Metal-Based Optical Probes for Live Cell Imaging of Nitroxyl (HNO)

A review. Nitroxyl (HNO) is a biol. signaling agent that displays distinctive reactivity compared to nitric oxide (NO). As a consequence, these two reactive nitrogen species trigger different physiol. responses. Selective detection of HNO over NO has been a challenge for chemists, and several fluorogenic mol. probes have been recently developed with that goal in mind. Common constructs take advantage of the HNO-induced redn. of Cu(II) to Cu(I). The sensing mechanism of such probes relies on the ability of the unpaired electron in a d orbital of the Cu(II) center to quench the fluorescence of a photoemissive ligand by either an electron or energy transfer mechanism. Exptl. and theor. mechanistic studies suggest that proton-coupled electron transfer mediates this process, and careful tuning of the copper coordination environment led to sensors with optimized selectivity and kinetics. The current optical probes cover the visible and near-IR regions of the spectrum. This palette of sensors comprises structurally and functionally diverse fluorophores such as coumarin (blue/green emission), boron dipyrromethane (BODIPY, green emission), benzoresorufin (red emission), and dihydroxanthenes (near-IR emission). Many of these sensors have been successfully applied to detect HNO prodn. in live cells. For example, copper-based optical probes have been used to detect HNO prodn. in live mammalian cells that have been treated with H2S and various nitrosating agents. These studies established a link between HSNO, the smallest S-nitrosothiol, and HNO. In addn., a near-IR HNO sensor has been used to perform multicolor/multianalyte microscopy, revealing that exogenously applied HNO elevates the concn. of intracellular mobile zinc. This mobilization of zinc ions is presumably a consequence of nitrosation of cysteine residues in zinc-chelating proteins such as metallothionein. Future challenges for the optical imaging of HNO include devising probes that can detect HNO reversibly, esp. because ratiometric imaging can only report equil. concns. when the sensing event is reversible. Another important aspect that needs to be addressed is the creation of probes that can sense HNO in specific subcellular locations. These tools would be useful to identify the organelles in which HNO was produced in mammalian cells and probe the intracellular signaling networks in which this reactive nitrogen species is involved. In addn., near-IR emitting probes might be applied to detect HNO in thicker specimens, such as acute tissue slices and even live animals, enabling the study of the roles of HNO in physiol. or pathol. conditions in multicellular systems.