Highly Specific Cys Fluorescence Probe for Living Mouse Brain Imaging via Evading Reaction with Other Biothiols

Depression is characterized by oxidative stress in the brain. As the crucial reductive biothiol, cysteine (Cys) directly regulates the occurrence of oxidative stress in the brain. Despite its significance, the precise exploration of Cys in mouse brains remains a challenge, primarily owing to the limitations of Cys-monitoring tools, especially the interference from unavoidable reaction with other biothiols. Thus, we developed a novel two-photon fluorescence probe for Cys based on a new specific recognition site, thiobenzoate. Encountering Cys, the carbon- sulfur double bond in the probe formed a stable five-membered ring via the selective nucleophilic addition reaction, triggering a remarkable fluorescence increase. Notably, this reaction cannot occur with other biothiols, which afford the probe unprecedented selectivity to Cys. With two-photon excitation at 754 nm, we achieved in situ visualization of the increased Cys in PC12 cells under dithiothreitol stimulation. Furthermore, we directly visualized the precipitous reduction of Cys in the brains of mice with depression phenotypes for the first time. This work opens up new vistas for Cys imaging and expands the understanding of pathogenesis of depression.