An Endoperoxide Reactivity-Based FRET Probe for Ratiometric Fluorescence Imaging of Labile Iron Pools in Living Cells

Iron is essential for sustaining life, as its ability to cycle between multiple oxidation states is critical for catalyzing chemical transformations in biological systems. However, without proper regulation, this same redox capacity can trigger oxidative stress events that contribute to aging along with diseases ranging from cancer to cardiovascular and neurodegenerative disorders. Despite its importance, methods for monitoring biological iron bound weakly to cellular ligands-the labile iron pool-to generate a response that preserves spatial and temporal information remain limited, owing to the potent fluorescence quenching ability of iron. We report the design, synthesis, and biological evaluation of FRET Iron Probe 1 (FIP-1), a reactivity-based probe that enables ratiometric fluorescence imaging of labile iron pools in living systems. Inspired by antimalarial natural products and related therapeutics, FIP-1 links two fluorophores (fluorescein and Cy3) through an Fe(II)-cleavable endoperoxide bridge, where Fe(II)-triggered peroxide cleavage leads to a decrease in fluorescence resonance energy transfer (FRET) from the fluorescein donor to Cy3 acceptor by splitting these two dyes into separate fragments. FIP-1 responds to Fe(II) in aqueous buffer with selectivity over competing metal ions and is capable of detecting changes in labile iron pools within living cells with iron supplementation and/or depletion. Moreover, application of FIP-1 to a model of ferroptosis reveals a change in labile iron pools during this form of cell death, providing a starting point to study iron signaling in living systems.