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 Cu 2+ and Zn 2+ 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.
A combination of density functional theory (DFT) and solution of the Poisson equation for continuum dielectric media was used to compute accurate redox potentials for several mononuclear transition metal complexes including iron, manganese and nickel. Progress was achieved by altering the B3LYP DFT functional (B4(XQ3)LYPapproach) and supplementing it with an empirical correction term, which is applied after the quantum-chemical DFT computations. Calculation of the 58 redox potentials of 48 different transition metal complexes shows a root mean square deviation from experimental values of 65 mV. The quality of the present approach becomes also evident by observing that the energetic order of the spin multiplicity is fulfilled for all considered transition metal complexes. For some transition metal complexes it was necessary to account for the dielectric environment before agreement with the corresponding measured spin multiplicities was obtained.
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