Single-Molecule Mechanics of Catechol-Iron Coordination Bonds

Metal coordination bonds are widely found in natural adhesives and load-bearing and protective materials, in which they are thought to be responsible for the high mechanical strength and toughness. However, it remains unknown how metal-ligand complexes could give rise to such superb mechanical properties. Here, we developed a single-chain nanoparticle based force spectroscopy to directly quantify the mechanical properties of individual catechol-Fe³⁺ complexes, the key elements accounting for the high toughness and extensibility of byssal threads of marine mussels. We found that catechol-Fe³⁺ complexes possess a unique combination of mechanical features, including high mechanical stability, fast reformation kinetics, and stoichiometry-dependent mechanics. Therefore, they can serve as sacrificial bonds to efficiently dissipate energy in the materials, quickly recover the mechanical properties when load is released, and respond to pH and Fe³⁺ concentrations. Especially, we revealed that the bis-catechol-Fe³⁺ complex is mechanically ∼90% stronger than the tris-catechol-Fe³⁺ complex. Quantum calculation study suggested that the distinction between mechanical strength and thermodynamic stability of catechol-Fe³⁺ complexes is due to their different mechanical rupture pathways. Our study provides the nanoscale mechanistic understanding of the coordination bond-mediated mechanical properties of biogenetic materials, and could guide future rational design and regulation of the mechanical properties of synthetic materials.