Total X-Ray Scattering of Spider Dragline Silk

Total x-ray scattering measurements of spider dragline silk fibers from Nephila clavipes, Argiope aurantia, and Latrodectus hesperus all yield similar structure factors, with only small variations between the different species. Wide-angle x-ray scattering from fibers orientated perpendicular to the beam shows a high degree of anisotropy, and differential pair distribution functions obtained by integrating over wedges of the equatorial and meridian planes indicate that, on average, the majority ($95\%$) of the atom-atom correlations do not extend beyond 1 nm. Futhermore, the atom-atom correlations between 1 and 3 nm are not associated with the most intense diffraction peaks at $Q=1–2{\AA }^{-1}$. Disordered molecular orientations along the fiber axis are consistent with proteins in similar structural arrangements to those in the equatorial plane, which may be associated with the silk’s greater flexibility in this direction.

Figure 1

2D x-ray scattering images of a Nephila clavipes spider silk orientated perpendicular to the incoming beam (top) and along the fiber axis (bottom). The long dashed lines represent the equatorial and meridional wedges used to compare the silks from different species. The short dashed lines represent narrower wedges taken to investigate the influence of individual Bragg peaks in Nephila clavipes, comprised of regions I and II in the equatorial plane and regions III, IV, and V in the meridional plane.

Figure 2

(a) X-ray structure factors for Nephila clavipes spider silks integrated over regions I to V, as illustrated in Fig. 1. The error bars are within the scatter of the points. (b) The corresponding atom-atom differential pair distribution functions, $D(r)$, for regions I to V obtained using independent atom x-ray form factors. These curves were obtained by a Fourier transform using a Lorch modification function using a $Q_{\max }$ of $17{\AA }^{-1}$ [34]. All curves are offset for clarity.

Figure 3

Differential distribution functions, $D(r)$, for Nephila clavipes (solid line), Argiope aurantia (dotted line), and Latrodectus hesperus (dashed line), measured with the silk fibers orientated perpendicular to the x-ray beam for the (a) equatorial and (b) meridian wedges. (c) Represents the $D(r)$ measured with the silk fibers orientated along the axis of the x-ray beam. The curves (which are offset for clarity) were obtained by a Fourier transform using a Lorch modification function with a $Q_{\max }$ of $14.2{\AA }^{-1}$ [34, 35]. The value of $\sigma =0.05$ (solid arrows) and $\sigma =0.02$ (dotted arrow) are given for Nephila clavipes as described in the text. The inset shows (c) (solid blue line) compared to the sum of regions I, II, and III (dashed black line) for Nephila clavipes..