Highly tunable bimane-based fluorescent probes: design, synthesis, and application as a selective amyloid binding dye

Small molecule fluorescent probes are indispensable tools for a broad range of biological applications. Despite many probes being available, there is still a need for probes where photophysical properties and biological selectivity can be tuned as desired. Here, we report the rational design and synthesis of a palette of fluorescent probes based on the underexplored bimane scaffold. The newly developed probes with varied electronic properties show tunable absorption and emission in the visible region with large Stokes shifts. Probes featuring electron-donating groups exhibit rotor effects that are sensitive to polarity and viscosity by "intramolecular charge transfer" (ICT) and twisted intramolecular charge transfer (TICT) mechanisms, respectively. These properties enable their application as "turn-on" fluorescent probes to detect fibrillar aggregates of the alpha-synuclein (alpha S) protein that are a hallmark of Parkinson's disease (PD). One probe shows selective binding to alpha S fibrils relative to soluble proteins in cell lysates and amyloid fibrils of tau and amyloid-beta. Finally, we demonstrate the diagnostic potential of the probe in selectively detecting alpha S fibrils amplified from PD with dementia (PDD) patient samples. Rational design of environmentally-sensitive bimane probes with tunable properties holds promise for studying neurodegenerative disease.