Sacrificial Ionic Bonds Need To Be Randomly Distributed To Provide Shear Deformability

Multivalent ions are known to allow for reversible cross-linking in soft biological materials, providing stiffness and extensibility via sacrificial bonds. We present a simple model where stiff nanoscale elements carrying negative charges are coupled in shear by divalent mobile cations in aqueous media. Such a shear coupling through a soft glue has, indeed, been proposed to operate in biological nanocomposites. While the coupling is elastic and brittle when the negative charges are periodically arranged, sufficient randomness in their distribution allows for large irreversible deformation

Metal-Tunable Self-Assembly of Hierarchical Structure in Mussel-Inspired Peptide Films

Bottom-up control over structural hierarchy from the nanoscale through the macroscale is a critical aspect of biological materials fabrication and function, which can inspire production of advanced materials. Mussel byssal threads are a prime example of protein-based biofibers in which hierarchical organization of protein building blocks coupled <i>via</i> metal complexation leads to notable mechanical behaviors, such as high toughness and self-healing. Using a natural amino acid sequence from byssal thread proteins, which functions as a pH-triggered self-assembly point, we created free-standing peptide films with complex hierarchical organization across multiple length scales that can be controlled by inclusion of metal ions (Zn²⁺ and Cu²⁺) during the assembly process. Additionally, analysis of film mechanical performance indicates that metal coordination bestows up to an order of magnitude increase in material stiffness, providing a paradigm for creating tunable polymeric materials with multiscale organizational structure

Self-Repair of a Biological Fiber Guided by an Ordered Elastic Framework

Incorporating sacrificial cross-links into polymers represents an exciting new avenue for the development of self-healing materials, but it is unclear to what extent their spatial arrangement is important for this functionality. In this respect, self-healing biological materials, such as mussel byssal threads, can provide important chemical and structural insights. In this study, we employ in situ small-angle X-ray scattering (SAXS) measurements during mechanical deformation to show that byssal threads consist of a partially crystalline protein framework capable of large reversible deformations via unfolding of tightly folded protein domains. The long-range structural order is destroyed by stretching the fiber but reappears rapidly after removal of load. Full mechanical recovery, however, proceeds more slowly, suggesting the presence of strong and slowly reversible sacrificial cross-links. One likely role of the highly ordered elastic framework is to bring sacrificial binding sites back into register upon stress release, facilitating bond reformation and self-repair

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

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

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

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 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

Induced Mineralization of Hydroxyapatite in Escherichia coli Biofilms and the Potential Role of Bacterial Alkaline Phosphatase

Biofilms appear when bacteria colonize a surface and synthesize and assemble extracellular matrix components. In addition to the organic matrix, some biofilms precipitate mineral particles such as calcium phosphate. While calcified biofilms induce diseases like periodontitis in physiological environments, they also inspire the engineering of living composites. Understanding mineralization mechanisms in biofilms will thus provide key knowledge for either inhibiting or promoting mineralization in these research fields. In this work, we study the mineralization of Escherichia coli biofilms using the strain E. coli K-12 W3110, known to produce an amyloid-based fibrous matrix. We first identify the mineralization conditions of biofilms grown on nutritive agar substrates supplemented with calcium ions and β-glycerophosphate. We then localize the mineral phase at different scales using light and scanning electron microscopy in wet conditions as well as X-ray microtomography. Wide-angle X-ray scattering enables us to further identify the mineral as being hydroxyapatite. Considering the major role played by the enzyme alkaline phosphatase (ALP) in calcium phosphate precipitation in mammalian bone tissue, we further test if periplasmic ALP expressed from the phoA gene in E. coli is involved in biofilm mineralization. We show that E. coli biofilms grown on mineralizing medium supplemented with an ALP inhibitor undergo less and delayed mineralization and that purified ALP deposited on mineralizing medium is sufficient to induce mineralization. These results suggest that also bacterial ALP, expressed in E. coli biofilms, can promote mineralization. Overall, knowledge about hydroxyapatite mineralization in E. coli biofilms will benefit the development of strategies against diseases involving calcified biofilms as well as the engineering of biofilm-based living composites