Dependence of Mechanical Properties of Lacewing Egg Stalks on Relative Humidity

Silk fibers are well known for their mechanical properties such as strength and toughness and are lightweight, making them an interesting material for a variety of applications. Silk mechanics mainly rely on the secondary structure of the underlying proteins. Lacewing egg stalk silk proteins obtain a cross-β structure with individual β strands aligned perpendicular to the fiber axis. This structure is in contrast with that of silks of spiders or silkworms with β strands parallel to the fiber axis and to that of silks of honeybees with α helices arranged in coiled coils. On the basis of the cross-β structure the mechanical properties of egg stalks are different from those of other silks concerning extensibility, toughness, and bending stiffness. Here we show the influence of relative humidity on the mechanical behavior of lacewing egg stalks and propose a model based on secondary structure changes to explain the differences on a molecular level. At low relative humidity, the stalks rupture at an extension of 3%, whereas at high relative humidity the stalks rupture at 434%. This dramatic increase corresponds to breakage of hydrogen bonds between the β strands and a rearrangement thereof in a parallel-β structure

Unraveling the Molecular Requirements for Macroscopic Silk Supercontraction

Spider dragline silk is a protein material that has evolved over millions of years to achieve finely tuned mechanical properties. A less known feature of some dragline silk fibers is that they shrink along the main axis by up to 50% when exposed to high humidity, a phenomenon called supercontraction. This contrasts the typical behavior of many other materials that swell when exposed to humidity. Molecular level details and mechanisms of the supercontraction effect are heavily debated. Here we report a molecular dynamics analysis of supercontraction in <i>Nephila clavipes</i> silk combined with <i>in situ</i> mechanical testing and Raman spectroscopy linking the reorganization of the nanostructure to the polar and charged amino acids in the sequence. We further show in our <i>in silico</i> approach that point mutations of these groups not only suppress the supercontraction effect, but even reverse it, while maintaining the exceptional mechanical properties of the silk material. This work has imminent impact on the design of biomimetic equivalents and recombinant silks for which supercontraction may or may not be a desirable feature. The approach applied is appropriate to explore the effect of point mutations on the overall physical properties of protein based materials

Flavonoid Insertion into Cell Walls Improves Wood Properties

Wood has an excellent mechanical performance, but wider utilization of this renewable resource as an engineering material is limited by unfavorable properties such as low dimensional stability upon moisture changes and a low durability. However, some wood species are known to produce a wood of higher quality by inserting mainly phenolic substances in the already formed cell walls – a process so-called heartwood formation. In the present study, we used the heartwood formation in black locust (<i>Robinia pseudoacacia</i>) as a source of bioinspiration and transferred principles of the modification in order to improve spruce wood properties (<i>Picea abies</i>) by a chemical treatment with commercially available flavonoids. We were able to effectively insert hydrophobic flavonoids in the cell wall after a tosylation treatment for activation. The chemical treatment reduced the water uptake of the wood cell walls and increased the dimensional stability of the bulk spruce wood. Further analysis of the chemical interaction of the flavonoid with the structural cell wall components revealed the basic principle of this bioinspired modification. Contrary to established modification treatments, which mainly address the hydroxyl groups of the carbohydrates with hydrophilic substances, the hydrophobic flavonoids are effective by a physical bulking in the cell wall most probably stabilized by π–π interactions. A biomimetic transfer of the underlying principle may lead to alternative cell wall modification procedures and improve the performance of wood as an engineering material

Global coordinate system and the Euler angles that describe the position of the molecular structures of collagen-like peptide and alpha helix.

<p>The directions of propagation of the incident and scattered beam (E_i and E_s) are represented by the red arrows parallel to the X axis while yellow bar represents the position of molecular structures. The “in plane” rotations are performed in the plane YZ and the “out of plane” rotations are performed in the ZX plane.</p

Averaged theoretical amide I response of collagen-like peptide for rotation around the main axis of the molecule.

<p>A) Amide I band response of a collagen-like peptide (ID: 1BKV) oriented (φ = 0°, θ = 0°,) plane (i.e. perpendicular to the ZY plane) rotated at different ω angles around the c-axis and its average response. B) Amide I band response of a collagen-like peptide (ID:1BKV) lying in the ZY plane (φ = 90°, θ = 0°) that has been rotated at different ω angles around the c-axis of the molecule and its average response. All graphs are plotted as functions of the polarization angle β of the incident laser beam (according to eq.3).</p

Theoretical prediction of the anisotropic response of amide I band for collagen-like and alpha helix molecules.

<p>Normalized anisotropic response of the amide I band of a collagen-like peptide molecule (ID:1CAG) and alpha helix (ID:1XQ8) located at A) (φ = 90°, θ = 0°,ω = 0°) on the plane ZY, B) (φ = 90°, θ = 90°, ω = 0°) on the plane ZY and C) (φ = 0°, θ = 0°,ω = 0°) on the plane ZX. For the collagen-like peptide structure located “in plane” (A and B) the maximum response of the amide I band is obtained when the polarization of the light is parallel to the molecule position, the opposite response is observed for the alpha helix. In the “out of plane” (C) response both structures give rise to a much more isotropic response of the amide I band.</p

<i>In situ</i> PRS mapping of the collagen orientation in −45 and 45 degrees tilted dried RTT.

<p>(A) and (C) show maps obtained by fitting thirteen Raman images collected with different polarization angles of the incident laser light. The direction of arrows indicates the orientation of collagen molecules, their length represents the amplitude of the fitting curve, and the color code is the average intensity of the amide I band. (B) and (D) are example of experimental points extracted from the area marked in (1) and (2), respectively.</p

Polarized Raman spectroscopy of RTT.

<p>Spectra taken at the same spot of the sample were collected with two different laser polarization orientations [parallel (laser X, blue line) and perpendicular (laser Z, red line) to the tendon axis]. A large anisotropy of the amide I band in the two different laser to fiber configurations is due to the preferential orientation of vibrational units along the main axis of the tendon.</p

Averaged theoretical amide I response of collagen-like peptide molecules for “out of plane” rotation.

<p>Normalized amide I response for four different collagen-like peptide structures (ID: 1BKV, 1CGD, 1QSU) that are rotated in the plane XZ [from (φ = 90°, θ = 90°) to (φ = 0°, θ = 90°)] <i>vs</i> the polarization angle of the incident light. The responses have been averaged at angles ω = 0°, ω = 90°, ω = 180°, ω = 270°. All the molecules exhibit a similar trend independent from which collagen-like peptide crystal structure. The average responses for all the selected structures are marked in bold.</p

Nanoconfined β‑Sheets Mechanically Reinforce the Supra-Biomolecular Network of Robust Squid Sucker Ring Teeth

The predatory efficiency of squid and cuttlefish (superorder Decapodiformes) is enhanced by robust Sucker Ring Teeth (SRT) that perform grappling functions during prey capture. Here, we show that SRT are composed entirely of related structural “suckerin” proteins whose modular designs enable the formation of nanoconfined β-sheet-reinforced polymer networks. Thirty-seven previously undiscovered suckerins were identified from transcriptomes assembled from three distantly related decapodiform cephalopods. Similarity in modular sequence design and exon–intron architecture suggests that suckerins are encoded by a multigene family. Phylogenetic analysis supports this view, revealing that suckerin genes originated in a common ancestor ∼350 MYa and indicating that nanoconfined β-sheet reinforcement is an ancient strategy to create robust bulk biomaterials. X-ray diffraction, nanomechanical, and micro-Raman spectroscopy measurements confirm that the modular design of the suckerins facilitates the formation of β-sheets of precise nanoscale dimensions and enables their assembly into structurally robust supramolecular networks stabilized by cooperative hydrogen bonding. The suckerin gene family has likely played a key role in the evolutionary success of decapodiform cephalopods and provides a large molecular toolbox for biomimetic materials engineering