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Unravelling the secrets of spider silk.

Biological fibrous materials have always inspired material scientists in the development of novel fibres and other highperformance materials. In recent years, the wide variety of task-specific silks produced by orb-weaving spiders have aroused considerable interest because of their broad divergent mechanical properties. In some cases, these spider silks combine a level of strength and toughness that could not be reproduced by artificial means.

Orb-weaving spiders are equipped with a set of specialized abdominal glands in which the silk proteins are synthesized and stored at high concentration in a gel-like state prior to being spun as an insoluble thread. Each of the seven types of silks produced by the spider Nephila clavipes, a species widely studied in the literature and common in southern U.S., is used for a specific or multiple functions. For example, spiders use the filament spun from the Major ampullate glands, referred to as dragline, as a lifeline to escape predators as well as to build the frame and radii of the web. In the former case, the thread needs to be strong enough to support the spider's body weight during its vertical descent. It also needs to be extensible to support the distorting forces induced by wind and rain. In comparison, the capture spiral, produced by the Flagelliform glands, exhibits a low Young modulus, but is exceptionally extensible. This type of silk can more than triple its length before breaking, which insures adequate dissipation of the kinetic energy transferred to the web when flying prey is captured.

The remarkable mechanical properties of natural silks are intimately related to their structural organization. Such broad functionality is achieved by changes in the proteins' amino acid sequence and processing conditions. These parameters affect the conformation and orientation of the proteins and are judiciously adapted to create a self-assembled, highly structural material.

The production of biopolymers that biomimic silk is an attractive way to capitalize on silk's high strength, stiffness, and extensibility. In addition, silk is a biodegradable material. Novel bio-inspired high-performance fibres would have numerous applications such as reinforcing support fibres in composite materials where light weight is an issue (aerospace), impact resistant textiles (bullet-proof vests, cables), suture material, and other biomedical related applications.

Recently, the advent of recombinant DNA technology has allowed manipulation of the biosynthetic routes to produce recombinant spider silk proteins of high molecular weight. Besides the considerable progress made toward the mass-production of silk analogues, the greatest challenge still remains to process these recombinant proteins into usable biomaterials with the desired mechanical properties. The achievement of this goal depends on our ability to mimic nature's perfection to self-assemble them into a thread. Therefore, a detailed understanding of the structural organization in native silk fibres is of paramount importance. Moreover, the combined study of the silk proteins conformation and orientation together with the silk thread mechanical properties should help to establish structure-property relationships. Such information is not only relevant to improve the artificial spinning techniques of silk-based biopolymers, but also for the design of new synthetic materials inspired by the organization of natural fibres.

Raman spectromicroscopy

Our research group at the Universite Laval is currently using Raman spectromicroscopy as well as other spectroscopy and microscopy techniques to study protein organization in a global multidisciplinary approach to fully unravel the secrets of silk. Raman spectromicroscopy is a powerful non-destructive technique that is well suited to investigate the molecular organization of silk. The use of a microscope to focus the laser beam on the sample and to collect the scattered light allows the in situ recording of high-quality spectra of single silk monofilaments having a diameter of less than 5 [micro]m. The bands caused by the vibrations of protein amide groups are conformation sensitive, particularly the amide I band in the 1575-1750 [cm.sup.-1] spectral region. These bands are therefore good probes of the secondary structures present in the sample. In addition, polarization measurements, with the incident and scattered light parallel and perpendicular to the fibre axis, provide information about the orientation of the secondary structure elements in the sample.

Figure 1 shows the stress-strain curves and the polarized Raman spectra of the dragline and the capture spiral silks produced by the spider Nephila clavipes. These stress-strain curves were obtained with a specially designed stretcher that allows the simultaneous measurement of the Raman spectra and the mechanical properties. The results clearly illustrate that the two types of silk have very different mechanical properties. The dragline shows a high modulus and an excellent breaking energy that comes from a good trade-off between stiffness and extensibility. It has a breaking stress (stress in GPa at the breaking point) comparable to steel and presents a toughness (i.e., the breaking energy given by the area under the stress-strain curve) that surpasses Kevlar[R] fibres. On the other hand, the capture spiral has a low modulus similar to tendons, but can be stretched several orders of magnitude more before rupturing.

[FIGURE 1 OMITTED]

The dragline is by far the most studied spider silk. It is described as a semi-crystalline biopolymer made of interconnected crystalline and amorphous regions that would be responsible for its high strength and extensibility, respectively. It is known that the crystalline fraction of silk is composed of nanosized inclusions made of repetitive amino-acid motifs that adopt the anti-parallel [beta]-sheet conformation. The position of the amide I band, appearing at 1668 [cm.sup.-1] in Figure 1, indeed confirms that [beta]-sheet is the predominant secondary structure (approximately 35 percent) in Nephila clavipes dragline silk. In addition, Figure 1 shows that the amide I band is much more intense when the incident laser beam and analyzer are polarized perpendicular to the fibre axis ([perpendicular to]) compared with the spectrum obtained with the parallel polarization (//). This result indicates that the amide carbonyl groups of the proteins are predominantly oriented perpendicular to the fibre axis, thus that the [beta]-sheets are mainly parallel to the fibre axis. The presence of well-aligned [beta]-sheets, more likely to be found in the crystalline inclusions, certainly accounts for the high tenacity of the dragline thread.

It is known that the capture spiral silk has a different primary structure and different mechanical properties compared to the well-characterized dragline silk. However, at the molecular level, the factors responsible for its elastomeric character remain unknown. The polarized Raman spectra, shown here for the first time, are surprisingly different showing a broad amide I band, centred around 1664 [cm.sup.-1], indicating no preferred conformation. In addition, there is no significant orientation dependence. According to these results, the exceptional extensibility of the silk produced by the Flagelliform gland is more likely due to the highly disordered state of the molecular chains that have no specific dihedral angle constraints--which would be the case in a given secondary structure.

The two examples given above illustrate that there is a close relationship between the structure of silk proteins and the mechanical properties of the fibres, and that Raman spectromicroscopy is quite efficient to study in situ the structure of proteins in thin monofilaments. Other types of silks are currently being investigated in our laboratory to better understand how the conformation of the proteins, their assembly into supramolecular structures, and their molecular orientation impact on the mechanical properties of the thread. There is certainly a bright future for bio-inspired novel materials, but scientists still have a lot to learn from Mother Nature.

The authors are members of the research group of professor and co-author Michel Pezolet, FCIC. Together they do research on spider silk in the chemistry department of the Universite Laval. They are also members of the Centre de recherche en sciences et ingenierie des macromolecules and the Centre de recherche sur la fonction, la structure et l'ingenierie des proteines.
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Author:Rousseau, Marie-Eve; Bedard, Sarah; Rioux-Dube, Jean-Francois; Lefevre, Thierry; Pezolet, Michel
Publication:Canadian Chemical News
Geographic Code:1CANA
Date:May 1, 2007
Words:1291
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