Physical and structural properties of [Cu(BOT1)Cl]Cl, a fluorescent imaging probe for HNO

Nitroxyl, or HNO, is involved in a number of important physiological processes, such as vascular relaxation and neuroregulation. Effective imaging tools are required in order to gain a deeper understanding of the in vivo mechanisms of these processes and to identify the endogenous sources of HNO. Here, we further investigate the physical properties of our previously reported fluorescent nitroxyl sensor, [Cu(BOT1)Cl]Cl (J. Am. Chem. Soc.2010, 132, 5536; BOT1 = BODIPY·triazole, a tetradentate ligand). A new high-yielding synthetic procedure for BOT1 is reported. The X-ray crystal structures of two Cu(II) complexes of BOT1 are described. These structural studies show that the BOT1 ligand can form Cu(II) coordination complexes of both square-pyramidal and trigonal–bipyramidal geometries. Cyclic voltammograms of [Cu(BOT1)Cl]Cl were acquired, revealing the presence of a quasi-reversible feature at 130 mV (vs the ferrocene/ferrocenium couple) in MeCN and at − 40 mV (vs Ag/AgCl) in aqueous buffer, which is assigned to the CuII/CuI couple. The reactivity of [Cu(BOT1)Cl]Cl with Angeli's salt, a stable source of HNO, was further investigated. A 1000-fold excess of Angeli's salt elicits an immediate 10-fold emission turn-on response of the sensor, consistent with our previous report. A new observation, reported here, is that the intensity of this turn-on emission diminishes at longer incubation times. Fluorescent imaging of nitroxyl by [Cu(BOT1)Cl]Cl in HeLa cells was carried out. Upon treatment of the cells with Angeli's salt, there was a modest 2-fold intracellular turn-on in emission intensity.

Graphical abstract[Cu(BOT1)Cl]Cl is a fluorescent probe capable of imaging HNO in solution as well as live cells. The present work describes an optimized synthesis of the probe and further characterization of its photophysical and structural properties.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Improved synthesis of BOT1. ► X-ray crystal structures of CuII(BOT1) perchlorate and tetrafluoroborate salts. ► Electrochemical properties of CuII(BOT1). ► HNO imaging and localization in live HeLa cells using CuII(BOT1).