Fiber of spiders

Eman youssif

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 protein-based 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.

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.

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.

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.

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.

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

beta-structured proteins, including those comprising muscle tissue, according to experimental proteomics data on protein structures in

the Protein Data Bank.

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.

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.

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.

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.

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.)

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).

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 so-called 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.

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]

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]

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

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!

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!

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.

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.

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.

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 so-called amorphous matrix consisting of helical and beta turn structures.

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.

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

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.

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.

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.

Spider webs are incredible! Spider web material is about one-tenth 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.

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 .

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.

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.

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.

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-tangled-proteins-mathematics.html#jCp

“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-webs-tangled-proteins-mathematics.html#jCp

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.

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.

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.

Fibrous proteins are long-chain molecules that serve as structural materials for the same reason that other polymers do. They can cross-link and inter-

twine to provide strength and flexibility. Examples are coiled-coil alpha-helices 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.

protein found in spider webs

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.

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.

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.

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, and 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.

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.

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.

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 .

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 β-sheet 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.

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.

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.

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.

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 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 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.

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

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.

Structural analysis revealed that oligopeptides with the sequence (GA)n/(A)n tend to form α-helices in solution and β-

sheet structures in assembled fibers

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.

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

these domains

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.

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.

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 structure-function relationship of individual spider silk proteins.

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 high-performing 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

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

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

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.

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.

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.

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.

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.

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.

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).

The characteristics of the silk are due to the order of the amino acid residues in the protein sequence of silk: the poly-(Glycine-

Alanine) 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 semi-

amorphous regions using beads connected by nonlinear springs.

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.

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.

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.

The 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.

Picture from reference 2Hydrogen 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 in easier formation of the crystalline regions.

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 keratin-like 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 extra-corporally 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.

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 beta-sheet 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 beta-sheet 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.

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

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 self-assembly to form silk but it may involve mechanical and frictional forces that

arise during passage through the spider’s spinning organs.

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2658765/

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