Coordination stoichiometry effects on the binding hierarchy of histamine and imidazole-M2+ complexes

Histidine-M2+ coordination is a recognized bond motif in biogenic materials with high hardness and extensibility, which has led to growing interest in their use in soft materials for mechanical function. However, the effect of different metal ions on the stability of the coordination complex remains poorly understood, complicating their implementation in metal-coordinated systems. Here, we use rheology experiments and density functional theory calculations to characterize the stability of coordination complexes and establish the binding hierarchy of histamine and imidazole with Ni2+, Cu2+ and Zn2+. We find that the binding hierarchy is driven by the specific affinity of the metal ions to different coordination states, which can be macroscopically tuned by changing the metal-to-ligand stoichiometry of the metal-coordinated network. These findings facilitate the rational selection of metal ions for optimizing the mechanical properties of metal-coordinated materials.