Our results are relevant in conditions in which histamine or imidazole ligands freely and exclusively bind to the M 2+ at a neutral pH, deviations from these predictions can occur in the presence of competing species in non-buffered conditions that may compete with the M 2+ binding, such as counterions and hydroxides, or in systems that exhibit higher-order self-assembly (for instance clustering of the metal-ligand complex). For instance, this higher-order clustering of the coordination complexes is reported to occur in imidazole-containing polymer systems reinforced by Zn 2+ , which can give rise to multivalent cross-linking and thus a slower mode of viscoelastic relaxation in these systems. This may explain the substantially higher reinforcement of viscoelasticity in imidazole-containing polymers by Zn 2+ than other M 2+ ions. The aggregate formation by Zn 2+ is a fascinating behavior-perhaps attributable to the formation of wellcontrolled tetrahedral coordination structures-which warrants future studies.