Hydroquinone-functionalized cyanine dye as reduction-sensitive probe for imaging of biological reducing species

The redox chemistry and status of different cell types and tissues has still not been well understood and thus represents a major focus for the development of novel responsive fluorophores. In this context, there is ongoing search for reduction-sensitive probes that can globally or specifically image reducing environments species in biological settings. Here, the synthesis and characterization of responsive cyanine dyes operating in the near-infrared (NIR) is reported based on the use of the hydroquinone/quinone and catechol/quinone couple. The introduction of a hydroquinone group into the meso-position of an indodicarbocyanine dye via a Suzuki cross coupling reaction created a water-soluble fluorescent "on-off-on " probe whose fluorescence can be modulated through oxidation and reduction. By adding biologically relevant reducing species, such as glutathione, cysteine or ascorbate, the "off "-probe could be rapidly turned on around 660 nm within 30 s. Reversible switching tween the "on " and "off " form was demonstrated. Furthermore, the sensor was applied in confocal microscopy studies as a reduction-sensitive probe for bioimaging in living cells.