Enzyme Control Over Ferric Iron Magnetostructural Properties

Fe3+ complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli‐driven change to Fe3+ speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe3+ magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μ‐oxo‐bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high‐spin Fe3+ chelate resulting in substantial T1‐relaxivity increase. The data communicated demonstrate proof of concept for a novel and effective strategy to exert biochemical control over aqueous Fe3+ magnetic, structural, and relaxometric properties. We introduce a first‐in‐class stimuli responsive Fe3+ complex that is switched between discrete antiferromagnetically coupled and high‐spin paramagnetic species through the action of a carboxylesterase enzyme. We also demonstrate how enzyme‐mediated switching of Fe3+ magnetic properties offers a mechanistic foundation for new bioresponsive magnetic resonance sensor technology.