The internal structure of the velvet worm projectile slime: A small-angle scattering study

Velvet worms capture prey and defend themselves by ejecting an adhesive slime which has been established as a model system for recyclable complex liquids. Triggered by mechanical agitation, the sticky fluid rapidly transitions into solid fibers. The assembly of slime proteins into stiff polymers is fully reversible and recyclable enabling the recovery of the soluble precursors. In order to understand the rapid and reversible mechanoresponsive behavior of this material, here, we study the nanostructural organization of slime components using small-angle scattering with neutrons and x-rays under physiological native conditions, after drying and re-hydration, and mechanical agitation. The scattering intensities are successfully described with a three-component model accounting for proteins of two dominant molecular weight fractions and for protein-based nanoglobules with a radius of ~40-45 nm, which is in line with the literature. However, in contrast to the previous assumption that high molecular weight (HMW) proteins -- the presumed building blocks of the fiber core -- are contained in the nanoglobules, we find that the majority of slime proteins exist as free proteins in solution, including the HMW fiber core precursors. Only less than 10 % of the slime proteins are contained in the nanoglobules, necessitating a reassessment of the previously proposed function of nanoglobules in fiber formation. Exploiting distinct differences in the x-ray and neutron scattering contrast of slime re-hydrated with light and heavy water (D2O) indicates that the majority of lipids available in the slime are contained in the nanoglobules, where they are homogeneously distributed. Surprisingly, mechanical agitation of slime in a completely filled container causes gelification; however, this neither leads to fiber formation nor alters the bulk structure of the slime significantly, suggesting that interfacial phenomena and directional shearing are required for the formation of stiff fibers in velvet worm slime.