4-Dimethylamino-1,8-Naphthalimide as a Fluorescence Signal Amplification for Site Selective and in Situ Probing of the Hydroxyl Radical.

Overproduction of reactive oxygen species (ROS) is involved in several diseases. Copper can be an efficient catalysts for ROS generation, as it was suggested for Cu bound to aberrant sites. Determination of rates and type of ROS production in more complex biological media is challenging and often only down-stream events can be monitored. Here, a selective detection of the hydroxyl radical (HO^•), by a demethylation of 4-dimethylamino-1,8-naphthalimide (NMI) fluorescence probe by HO^• resulting in ∼25-fold increase in quantum yield is reported. To investigate potential applications, the NMI was coupled to the Cu-binding domain of amyloid-beta (Aβ16). This allowed the in situ fluorescence detection of HO^• produced by Aβ16 bound Cu due to the Fenton-type reaction. In a more complex medium with scavengers present including in cell culture conditions, the HO^• produced in the bulk was not detected, in line with the short life-time of HO^• in the presence of other molecules. Thus, this is a proof of concept to show that NMI reaction-based HO^• detection could be a useful and a versatile tool to investigate in situ the role of HO^• in a defined location such as Cu bound to Aβ16 in biologically relevant complex systems.