Insights into the Hierarchical Structure of Spider Dragline Silk Fibers: Evidence for Fractal Clustering of $\beta$-Sheet Nano-Crystallites

TL;DR

This study reveals the fractal hierarchical organization of $eta$-sheet nano-crystallites in spider dragline silk, linking nano-structure to its exceptional mechanical properties through combined SAXS/WAXS analysis.

Contribution

First detailed investigation demonstrating the fractal clustering of nano-crystals in spider silk using combined scattering techniques.

Findings

Nano-crystals form 10-50 nm clusters with long-range order.

Evidence of lamellar peaks indicating strong nano-crystal ordering.

Hierarchical fractal structure explains differences in sound velocities.

Abstract

Spider dragline silk is one of the toughest materials known and understanding the hierarchical structure is a critical component in the efforts to connect structure to function. In this paper, we take the first step in elucidating the hierarchical fractal structure of $\beta$-sheet nano-crystallites, which form a robust self-similar network exhibiting an non-linear mechanical property. A combined small angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) study of the nano-crystalline component in dragline silk fibers from several species of spiders including, Latrodectus hesperus, Nephila clavipes, Argiope aurantia and Araneus gemmoides is presented. SAXS structure factors exhibit a `lamellar peak' in the q-range from 0.60 to 0.82 nm$^{-1}$ for various spider dragline silk fibers, indicating the presence of strong nano-crystal ordering on the $>$10 nm length-scale. The stochastically reconstructed electron density maps indicate that the $\beta$-sheet crystals are hierarchically structured as mass fractals and that nano-crystals tend to form 10 to 50 nm sized clusters with long-range crystalline ordering. This nano-crystal ordering along the fiber axis also helps to explain the difference between axial and radial sound velocities recently measured by Brillouin spectroscopy.