Photophysical insights and guidelines for blue “turn‐on” fluorescent probes for the direct detection of nitric oxide (NO^•) in biological systems

Abstract

Synthesis and photophysical properties of a family of five blue fluorescent “turn‐on” probes for the direct detection of nitric oxide (NO•) is reported. The probes CB1‐5 feature a substituted 7‐hydroxy coumarin chromophore coupled to 2‐methyl‐8‐aminoquinoline, which act as tridentate ligand for Cu(II) and active site for monitoring NO• using a replacement strategy. The UV/vis absorption and fluorescence emission characteristics of the probes are significantly influenced by the substitution pattern on the coumarin ring, as well as by solvent polarity and pH. Time‐dependent density functional theory (TD‐DFT) calculations for CB4 and CB5 showed that the absorptions are due to π‐π* transitions localised on coumarin, with a small charge transfer contribution from the quinoline system at higher pH where the 7‐hydroxy coumarin moiety is deprotonated, which leads to a bathochromic shift of the absorption. Complexation of the probes with Cu(II) quenches the fluorescence, which is turned back on upon reaction with NO•, allowing selective detection of this important signalling molecule in μM concentrations. Preliminary experiments revealed that CB4 and CB5 enable to monitor endogenously produced NO• in living bacterial cells in multi‐dye imaging experiments.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

“…The reduction of Cu 2+ to diamagnetic Cu + by NO is concomitant with nitrosylation of the ligand, resulting in ligand dissociation from the reduced copper(I) center and thus fluorescence turn-on. This seminal work has stimulated the development of copper(II) complexes as fluorogenic probes for NO, − offering valuable tools for investigating the physiological and pathological functions of NO in living systems.…”

Combining nitric oxide and calcium sensing for the detection of endothelial dysfunction

“…c General sensing method for NO by metal-based fluorescent probes. d Molecular formulae of selected examples of transition metal-based NO sensors 191 , 196 . e Selected nitric oxide sensors useful for sensing endothelial dysfunction.…”