygroscopic properties and helps to keep the thread moist. It
occurs in especially high concentration in glue threads.
Potassium hydrogen phosphate releases protons in aqueous
solution, resulting in a pH of about 4, making the silk acidic and
thus protecting it from fungi and bacteria that would otherwise
digest the protein. Potassium nitrate is believed to prevent the
protein from denaturing in the acidic milieu.

25. This first very basic model of silk was introduced by Termonia in
1994[30] suggested crystallites embedded in an amorphous
matrix interlinked with hydrogen bonds. This model has refined
over the years: Semi-crystalline regions were found[25] as well
as a fibrillar skin core model suggested for spider silk,[31] later
visualized by AFM and TEM.[32] Sizes of the nanofibrillar
structure and the crystalline and semi-crystalline regions were
revealed by neutron scattering

26. Non-protein composition[edit]
Various compounds other than protein are found in spider
silks, such as sugars, lipids, ions, and pigments that might affect
the aggregation behaviour and act as a protection layer in the
final fiber.

27. The production of silks, including spider silk, differs in an important respect
from the production of most other fibrous biological materials: rather than
being continuously grown as keratin in hair, cellulose in the cell walls of
plants, or even the fibers formed from the compacted faecal matter of
beetles,[16] it is "spun" on demand from liquid silk precursor sometimes
referred to as unspun silk dope, out of specialised glands.

28. As discussed in the Structural section of the article, the
molecular structure of unspun silk is both complex and
extremely long. Though this endows the silk fibers with their
desirable properties, it also makes replication of the fiber
somewhat of a challenge. Various organisms have been used as a
basis for attempts to replicate some components or all of some
or all of the proteins involved. These proteins must then be
extracted, purified and then spun before their properties can be
tested. The table below shows the results including the true gold
standard- actual stress and strain of the fibers as compared to
the best spider dragline.

29. Spider webs are incredible! Spider web material is about onetenth the diameter of a human hair, but it has incredible
strength. In fact it is ten times stronger than a steel strand of the
same weight.

30. The dragline silk which makes up the spokes of the spider’s web
is an amazing chemical design. Two proteins are used in these
strands. Each protein contains three regions with distinct
properties. The first takes a form that is called amorphous. An
amorphous material is a plastic material like bubble gum that is
stretchable. This is what gives the spider’s web its huge elasticity
so when the spider’s prey hits the web, it stretches and does not
break.

31. Embedded in the amorphous material are two kinds of
crystalline material. These crystalline materials toughen the web.
They are tightly pleated and resist stretching, but only one of
them is truly rigid. The pleats of the less rigid material anchor
the rigid crystals to the matrix producing massive strength.

32. Chemists are learning new things from examining the web
materials. One reason why scientists have studied spider webs is
that all of the chemical design found in the spider web has
enormous applications for new fabrics and fibers, bullet-proof
vests and even artificial tendons.

33. It is obvious that a master chemist designed something to meet
the needs of arachnid creatures. Man’s ability to copy what God
has done continues to give us solutions to the needs we have.

34. Since joining the MIT faculty in 2006, Buehler has focused on
understanding biological materials such as spider silk and the tangled
masses of protein known as amyloids — primarily as a way to
understand how their complex structures could improve the functional
properties of manmade materials. He has also collaborated with his
wife, whose experimental research at Harvard University focused on
the interactions of cells with materials. “She has taught me a lot about
biology, and how simulations might contribute to the field,” he says.
But “our focus is not to just copy nature” or the kinds of materials
nature produces, Buehler says. Rather, he’d like to learn the underlying
principles of how complex, hierarchical structures with useful
properties can be assembled from the simplest of building blocks —
and how engineers can actually apply such knowledge in different
materials or in different problems altogether.
Read more at: http://phys.org/news/2012-04-spider-webs-tangledproteins-mathematics.html#jCp

35. “Here, I feel like I have an opportunity to do something new,”
Buehler says. “At MIT, we don’t believe in keeping things the
same, we continue to push the boundaries of innovation.” Civil
and environmental engineering research “is no longer just about
building bridges, it’s about using nanotechnology and scalability
to improve the materials we use to build and maintain our
infrastructure, and to improve the interface between the natural
and built world from the tiny atoms to the tallest of structures.
It’s exciting to be part of redefining this field.”
Read more at: http://phys.org/news/2012-04-spider-webstangled-proteins-mathematics.html#jCp

36. Spiderweb capture-silk threads
In order to learn why spider web silk threads are so remarkably strong and
elastic, we studied spider web capture-silk threads: the very sticky, strong and
elastic spiraled material in the webs of orb-weaving spiders. We studied them
with AFM force spectroscopy experiments, as force spectroscopy enables the
mechanical testing of capture silk alone, which is otherwise difficult to
separate from dragline silk.

37. Molecular nanosprings in spider capture-silk threads. Nature Materials
2, 278-283 (2003)].
After force spectroscopy experiments with the Atomic Force
Microscope, pulling bulk threads, and amino acid sequencing, we we able to
present initial models for capture silk's molecular and multimolecular
structure. We had found that spider capture silk requires exponential force
increases as it is pulled (In each capture-silk pull, the successive rupture peaks
usually occurred at increasing forces). Intact spider dragline silk does not
show an exponential force increase when stretched. We also found evidence
that capture silk molecules contain sacrificial bonds and hidden length that
reform quickly if the protein is allowed to relax after it is extended. We
proposed two models for why the pulling force increased exponentially:
simply, as intact capture silk is composed of an assembly of molecules, either
that the molecules successively rupture or detach, sometimes even in
multiple steps because of looping, or that the molecular assembly is
networked by crosslinking springs. We discussed the possible nature of the
cross-linking springs, proposing that they may be a part of the glue that is
known to coat capture-silk.

38. Our β-spiral models for relaxed and extended protein conformations in spider
capture silk - side and end views. a) Araneus gemmoides, 85 amino acids
long, in its relaxed state. b) Nephila clavipes, 75 amino acids long - relaxed
state. c) Nephila clavipes stretched to its maximum extension without
deforming bond angles. The capture silk from both these species of spider
contain high percentages of glycine and proline, often in the repetitive
GPGGXn motifs characteristic of β-spirals.

39. Fibrous proteins are long-chain molecules that serve as structural materials
for the same reason that other polymers do. They can cross-link and intertwine to provide strength and flexibility. Examples are coiled-coil alphahelices in muscles, a triple helix in collagen and beta-sheets in silks and
spiders' webs. The figure below shows the composition of fibers found in a
spiders' web in which beta-sheets stack up to form microcrystals interspersed
with regions of less-ordered (random coil) structures.

40. protein found in spider webs

41. Interestingly, these abundant material constituents (such as H-bonds) are
often functionally inferior and extremely weak. Yet, materials such as
silk, collagen in tendon and bone, or intermediate filament proteins that
make up cells and hair are highly functional, mutable, and some even
stronger than steel. It is therefore an elementary question how Nature can
achieve such functional material properties in spite of severe environmental
constraints.

42. By incorporating concepts from structural engineering, materials
science and biology our lab's research has identified the core
principles that link the fundamental atomistic-scale chemical
structures to functional scales by understanding how biological
materials achieve superior mechanical properties through the
formation of hierarchical structures, via a merger of the concepts
of structure and material. Our work has demonstrated that the
chemical composition of biology's construction materials plays a
minor role in achieving functional properties. Rather, the way
components are connected at distinct scales defines what
material properties can be achieved, how they can be altered to
meet functional requirements, and how they fail in disease
states.

43. Similar to conventional engineering testing of materials (e.g. by
exposing them to severe stress to break them) our research
approach is based on using the study of materials failure as a
tool to elucidate the design principles of how functional material
properties are achieved, and how they are lost. We apply an
experimentally validated multi-scale modeling and simulation
approach that considers the structure-process-property
paradigm of materials science and the architecture of proteins at
multiple levels, from the atomistic (chemistry, molecular) scale
up to the overall structural scale (material, tissue, spider web).
My research has resulted in an engineering paradigm that
facilitates the analysis and design of sustainable
materials, starting from the molecular level, which mimic and
exceed the properties of biological ones while being constructed
from abundant and intrinsically poor material constituents.

44. Biological protein materials provide the foundation for the
integration of structure and material, thereby enabling the
inclusion of molecular and multi-scale features in the material
design process to realize a unique combination of properties
despite the intrinsic weakness and simplicity of constituents.
We are in particular interested in the combination of disparate
properties in biological materials, such as
strength, robustness, deformability, adaptability, changeability, a
nd evolvability as well as mutability. Our lab uses principles
developed in civil engineering and architecture applied at all
scales inlcuding the nanoscale in the analysis and design of
materials.

45. A spider-web’s ability to catch insects is due to the
silk’s unique combination of mechanical properties:
strength, extensibility (up to 30%) and, most
importantly, toughness, or resistance to breakage.
Spider silk may be six times stronger than steel by
weight, but it is its toughness that makes it so
special, as it allows it to absorb a large amount of
energy without breaking. Man-made materials such
as Kevlar are strong, but lack this specificity.
Moreover, unlike Kevlar, spider silk is biodegradable
and recyclable: when repairing their webs, spiders
frequently eat damaged parts of the web and absorb
the nutrients.

46. These special characteristics make spider silk of interest to many
different research fields. A polymer based on spider silk could be
used in medicine, as a high-strength, non-toxic suture, or in
ligament repair, because the fibre not only does not tire when
frequently flexed, but also can withstand regular impact and
great pressure. The military sector is also investigating this
material because its ability to dissipate energy could make it
ideal for lightweight armour.

47. But before we can produce and use artificial spider silk, we need to
understand what confers its unique mechanical properties. Recent
experiments at the Institut Laue-Langevin (ILL) and the European Synchrotron
Radiation Facility (ESRF) in Grenoble, France, have used neutron scattering
and synchrotron radiation to investigate the microscopic characteristics of
spider silk. This has provided researchers with a new insight into the silk’s
structure, which in turn confers its mechanical properties. The two
techniques, neutron scattering and synchrotron radiation, complement each
other. Whereas synchrotron radiation, a type of very high-energy X-ray
irradiationw1, enables a single silk fibre to be studied as it is extruded from a
living spider, neutron scattering allows us to identify differences in the
organisation of proteins and their accessibility to water, which has a strong
influence on its mechanical properties.

48. arises from β-sheet nanocrystals that universally consist of highly
conserved poly-(Gly-Ala) and poly-Ala domains. This is
counterintuitive because the key molecular interactions in βsheet nanocrystals are hydrogen bonds, one of the weakest
chemical bonds known. Here we report a series of large-scale
molecular dynamics simulations, revealing that β-sheet
nanocrystals confined to a few nanometres achieve higher
stiffness, strength and mechanical toughness than larger
nanocrystals. We illustrate that through nanoconfinement, a
combination of uniform shear deformation that makes most
efficient use of hydrogen bonds and the emergence of
dissipative molecular stick–slip deformation leads to significantly
enhanced mechanical properties. Our findings explain how size
effects can be exploited to create bioinspired materials with
superior mechanical properties in spite of relying on
mechanically inferior, weak hydrogen bonds.

49. The dragline silk of the Golden Orb-Weaving spider is the most studied in
scientific research. Spider silk is a natural polypeptide, polymeric protein and
is in the scleroprotein group which also encompasses collagen (in ligaments)
and keratin (nails and hair). These are all proteins which provide structure.
The protein in dragline silk is fibroin (Mr 200,000-300,000) which is a
combination of the proteins spidroin 1 and spidroin 2. The exact composition
of these proteins depends on factors including species and diet. Fibroin
consists of approximately 42% glycine and 25% alanine as the major amino
acids. The remaining components are mostly
glutamine, serine, leucine, valine, proline, tyrosine and arginine. Spidroin 1
and spidroin 2 differ mainly in their content of proline and tyrosine.

50. Structure of spidroinHelix
Spidroin contains polyalanine regions where 4 to 9 alanines are linked
together in a block. The elasticity of spider silk is due to glycine-rich regions
where a sequence of five amino acids are continuously repeated. A 180° turn
(b-turn) occurs after each sequence, resulting in a b-spiral. Capture silk, the
most elastic kind, contains about 43 repeats on average and is able to extend
2-4 times (>200%) its original length whereas dragline silk only repeats about
nine times and is only able to extend about 30% of its original length. There
are also glycine-rich repeated segments which consist of three amino acids.
These turn after each repeat to give a tight helix and may act as a transitional
structure between the polyalanine and spiral regions.

51. Structure of spider silk
Liquid crystalline solutionThe fluid dope is a liquid crystalline solution where
the protein molecules can move freely but some order is retained in that the
long axis of molecules lie parallel, resulting in some crystalline properties. It
is thought that the spidroin molecules are coiled in rod-shaped structures in
solution and later uncoil to form silk.

52. During their passage through the narrowing tubes to the spinneret the
protein molecules align and partial crystallisation occurs parallel to the fibre
axis. This occurs through self-assembly of the molecules where the
polyalanine regions link together via hydrogen bonds to form pleated bsheets (highly ordered crystalline regions). These b-sheets act as crosslinks
between the protein molecules and imparts high tensile strength on the silk.

53. It is not purely coincidence that the major amino acids in spider silk are alanine and
glycine. They are the smallest two amino acids and do not contain bulky side groups
so are able to pack together tightly, resulting in easier formation of the crystalline
regions.
The crystalline regions are very hydrophobic which aids the loss of water during
solidification of spider silk. This also explains why the silk is so insoluble - water
molecules are unable to penetrate the strongly hydrogen bonded b-sheets.
General structure of spider silkThe glycine-rich spiral regions of spidroin aggregate to
form amorphous areas and these are the elastic regions of spider silk. Less ordered
alanine-rich crystalline regions have also been identified and these are thought to
connect the b-sheets to the amorphous regions. Overall, a generalised structure of
spider silk is considered to be crystalline regions in an amorphous matrix. Kevlar has a
similar structure.
It is not entirely clear how the protein molecules align and undergo self-assembly to
form silk but it may involve mechanical and frictional forces that arise during passage
through the spider’s spinning organs.

54. Spider silk primarily consists of proteins that possess large
quantities of nonpolar and hydrophobic amino acids like glycine
or alanine, but for example, no or only very little
tryptophan.4,13,14 In comparison to common cellular
enzymes, it is evident that silk proteins exhibit a quite aberrant
amino acid composition (Fig. 2A). Furthermore, spider silk
proteins contain highly repetitive amino acid
sequences, especially in their large core domain

55. The repetitive sequences often account for more than 90% of
the whole spider silk protein and are composed of short
polypeptide stretches of about 10–50 amino acids. These motifs
can be repeated more than a hundred times within one
individual protein. Each polypeptide repeat therefore has distinct
functional features resulting in the outstanding mechanical
properties of spider silk threads.

56. Structural analysis revealed that oligopeptides with the
sequence (GA)n/(A)n tend to form α-helices in solution and βsheet structures in assembled fibers

57. Due to conserved cysteine residues, these domains can establish
intermolecular disulfide bonds and are thus able to stabilize
dimers and multimers under oxidizing conditions.
Therefore, these domains are thought to initiate and specify
assembly of silk proteins.

58. Several carboxy-terminal nonrepetitive sequences of different silks and
spiders have been identified, revealing a high sequence homology amongst
these domains

59. After secretion from the silk glands, silk proteins are in aqueous
solution and lack considerable secondary or tertiary structure.37
Particularly in their repetitive core domains, however, the long
repetitive sequences permit weak but numerous intra- and
intermolecular interactions between neighboring domains and
proteins upon passage through the spinning duct. These
interactions result in the formation of secondary, tertiary and
quaternary structure. Roentgen diffraction analysis of the final
structure of MA silk threads led to the identification of areas of
high electron density embedded in areas with low electron
density (Fig. 3).2,3,38 In a postulated model of this structure, the
high electron density regions comprise crystalline sub-structures
with high β-sheet content.

60. These sub-structures are thought to be responsible for the
mechanical strength of the silk thread. The elasticity of silk is
based on the areas with low electron density, which are
characterized by amorphous structures with few defined
elements of secondary or supersecondary structure.40,41 Such
arrangement closely resembles that of protein hydrogels.42
Upon tensile loading, the hydrogel-like areas can partially
deform, contributing to the elasticity and flexibility of the thread.

61. Different types of silk reveal different structural distributions
(e.g., different compositions of crystalline-and hydrogel-parts).
MA silk which is used for constructing the frame of the web
contains a high amount of crystalline (β-sheet) structures. In
contrast, the much more flexible Flag silk consists almost
exclusively of amorphous hydrogel-like regions. Thus, the
correlation between the structure and function of individual
spider silk proteins becomes evident. However, in the future
more detailed analysis is necessary to characterize the structurefunction relationship of individual spider silk proteins.

62. It's no wonder that scientists and engineers try to emulate the production of
spider silk technologi-cally in research laboratories and industrial facilities.
There are many conceivable applications, rang-ing from novel fibers for highperforming textiles to innovative materials for vehicle construction or medical
technology. Spider silk has the additional advantage of being biologically
compatible with the human body and it is completely biodegradable.
"From a technological perspective, the production of spider silk works quite
well already. So far, however, the outstanding mechanical properties of
natural spider silk have not been attained in this way," says biotechnologist
Neuweiler. And he knows a reason for this: We still do not understand the
molecular mechanisms in the natural spinning process well enough to imitate
them perfectly.
Read more: New findings on spider silk
http://www.nanowerk.com/news2/biotech/newsid=33259.php#ixzz2skJtL5z5
Follow us: @nanowerk on Twitter

63. What the Würzburg researcher finds particularly fascinating about the
spinning process is the speed with which individual protein molecules in the
spider arrange themselves into long threads. He examined this aspect in
greater detail – after all, his research team specializes in the visualization of
protein dynamics. This research requires the application of special optical
methods.
Neuweiler and his associates have now analyzed a certain section of a silk
protein from the nursery web spider Euprosthenops australis. "This section is
very interesting, because it connects the termi-nal areas of the proteins that
link to form silk threads," says Neuweiler.
Read more: New findings on spider silk
http://www.nanowerk.com/news2/biotech/newsid=33259.php#ixzz2skK879K
5
Follow us: @nanowerk on Twitter

64. The rapid production of silk threads in spiders
involves unusual electrostatic interactions
between the proteins
Read more: New findings on spider silk
http://www.nanowerk.com/news2/biotech/new
sid=33259.php#ixzz2skKFfel8
Follow us: @nanowerk on Twitter

65. The main thing that distinguishes spiders from the rest of the
animal kingdom is their ability to spin silk, an extremely strong
fiber. A few insects produce similar material (silkworms, for
example), but nothing comes close to the spinning capabilities of
spiders. Most species build their entire lives around this unique
ability.

66. Scientists don't know exactly how spiders form silk, but they do
have a basic idea of the spinning process. Spiders have special
glands that secrete silk proteins (made up of chains of amino
acids), which are dissolved in a water-based solution. The spider
pushes the liquid solution through long ducts, leading to
microscopic spigots on the spider's spinnerets. Spiders typically
have two or three spinneret pairs, located at the rear of the
abdomen.

67. Each spigot has a valve that controls the thickness and speed of
the extruded material. As the spigots pull the fibroin protein
molecules out of the ducts and extrude them into the air, the
molecules are stretched out and linked together to form long
strands. The spinnerets wind these strands together to form the
sturdy silk fiber.

68. Most spiders have multiple silk glands, which secrete different
types of silk material optimized for different purposes. By
winding different silk varieties together in varying
proportions, spiders can form a wide range of fiber material.
Spiders can also vary fiber consistency by adjusting the spigots to
form smaller or larger strands.

69. Some silk fibers have multiple layers -- for example, an inner core
surrounded by an outer tube. Silk can also be coated with
various substances suited for different purposes. Spiders might
coat fiber in a sticky substance, for example, or a waterproof
material.

70. Spider silk is incredibly strong and flexible. Some varieties are
five times as strong as an equal mass of steel and twice as strong
as an equal mass of Kevlar. This has attracted the attention of
scientists in a number of fields, but up until recently, humans
haven't been able to get much out of this natural resource. It's
simply too hard to extract silk from spiders, and each spider has
only a small amount of it.

71. This may change in the near future. Researchers at a company
called Nexia Biotechnologies have genetically modified goats
using silk-producing genes from spiders. The hope is that a small
number of goats will be able to produce a large amount of silk
material in their milk. Engineers will be able to put this material
to work in aircraft, bulletproof vests and artificial limbs, among
other things (check out this page for more information).

73. The characteristics of the silk are due to the order of the amino
acid residues in the protein sequence of silk: the poly-(GlycineAlanine) and poly-Alanine domain makeup of the beta-sheet
nanocrystals.
One of the current ways to analyze the properties of spider silk is
by using a one-dimensional coarse-grained model, which
essentially models the beta-sheet nanocrystal and semiamorphous regions using beads connected by nonlinear springs.

74. Through a number of molecular dynamics simulations, it has been
determined that the physical size of the beta-sheet nanocrystals affects the
tensile strength of the silk as a whole. When the beta-sheet nanocrystals are
smaller in size is when the spider silk is the toughest mechanically, highest in
strength, and stiffest. The importance of the beta-sheet nanocrystals is highly
evident when stretching of the silk occurs; they reinforce the partially
extended macromolecular chains by forming interlocking regions that transfer
the force load between the chains, which allows for greater extensibility of
the amorphous region. The one-dimensional model that will be explained in
greater detail reveals that the semi-amorphous region dominates
deformation at small deformation levels (the region starts to unravel when
the silk is stretched), but the beta-sheet nanocrystals dominate after the
semi-amorphous region has essentially stretched to capacity. When an axial
force is applied to spider silk, the silk experiences four distinct regimes that
include a high tangent modulus, a softening effect, a stiffening effect, and
then finally complete failure.

75. The dragline silk of the Golden Orb-Weaving spider is the most
studied in scientific research. Spider silk is a natural
polypeptide, polymeric protein and is in the scleroprotein group
which also encompasses collagen (in ligaments) and keratin
(nails and hair). These are all proteins which provide structure.
The protein in dragline silk is fibroin (Mr 200,000-300,000) which
is a combination of the proteins spidroin 1 and spidroin 2. The
exact composition of these proteins depends on factors including
species and diet. Fibroin consists of approximately 42% glycine
and 25% alanine as the major amino acids. The remaining
components are mostly
glutamine, serine, leucine, valine, proline, tyrosine and arginine.
Spidroin 1 and spidroin 2 differ mainly in their content of proline
and tyrosine.

76. Spidroin contains polyalanine regions where 4 to 9 alanines are
linked together in a block. The elasticity of spider silk is due to
glycine-rich regions where a sequence of five amino acids are
continuously repeated. A 180° turn (b-turn) occurs after each
sequence, resulting in a b-spiral. Capture silk, the most elastic
kind, contains about 43 repeats on average and is able to extend
2-4 times (>200%) its original length whereas dragline silk only
repeats about nine times and is only able to extend about 30% of
its original length. There are also glycine-rich repeated segments
which consist of three amino acids. These turn after each repeat
to give a tight helix and may act as a transitional structure
between the polyalanine and spiral regions.

77. molecules can move freely but some order is retained in that the long
axis of molecules lie parallel, resulting in some crystalline properties.
It is thought that the spidroin molecules are coiled in rod-shaped
structures in solution and later uncoil to form silk.
Picture from reference 2
Hydrogen bonds in a beta sheetSchematic diagram of a beta
sheetDuring their passage through the narrowing tubes to the
spinneret the protein molecules align and partial crystallisation occurs
parallel to the fibre axis. This occurs through self-assembly of the
molecules where the polyalanine regions link together via hydrogen
bonds to form pleated b-sheets (highly ordered crystalline regions).
These b-sheets act as crosslinks between the protein molecules and
imparts high tensile strength on the silk.
It is not purely coincidence that the major amino acids in spider silk are
alanine and glycine. They are the smallest two amino acids and do not
contain bulky side groups so are able to pack together tightly, resulting

78. Biopolymers fulfil a variety of different functions in nature. They conduct various
processes inside and outside cells and organisms, with a functionality ranging from
storage of information to stabilization, protection, shaping, transport, cellular
division, or movement of whole organisms. Within the plethora of biopolymers, the
most sophisticated group is of proteinaceous origin: the cytoskeleton of a cell is made
of protein filaments that aid in pivotal processes like intracellular
transport, movement, and cell division; geckos use a distinct arrangement of keratinlike filaments on their toes which enable them to walk up smooth surfaces, such as
walls, and even upside down across ceilings; and spiders spin silks that are extracorporally used for protection of offspring and construction of complex prey traps. The
following tutorial review describes the hierarchical organization of protein fibers, using
spider dragline silk as an example. The properties of a dragline silk thread originate
from the strictly controlled assembly of the underlying protein chains. The assembly
procedure leads to protein fibers showing a complex hierarchical organization
comprising three different structural phases. This structural organization is responsible
for the outstanding mechanical properties of individual fibers, which out-compete
even those of high-performance artificial fibers like Kevlar. Web-weaving spiders
produce, in addition to dragline silk, other silks with distinct properties, based on
slightly variant constituent proteins—a feature that allows construction of highly
sophisticated spider webs with well designed architectures and with optimal
mechanical properties for catching prey.

79. Since its development in China thousands of years ago, silk from
silkworms, spiders and other insects has been used for high-end, luxury
fabrics as well as for parachutes and medical sutures. Now, MRSEC supported
researchers are untangling some of its most closely guarded secrets, and
explaining why silk is so strong, a question that has remained unresolved.
Buehler and co-workers of the MIT MRSEC IRG-II have found that the key to
silk's pound-for-pound toughness, which exceeds that of steel, is its betasheet crystals, the nano-sized cross-linking domains that hold the material
together. Using computer models to simulate exactly how the components of
beta sheet crystals move and interact with each other, they found that an
unusual arrangement of hydrogen bonds -- the “glue” that stabilizes the betasheet crystals--play an important role in defining the strength of silk.
Buehler and his team are now looking at the possibility of synthesizing
materials that have a similar structure to silk, but using molecules that have
inherently greater strength, such as carbon nanotubes.

80. Along with H-bonds, disulfide cross-links also play a fundamental role in
defining the mechanical properties of fibrous protein materials such as
hair, feather and wool, as well as other natural materials like bread making
dough and rubber. We are currently investigating the rupture of disulfide
bonds under various chemical conditions combined with mechanical stress, to
understand the role of structural and environmental factors on the mechanics
of biological and bioinspired materials at multiple length scales. Insight
gained from this work will lead to new ideas for the design of novel
bioinspired adaptive materials with tunable structures and mechanical
properties (Keten et al. JMBB

81. The glycine-rich spiral regions of spidroin aggregate to form amorphous areas
and these are the elastic regions of spider silk. Less ordered alanine-rich
crystalline regions have also been identified and these are thought to connect
the b-sheets to the amorphous regions. Overall, a generalised structure of
spider silk is considered to be crystalline regions in an amorphous matrix.
Kevlar has a similar structure.
It is not entirely clear how the protein molecules align and undergo selfassembly to form silk but it may involve mechanical and frictional forces that
arise during passage through the spider’s spinning organs.

82. references
http://en.wikipedia.org/wiki/Spider_silk
http://www.dandydesigns.org/id41.html
http://tigger.uic.edu/classes/phys/phys461/phys45
0/ANJUM05/
http://web.mit.edu/mbuehler/www/
http://web.mit.edu/mbuehler/www/

83. http://www.chm.bris.ac.uk/motm/spider/page3.htm
http://www.sciencedaily.com/releases/2013/01/130128104741.htm
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2658765/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2658765/
http://www.nanowerk.com/spotlight/spotid=15880.php
http://www.chm.bris.ac.uk/motm/spider/page3.htm
http://web.mit.edu/newsoffice/2012/spider-web-strength-0202.html