The DFT simulations also reveal the geometry of the metal-coordination complexes, which can provide insight into the structural origins of the binding hierarchy exhibited by the metals (Fig. ). The identified coordination geometries of the complexes are generally in agreement with coordination geometries found in other work. For histamine complexes, Cu 2+ and Zn 2+ have been reported to undergo ML2 coordination through square-pyramidal and tetrahedral structures, respectively, while Ni 2+ is reported to undergo ML2 and ML3 coordination with histamine through an octahedral structure. For imidazole complexes, Cu 2+ has been reported undergo ML4 or ML6 coordination in square planar or octahedral geometries, and Ni 2+ and Zn 2+ have been reported to undergo ML4 coordination in a tetrahedral structure, though the coordination number can be lower in biological protein systems due to metal ion interactions with other coordinating amino acids or water. The lowest energy structures from DFT (Fig. ) can lend some insight into how the coordination geometries may influence binding hierarchies of the different transition metals. Ni 2+ is found to preferentially bind in an octahedral geometry, whereas Cu 2+ and Zn 2+ are shown to preferentially bind in square planar or tetrahedral geometries. These preferential bindings may explain the higher stability of histamine with Ni 2+ compared to the Cu 2+ and Zn 2+ in the ML3 complex, and the higher stability of histamine with Cu 2+ and Zn 2+ compared to Ni 2+ in the ML2 complex. This explanation does not explain the high ML4 stability of imidazole with Ni 2+ compared to Cu 2+ and Zn 2+ , and high ML2 stability of imidazole with Ni 2+ compared to Zn 2 . In our calculations, we find that octahedral coordination is not preferred by imidazole ML4 complexes due to the steric crowding of imidazole in octahedral geometries in the presence of water, and find that imidazole with Ni 2+ shows a higher stability than Cu 2+ and Zn 2+ in the ML4 complex, even though imidazole ML4 is in a tetrahedral geometry. This might reflect the limitations of a purely structural perspective on the binding hierarchies, and the energetics of binding may also play a role in the binding hierarchy of the metal ions. For instance, the strong preference of imidazole to Ni 2+ could be explained using crystal field theory, which evaluates the net stabilization of electrons in the d orbitals of metal ions depending on specific ligand field geometry. From Ni 2+ (d 8 ) to Cu 2+ (d 9 ) and Zn 2+ (d 10 ), the incremental electrons would occupy the T2g orbitals with higher energy, thus destabilizing the coordination compound. A more quantitative comparison of the experimental and computational results is shown in Fig. , where 𝜂 + 211 (which is the measured viscosity values normalized by 𝜂 + of the 4-PEG-histamine and 4-PEG-imidazole polymer solutions without ions) is compared with the binding energy calculations. As expected, a positive correlation between binding energy (Δ𝐺 + ) and 𝜂 + 211 exists, though the correlation appears to be system-specific. For example, we see that the viscosities of the ML4-coordinated 4-PEG-imidazole networks are only marginally higher compared to the ML4coordinated 4-PEG-imidazole networks, despite the substantial difference in the binding energies of the two coordination states. The reason for this is not immediately clear, although a possible hypothesis for this might be that the weak and dynamic coordination between imidazole and M 2+ species may not fully prevent the precipitation of M(OH)2 species, resulting in an overall lower population of M 2+ species especially in the ML2 state. We also cannot neglect the fact that the simulations do not consider any of the counter-ions present in the experimental system such as the MOPS buffer, and that some quantitative differences may arise as a result.