Abstract
Natural biopolymeric materials often possess properties superior to their individual components. In mussel byssus, reversible histidine (His)–metal coordination is a key feature, which mediates higher-order self-assembly as well as self-healing. The byssus structure, thus, serves as an excellent natural blueprint for the development of self-healing biomimetic materials with reversibly tunable mechanical properties. Inspired by byssal threads, we bioengineered His–metal coordination sites into a heterodimeric coiled coil (CC). These CC-forming peptides serve as a noncovalent cross-link for poly(ethylene glycol)-based hydrogels and participate in the formation of higher-order assemblies via intermolecular His–metal coordination as a second cross-linking mode. Raman and circular dichroism spectroscopy revealed the presence of α-helical, Zn2+ cross-linked aggregates. Using rheology, we demonstrate that the hydrogel is self-healing and that the addition of Zn2+ reversibly switches the hydrogel properties from viscoelastic to elastic. Importantly, using different Zn2+:His ratios allows for tuning the hydrogel relaxation time over nearly three orders of magnitude. This tunability is attributed to the progressive transformation of single CC cross-links into Zn2+ cross-linked aggregates; a process that is fully reversible upon addition of the metal chelator ethylenediaminetetraacetic acid. These findings reveal that His–metal coordination can be used as a versatile cross-linking mechanism for tuning the viscoelastic properties of biomimetic hydrogels.
Highlights
Histidine (His)–metal coordination is one of nature’s powerful means to mechanically reinforce biological materials, such as spider fangs, sandworm jaws or mussel byssal threads [1,2]
Zn2+ cross-linked aggregates inside the hydrogel. As both the coiled coil (CC) and the His–Zn2+ cross-links are reversible, this directly results in a hydrogel with two self-healing modes. We demonstrate that this additional cross-linking and self-healing mode allows for reversibly and dynamically tuning the viscoelastic properties of the hydrogel as a function of the Zn2+ concentration
The His–metal coordination sites were rationally designed into the solvent-exposed f positions of a well characterized heterodimeric CC
Summary
Histidine (His)–metal coordination is one of nature’s powerful means to mechanically reinforce biological materials, such as spider fangs, sandworm jaws or mussel byssal threads [1,2]. Mussel byssal threads are highly organized, self-assembled proteinaceous fibers. They function as a holdfast for marine mussels, such as Mytilus edulis, which live in the coastal intertidal zone. The HRDs are known to coordinate transition metal ions, such as Ni2+ , Cu2+ and Zn2+ [4,5] These His–metal coordination sites act as reversible cross-links, which break when dynamically loaded (e.g., by crashing waves) and dissipate high quantities of mechanical energy [6]. The coordination bonds recover, enabling self-healing of the thread [7,8]. This behavior was found to be intimately
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