The outstanding strength and toughness of certain spider and lepidopteran silkst [1] has aroused considerable interest in recent years, [2] with research focusing primarily on the rela-tionship between molecular structure and mechanical proper-ties. [3] Environmental conditions such as ambient humidity, [4] acidity, [5] and UV radiation [6] a11 affect the mechanical proper-ties of native silks to some degre. [7] pronounced differences in mechanical properties were also observed when conditions such as the speed or temperature at spinning were varied [8] or when the silk was (or had been) submerged in solvents such as water, urea solution, or a range of alcohols, where it con-tracts in varying degrees. [9]

The spinning mechanism of natural silk has been an open issue. In this study, both the conformation transition from random coil to b sheet and the b sheet aggregation growth of silk fibroin are identified in the B. mori regenerated silk fibroin aqueous solution by circular dichroism (CD) spectroscopy. A nucleation-dependent aggregation mechanism, similar to that found in prion protein, amyloid b (Ab) protein, and a-synuclein protein with the conformation transition from a soluble protein to a neurotoxic, insoluble b sheet containing aggregate, is a novel suggestion for the silk spinning process. We present evidence that two steps are involved in this mechanism: (a) nucleation, a ratelimiting step involving the conversion of the soluble random coil to insoluble b sheet and subsequently a series of thermodynamically unfavorable association of b sheet unit, i. e. the formation of a nucleus or seed; (b) once the nucleus forms, further growth of the b sheet unit becomes thermodynamically favorable, resulting a rapid extension of b sheet aggregation. The aggregation growth follows a first order kinetic process with respect to the random coil fibroin concentration. The increase of temperature accelerates the b sheet aggregation growth if the b sheet seed is introduced into the random coil fibroin solution. This work enhances our understanding of the natural silk spinning process in vivo.

We examined the mechanical characteristics of four major ampullate (MA) dragline silks during and after submersion in a range of solvents. The silks were reeled from four very different spiders: Araneus diadematus, Nephila edulis, Latrodectus mactans and Euprosthenops sp. They displayed significant differences in behaviour in the native state as well as during and after supercontraction in solvents such as water, urea solution and a set of alcohols. The different polarities of the solvents are thought to affect different regions of the silk's molecular conformation. We hypothesise that the observed mechanical properties of dragline silks are those of a hard elastic polymer; and we explain the supercontraction of the silks as changes of orientation in the molecular chains. q1999 Elsevier Science Ltd. All rights reserved.

We used wel-defined, fluorescence-free Raman spectra of single silk fibres to study silk ultrastructure. Major ampullate (MA) silk was reeled from the Araneus diadematus spider under controlled conditions. With acustom-built stress-strain gauge, we examined the mechanical properties of this silk both before and after supercontraction in a range of solvents. The solvents were found to modify the material properties considerably. We suggest that the solvents with their different polarities affect different regions of the silk's composite microstructure, in particular the conformation of the molecular chains.

With the step-growth polymerization of bis (oligopeptides) and diisocyanates, two silkproteinlike multiblock polymers (P1, P2), containing-(Ala)4-and-(GlyAlaGlyAla)-sequence derived from the crystalline region of spider dragline silk and silkworm (Bombyx mori) silk respectively, has been synthesized successfully. The intrinsic viscosities of P1 and P2 measured in dichloroacetic acid at 25℃ were 0.31 and 0.26dL/g. FT-IR, 13C CP/MAS NMR and WAXD measurements revealed the oligopeptide segments in these multiblock polymers could aggregate spontaneously into β-sheet structure in solid state. In addition, it was also found there were non-β-sheet structures in the synthetic polymers, which indicated such polymers had a solid-state structure similar to that of natural silks.