The molecular origin of the exceptional mechanical properties of spider silk is unclear. This paper presents solid-state 2H nuclear magnetic resonance data from unoriented, oriented, and supercontracted fibers, indicating that the crystalline fraction of dragline silk consists of two types of alanine-rich regions, one that is highly oriented and one that is poorly oriented and less densely packed. A new model for the molecular-level structure of individual silk molecules and their arrangement in the fibers is proposed. These data suggest that it will be necessary to control the secondary structure of individual polymer molecules in order to obtain optimum properties in bio-inspired polymers.

The fiber morphology of the dragline silk of Nephila clavipes has been investigated by the detailed analysis of wide-angle X-ray diffraction (WAXD) patterns. WAXD gives the crystal lattice dimensions, the orientation distribution, the crystalline fraction, and an estimate of the crystal size. It is found that the crystals are very small and well oriented. The mean (minimum) crystal dimensions are 2 × 5 × 7 nm, and the angle, φ, between the molecular chains in the crystals and the fiber axis has a full width at half-maximum (fwhm) of 15.7° and an orientation function f = 0.981. The X-ray crystallinity is in the range 10−15%, and the amorphous diffraction is divided 60:40 between an isotropic ring and an oriented halo with fwhm 30°. This means one-third of the material is in the oriented amorphous state, with a chain orientation of fwhm 43° and f = 0.87. When the fiber is extended up to 10%, the orientation of the crystals increases as predicted for affine deformation at constant volume. There is no observable change in crystallinity and apparently a small reduction in the lateral crystal size on deformation.

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