Metal-coordination is an important molecular motif in biology wherein metal-mediated bonds play dynamic mechanical roles in protein folding and unfolding and shock absorbance in biological tissues such as mussel threads. These bonds are reversible and tunable, unlike more covalent chemical crosslinks that permanently break upon fracture. Because they break and reform after rupture, metal-coordination bonds can act as sacrificial bonds that help increase a material’s fracture toughness. In synthetic hydrogels, the incorporation of sacrificial metal bonding can result in tunable dynamic mechanical properties which can be controlled over multiple decades in time. Although metal-coordination hydrogels potentially represent a new class of materials with advanced functions for biomedical and structural applications, a fundamental understanding of the relationship between the metal-coordination chemistry, effective bond properties, and overarching macroscale mechanical properties of such systems is yet to be developed.<br/><br/>This study combines multiscale computational modeling with experiment to characterize the bonding energy landscape and kinetics of various metal-coordination complexes and thereby enhance our chemical understanding of ligand design. 4-arm PEG end-functionalized with histidine derivatives and crosslinked with divalent metal cations (Ni2+, Cu2+) are used as a model transient hydrogel. Density functional theory and molecular dynamics (MD) are used to characterize the equilibrium bonding configuration and quantum properties of the metal cations with the histidine ligands. The energy landscape of the metal-coordinate bonds is probed through metadynamics calculations and MD. Simulation results are compared with rheological and spectroscopic characterizations of the hydrogels. Specific comparisons are drawn between monodentate ligands such as imidazole, naturally found in mussel byssal threads, and bidentate ligands, used in synthetic studies, to demonstrate a clear transition between biology and synthetic materials and to illustrate the quantum properties of ligand chemistry and their mechanical consequences. Based on new insights gained into the role of ligand-metal binding in the polymer hydrogel network mechanics from these calculations and experiments, this work presents key ligand design criteria for how metal-coordination hydrogels can be further advanced towards useful biological and environmental applications.

Molecular understanding of ligand design in metal-coordination transient hydrogels using simulation and experiment

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.