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 degrees FWHM, similar to the orientation of the crystals. The minimum breadth of the streak indicates that the scattering objects are 0.1 mu m 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.