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.