Ratiometric quantitation of redox status with a molecular Fe 2 magnetic resonance probe.

We demonstrate the ability of a molecular Fe ₂ complex to enable magnetic resonance (MR)-based ratiometric quantitation of redox status, namely through redox-dependent paramagnetic chemical exchange saturation transfer (PARACEST). Metalation of a tetra(carboxamide) ligand with Fe ^II and/or Fe ^III in the presence of etidronate ion affords analogous FeII2, Fe ^II Fe ^III , and FeIII2 complexes. Both FeII2 and Fe ^II Fe ^III complexes give highly-shifted, sharp, and non-overlapping NMR spectra, with multiple resonances for each complex corresponding to exchangeable carboxamide protons. These protons can be selectively irradiated to give CEST peaks at 74 and 83 ppm vs. H ₂ O for the Fe ^II Fe ^III complex and at 29, 40 and 68 ppm for the FeII2 complex. The CEST spectra obtained from a series of samples containing mixtures of FeII2 and Fe ^II Fe ^III are correlated with independently-determined open-circuit potentials to construct a Nernstian calibration curve of potential vs. CEST peak intensity ratio. In addition, averaged intensities of phantom images collected on a 9.4 T MRI scanner show analogous Nernstian behavior. Finally, both the FeII2 and Fe ^II Fe ^III forms of the complex are stable to millimolar concentrations of H ₂ PO ₄ ^- /HPO ₄ ^2- , CO ₃ ^2- , SO ₄ ^2- , CH ₃ COO ^- , and Ca ²⁺ ions, and the FeIII2 form is air-stable in aqueous buffer and shows >80% viability in melanoma cells at millimolar concentration. The stability suggests the possible application of this or related complexes for in vivo studies. To our knowledge, this concentration-independent method based on a single Fe ₂ probe provides the first example of MR-based ratiometric quantitation of redox environment.