Oxime Ligands for Fe(III) MRI Contrast Agents and for Sensing Fe(III) and Fe(II)
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Fe(III) macrocyclic complexes are a promising new alternative to Gd(III) complexes as MRI contrast agents. HOPO (hydroxypyridinones) ligands, a class of compounds that are heterocyclic and possess a hydroxyl group ortho to a ketone, are known iron chelators. Oxime ligands which bear similar properties to HOPOs were used in studies for ternary complex formation to impact the relaxivity and solubility of a Fe(III) macrocyclic contrast agent that contains an open coordination site. A library of HOPOs and oxime ligand L1 were used for this study. Oximes and HOPOs have been studied in literature and are known to bind to Fe(III) in a bidentate interaction leading to tris-complexes. Binding of oxime ligand L1 and Fe(II) in the presence and absence of O2 brings about an oxidative conversion of Fe(II) to Fe(III). Studies are presented in which UV-vis spectroscopic measurements are conducted to monitor the interaction between oxime ligand L1 and Fe(II) or Fe(III). In addition, Ga(III), in terms of its coordination chemistry, is an excellent diamagnetic analogue of the biologically important (but paramagnetic) Fe(III) ion and as such enables NMR investigations for structure elucidation purposes. Data will be presented to investigate the stoichiometry of the K3[Fe(L1**)3] complex and the interaction between Ga(III) and oxime ligand L1 under different conditions.