Probing oxidative stress: Small molecule fluorescent sensors of metal ions, reactive oxygen species, and thiols.

Oxidative stress is a common feature shared by many diseases, including neurodegenerative diseases. Factors that contribute to cellular oxidative stress include elevated levels of reactive oxygen species, diminished availability of detoxifying thiols, and the misregulation of metal ions (both redox-active iron and copper as well as non-redox active calcium and zinc). Deciphering how each of these components interacts to contribute to oxidative stress presents an interesting challenge.

A Cell-Permeable Fluorescent Prochelator Responds to Hydrogen Peroxide and Metal Ions by Decreasing Fluorescence.

Described here is the development of two boronic ester-based fluorescent prochelators, FloB (2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)-4(5)-[2-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-benzylidene-hydrazinocarbonyl]-benzoic acid) and FloB-SI (2-(6-hydroxy-3-oxo-3Hxanthen-9-yl)-4(5)-[2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyloxy)-benzylidene-hydrazinocarbonyl]-benzoic acid) that show a fluorescence response to a variety of transition metal ions only after reaction with H(2)O(2). Both prochelators' boronic ester masks are oxidized by H(2)O(2) to reveal a fluorescein-tagged metal chelator, FloS (4(5)-(2-hydroxy-benzylidenehydrazinocarbonyl)-2-(6-hydroxy-3-oxo-3H-xanthen-9-yl)-benzoic acid). Chelation of Fe(3+) or Cu(2+) elicits a 70% decrease in the emission signal of FloS, while Zn(2+), Ni(2+), and Co(2+) produce a more modest fluorescence decrease.

Toward the detection of cellular copper(II) by a light-activated fluorescence increase.

A new type of Cu fluorescent sensor, coucage, has been prepared with a photosensitive nitrophenyl group incorporated into the backbone of a coumarin-tagged tetradentate ligand. Coucage provides a selective fluorescence response for Cu over other biologically relevant metal ions. Coordination of Cu dims the fluorescence output until irradiation with UV light cleaves the ligand backbone, which relieves the copper-induced quenching to provide a turn-on response.

Toward the development of prochelators as fluorescent probes of copper-mediated oxidative stress.

A fluorescent sensor prochelator, FlamB (fluorescein hydrizido 2-imidophenylboronic ester), has been developed that selectively probes for copper under conditions of oxidative stress. High levels of hydrogen peroxide trigger the release of a boronic ester masking group from the prochelator to unveil a metal chelator, FlamS (fluorescein hydrizido 2-imidophenol), that provides a modest fluorescence increase in response to Cu(2+) but not other metal ions. X-Ray crystal structures of FlamB, FlamS, and Cu-bound FlamS are all reported.