Enzymatically Shifting Nitroxides for EPR Spectroscopy and Overhauser‐Enhanced Magnetic Resonance Imaging

In vivo investigations of enzymatic processes using non-invasive approaches are a long-lasting challenge. Recently, we showed that Overhauser-enhanced MRI is suitable to such a purpose. A β-phosphorylated nitroxide substrate prototype exhibiting keto–enol equilibrium upon enzymatic activity has been prepared. Upon enzymatic hydrolysis, a large variation of the phosphorus hyperfine coupling constant (ΔaP=4 G) was observed. The enzymatic activities of several enzymes were conveniently monitored by electronic paramagnetic resonance (EPR). Using a 0.2 T MRI machine, in vitro and in vivo OMRI experiments were successfully performed, affording a 1200 % enhanced MRI signal in vitro, and a 600 % enhanced signal in vivo. These results highlight the enhanced imaging potential of these nitroxides upon specific enzymatic substrate-to-product conversion.