3.3 Spider silk
The presence of regions of helical and sheet-like structures within a protein will affect its properties in different ways: a particularly striking example of this is provided by spider silk.
Imagine a polymer that forms fibres stronger, by mass, than steel and can be processed from water at ambient temperature and low pressure. As a consequence of its biological origin, it is extremely environmentally friendly and totally recyclable. It may sound like science fiction, but this is exactly what we have in spider silk. Mind you, it has taken 400 million years to develop!
Spider silk is composed of proteins. Spiders make their webs and perform other tasks using up to seven different types of silk fibre. Each silk has distinct mechanical properties, which arise directly from the primary and higher-order structure of the proteins from which they are constructed. Table 4 lists three types of silk from the golden orb web-weaving spider (Nephila clavipes) shown in Figure 15 and compares their mechanical properties to some synthetic polymer materials.
Table 4 Comparison of mechanical properties of spider silk and other polymer fibres
|Material||Use||Tensile strength / MPa||Extension at breaking point / %||Energy to break / kJ kg−1|
|Dragline silk||web frame and radii, support when climbing or falling||4000||35||100|
|Flagelliform silk||core fibres of adhesive spiral||1000||>200||100|
|Silk from the minor ampullate gland||web reinforcement||1000||5||30|
|Nylon 6||wide range of uses||70||200||60|
Not surprisingly, given its impressive properties, spider silk has been the focus of much research. Scientists have identified the major proteins that make up the different silks and have mapped their amino acid sequences. It turns out that there are just a few key repeating patterns, which in turn lead to particular secondary structures. These are summarised in Table 5: the letters G, P and A represent the amino acids glycine, proline and alanine respectively, with X as a ‘wild card’. There are two helical structures present here, known as a β-turn spiral and a 310 helix; you can think of these as acting like a loose and a tight spring. The structures also include spacers to separate the repeating units into clusters, but these aren't included in the chart.
Table 5: Structural modules found in spider silk from the golden orb spider; read across the rows to see the composition of each type
|Elastic β‑turn spiral||Crystalline β‑sheet||310 helix|
|Amino acid sequence||GPGXX||(GA)n or An||GGX|
|Dragline silk||protein 1||√||√|
|protein 2||√(~5 repeats)||√|
|Flagelliform silk||√(~50 repeats)||√|
|Silk from the minor ampullate gland||protein 1||√||√|
The dragline silk, which forms the main structure of the web and also supports the spider when climbing or dangling on a thread, is exceptionally strong and reasonably elastic, making it a very tough fibre comparable to the best that modern polymer technology can offer. By contrast, the flagelliform silk of the spiral part of the web is not only sticky but very stretchy, so that it can absorb the energy from a collision with a large insect without breaking, and keep the insect trapped in the web. The silk from the minor ampullate gland is as strong as the flagelliform silk but has much lower elasticity and is used for structural reinforcement.
The correlation between the secondary structures within the proteins and the physical properties of each silk is striking, as you'll see by working through SAQ 6.
Use Tables 4 and 5 to answer these questions.
(a) Which two types of silk have the highest extensibility?
(b) What structural feature do they have in common?
(c) To what would you attribute the difference in extensibility between the two?
(d) Why would you expect the β-sheet arrangement to convey strength?
(e) Suggest why a particularly regular amino acid sequence might be needed to form a β-sheet.
(f) How might you account for the high strength but low extensibility of the silk from the minor ampullate gland, by considering its structure?
(a) Flagelliform silk and dragline silk.
(b) The presence of helical structures: the elastic β-turn spiral and the 310 helix.
(c) The more elastic flagelliform silk has many more units in the elastic β-turn spiral component, i.e. it contains longer loose springs. Also there are no stiff β-sheet regions in the flagelliform silk.
(d) The β-sheet conveys strength because of the large density of hydrogen bonds between adjacent chains, which keep them fixed in position.
(e) The polypeptide chains are packed closely together in the β-sheet and the presence of different side groups would disrupt this regular arrangement.
(f) There are no elastic β-turn spiral modules in the minor ampullate silk to provide elasticity, but β-sheet regions are present to convey strength.
It isn't just the composition of spider silk that is of interest to technologists, but also the spinning process itself. The formation of secondary structural motifs such as those described result in a liquid crystalline solution, which reduces the amount of force needed by the spider to achieve high orientation. The fibres are extruded through a spinneret in the spider's abdomen, which involves a novel internal stretching process not seen in current industrial fibre processing.
The very close link between molecular-scale structure and large-scale properties seen in spider silk suggests that by carefully controlling the sequence of monomer units along a polymer chain we should be able to design materials with very closely specified properties. Indeed, polymer chemists have been doing this in a crude way for many years by designing block copolymers for specific applications. Synthetic block copolymers typically contain only two or three different monomer units, but a wide range of properties can be achieved. The potential degree of control over properties in a polypeptide chain with 20 different monomer options is much greater.