Thus, the experimental data indicates that there is a coordination number-dependent hierarchy of thermodynamic affinity between histamine and Ni 2+ , Cu 2+ , and Zn 2+ , where the ordering follows Ni 2+ > Cu 2+ > Zn 2+ in a ML3 configuration but the ordering changes to Cu 2+ > Zn 2+ > Ni 2+ in a ML2 configuration. While these results provide some clarity on the different contexts in which Ni 2+ and Cu 2+ provide the strongest binding to histamine, we recognize that these results cannot be easily compared to results from biological systems. This is because the amine group of the histamine amino acid is incorporated into the backbone of proteins and peptides, and thus it is rarely involved in metal-coordination interactions in metalloproteins, except in a few some cases with Cu 2+ or Cd 2+ . When the amine group cannot participate in the metal-coordination, the binding of histamine groups with M 2+ species are likely to be dominated by the interaction with coordinating nitrogen atoms on imidazole. Thus, a more suitable comparison to binding hierarchy exhibited in biological systems may come from studying the hierarchy of imidazole-M 2+ binding affinity. We thus perform a repeat of our experimental characterization of the complex shear viscosity 𝜂 * with 4-PEG-imidazole-M 2+ networks (Fig. ). We study the dynamics of polymer networks formed under ML2 and ML4 imidazole-M 2+ coordination stoichiometry, as metalloproteins rarely have more than four coordinated histidine amino acids unless there are two metals in the metal binding site. For ML4 coordination, a Ni 2+ > Cu 2+ > Zn 2+ hierarchy of binding affinity is observed, while for ML2 coordination, a Cu 2+ > Ni 2+ > Zn 2+ hierarchy is observed (Fig. ). A summary of the experimentally-observed binding hierarchies of histamine and imidazole with Ni 2+ , Cu 2+ , and Zn 2+ are illustrated in Fig. .