A Physico-Chemical Study of the Supercontraction of Spider Major Ampullate Silk Fibers

Abstract

Methanol, ethanol, and isopropanol do not radially swell or produce supercontrac tion (SC) of spider major ampullate silk fibers. Their aqueous solutions do so at rates associated with water concentrations, but produce no levels of SC ratios other than those expected of water Itself. Saturated aqueous solutions of NaSCN and LiBr produce swelling and lowered birefringences of axially restrained fibers. On unre strained fibers NaSCN produces further SC. After SC, the wetted fibers can be stretched to their original lengths, and on drying they are in a stable state from which further SC and re-stretching cycles can be repeated. On both unrestrained major and minor ampullate fibers, LiBr causes irreversible gellation. X-ray diffraction measure ments show that the crystallite unit cell rotates due to macro distortion during SC, but is otherwise unchanged. The suggestion is that water ruptures hydrogen bonds between amino and carboxy groups, in the anti-parallel rippled sheet model of silk, to produce radial swelling and allow SC. The internal axial retractive force present in the wetted fibers, which is the driving force for SC, provides axial tension in the restrained condition and in turn prevents access to LiBr, which in the unrestrained condition brings about gellation.

Stretching of supercontracted fibers: a link between spinning and the variability of spider silk

“…The stress-strain curve of supercontracted FS fibers tested in water (SCW) shows an elastomeric behaviour, which can be explained as the result of water molecules acting as a plasticizer (Gosline et al, 1984), by disrupting the network of hydrogen bonds. This explanation is supported by X-ray diffraction (Work and Morosoff, 1982;Grubb and Gending, 1999) and Raman spectroscopy data (Shao et al, 1999a,b). The chain conformation of proteins would be kept highly flexible in supercontracted fibers, but it could be frozen after drying by promoting the formation of protein-protein hydrogen bonds, thus re-establishing the hydrogen bond network and leading to behaviour observed in MS fibers.…”

The molecular structure of spider dragline silk: Folding and orientation of the protein backbone

“…They found a 40:60 mixture of a highly oriented part [full width half height (FWHH) ϭ 5°] and a less-oriented part (FWHH ϭ 75°) and obtained a value of the orientation function ͗cos 2 Ј͘ ϭ 0.760. ‡ ‡ From x-ray data of major ampulate silk from Araneus marmoreus and Nephila cruentata, the orientation function ͗cos 2 Ј͘ was calculated to be 0.776 and 0.788, respectively (44). If we reduce our distribution to the angular correlation function, we obtain 0.742.…”

Humidity affects the extensibility of an orb-weaving spider's viscous thread droplets

“…Although most of a thread's aggregate gland material is consolidated into droplets, a sheath of aqueous material covers axial fibers in inter-droplet regions. This serves to hydrate these fibers, maintaining their super-contracted state and increasing their tension (Work, 1981;Work, 1982;Vollrath and Edmonds, 1989;Edmonds and Vollrath, 1992;Gosline et al, 1994;Savage and Gosline, 2008;Guinea et al, 2010). This mechanism helps maintain web tension and better equips an orb web to absorb the force of an insect strike (Vollrath and Edmonds, 1989;Edmonds and Vollrath, 1992).…”