Small-Angle X-ray Scattering of Spider Dragline Silk

As part of a general study of the structure of spider silk fibers, major ampullate gland silk fibers were collected from Nephila clavipes spiders, and SAXS patterns were obtained from loops of fibers under a variety of conditions. Two orders of lamellar reflection were seen, with a long spacing of 8.4 nm. This increased reversibly by 4% when the fiber was stretched by 10% and shrank to 5.8 nm when the fiber itself shrank 45% on wetting. A strong equatorial streak had a bimodal orientation distribution similar to that seen in NMR and WAXD. The sharper component had a lateral size scale (radius of gyration) of 2.5 nm and a misorientation of 10° FWHM, similar to the orientation of the crystals. The minimum breadth of the streak indicates that the scattering objects are 0.1 μm long. There is an isotropic central scattering, probably caused by voids. On wetting, the lamellar peaks became more intense; more dramatically, the equatorial scattering strengthens and extends to higher angles, almost halving the lateral size scale to 1.4 nm. An equatorial maximum appears at a spacing of 6 nm, indicating a degree of order in the fibrillar structure.