Mechanical Rupture of Mono- and Bivalent Transition Metal Complexes in Experiment and Theory

Biomolecular systems are commonly exposed to a manifold of forces, often acting between multivalent ligands. To understand these forces, we studied mono- and bivalent model systems of pyridine coordination complexes with Cu²⁺ and Zn²⁺ in aqueous environment by means of scanning force microscopy based single-molecule force spectroscopy in combination with ab initio DFT calculations. The monovalent interactions show remarkably long rupture lengths of approximately 3 Å that we attribute to a dissociation mechanism involving a hydrogen-bound intermediate state. The bivalent interaction with copper dissociates also via hydrogen-bound intermediates, leading to an even longer rupture length between 5 and 6 Å. Although the bivalent system is thermally more stable, the most probable rupture forces of both systems are similar over the range of measured loading rates. Our results prove that already in small model systems the dissociation mechanism strongly affects the mechanical stability. The presented approach offers the opportunity to study the force-reducing effects also as a function of different backbone properties.