Time-Resolved Torsional Relaxation of Spider Draglines by an Optical Technique

The sensitivity of the torsional pendulum demonstrates the self-shape-memory effect in different types of spider draglines. Here we report the time-resolved noncovalent bonds recovery in the protein structure. The torsional dynamics of such multilevel structure governed by reversible interactions are described in the frame of a nested model. Measurement of three different relaxation times confirms the existence of three energy storage levels in such two protein spidroin systems. Torsion opens the way to further investigations towards unraveling the tiny torque effects in biological molecules.

Figure 1

Experimental setup. A rod made of plastic or copper, mimicking the spider, is suspended to a 10-cm-long spider dragline or man-made threads in a torsion pendulum experiment. The whole experiment is settled on an antivibration honeycomb core optical table. Hydrometry: $(40\pm 1)\%$, temperature: $(20.0\pm 0.1)°\mathrm{C}$. Inset: photograph of a typical gold-coated spider dragline taken with a scanning electronic microscope (Cambridge Instruments).

Figure 2

Experimental torsion pendulum relaxation dynamics of four threads. Red arrows: end of the applied torque and beginning of the free relaxation period. (a) Araneus diadematus (suspended mass 0.1 g) and zoom of the beginning of the relaxation dynamics (inset) and Nephila (suspended mass 0.5 g) spider threads. (b) $10\mu \mathrm{m}$-diameter polyparaphenylene terephthalamide (Kevlar 29) thread (suspended mass 0.5 g, torsional constant $C={10}^{-9}\mathrm{Nm}/\mathrm{rad}$) and $50\mu \mathrm{m}$-diameter copper thread. Blue dotted line: new equilibrium position.

Figure 3

Experimental torsional relaxation dynamics of (a) a RRAS both in pulsed excitation regime and in constant excitation regime, (b) a $100\mu \mathrm{m}$-diameter shape-memory nitinol alloy thread (Dynalloy, Inc.). To relax to its original shape, the alloy needs to be heated to 90 °C.

Figure 4

Torsional logarithm relaxation behavior of (a) the RRAS of Fig. 3a, (b) the Araneus diadematus spider dragline of Fig. 2a. Black points: experimental measurements, red line: theoretical fit with a function of the form of Eq. (3), blue dotted straight lines: time constants associated with the different torsional levels. Inset: an example of a tertiary protein structure composed of secondary structures (here $\alpha$ helices in red). The quaternary structure involves more than one amino-acid chain. Such nm-scale proteins are present in one section of a spider thread.