Complexes of Fe3+ engage in rich aqueous solution speciation chemistry in which discrete molecules can react with solvent water to form multinuclear μ-oxo and μ-hydroxide bridged species. Here we demonstrate how pH- and concentration-dependent equilibration between monomeric and μ-oxo-bridged dimeric Fe3+ complexes can be controlled through judicious ligand design. We purposed this chemistry to develop a first-in-class Fe3+-based MR imaging probe, Fe-PyCy2AI, that undergoes relaxivity change via pH-mediated control of monomer vs dimer speciation. The monomeric complex exists in a S = 5/2 configuration capable of inducing efficient T 1-relaxation, whereas the antiferromagnetically coupled dimeric complex is a much weaker relaxation agent. The mechanisms underpinning the pH dependence on relaxivity were interrogated by using a combination of pH potentiometry, 1H and 17O relaxometry, electronic absorption spectroscopy, bulk magnetic susceptibility, electron paramagnetic resonance spectroscopy, and X-ray crystallography measurements. Taken together, the data demonstrate that PyCy2AI forms a ternary complex with high-spin Fe3+ and a rapidly exchanging water coligand, [Fe(PyCy2AI)(H2O)]+ (ML), which can deprotonate to form the high-spin complex [Fe(PyCy2AI)(OH)] (ML(OH)). Under titration conditions of 7 mM Fe complex, water coligand deprotonation occurs with an apparent pK a 6.46. Complex ML(OH) dimerizes to form the antiferromagnetically coupled dimeric complex [(Fe(PyCy2AI))2O] ((ML) 2 O) with an association constant (K a) of 5.3 ± 2.2 mM–1. The relaxivity of the monomeric complexes are between 7- and 18-fold greater than the antiferromagnetically coupled dimer at applied field strengths ranging between 1.4 and 11.7 T. ML(OH) and (ML) 2 O interconvert rapidly within the pH 6.0–7.4 range that is relevant to human pathophysiology, resulting in substantial observed relaxivity change. Controlling Fe3+ μ-oxo bridging interactions through rational ligand design and in response to local chemical environment offers a robust mechanism for biochemically responsive MR signal modulation.
Electrocatalytic and light-driven CO2 reduction by nickel cyclam is strongly dependent on the nature of the buffering agent.
Fe 3 + complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli-driven change to Fe 3 + speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe 3 + magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μoxo-bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high-spin Fe 3 + chelate resulting in substantial T 1 -relaxivity increase. The data communicated demonstrate proof of concept for a novel and effective strategy to exert biochemical control over aqueous Fe 3 + magnetic, structural, and relaxometric properties.
The nickel–iron carbon monoxide dehydrogenase (CODH) enzyme catalyzes the reversible and selective interconversion of carbon dioxide (CO2) to carbon monoxide (CO) with high rates and negligible overpotential. Despite decades of research, many questions remain about this complex metalloenzyme system. A simplified model enzyme could provide substantial insight into biological carbon cycling. Here, we demonstrate reversible electron transfer and binding of both CO and cyanide, a substrate and an inhibitor of CODH, respectively, in a Pyrococcus furiosus (Pf) ferredoxin (Fd) protein that has been reconstituted with a nickel–iron sulfide cluster ([NiFe3S4] Fd). The [NiFe3S4] cluster mimics the core of the native CODH active site and thus serves as a protein-based structural model of the CODH subsite. Notably, despite binding cyanide, no CO binding is observed for the physiological [Fe4S4] clusters in Pf Fd, providing chemical rationale underlying the evolution of a site-differentiated cluster for substrate conversion in native CODH. The demonstration of a substrate-binding metalloprotein model of CODH sets the stage for high-resolution spectroscopic and mechanistic studies correlating the subsite structure and function, ultimately guiding the design of anthropogenic catalysts that harness the advantages of CODH for effective CO2 reduction.
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