Coumarin-3-aldehyde as a scaffold for the design of tunable PET-modulated fluorescent sensors for neurotransmitters.

NeuroSensor 521 (NS521) is a fluorescent sensor for primary-amine neurotransmitters based on a platform that consists of an aryl moiety appended to position C4 of the coumarin-3-aldehyde scaffold. We demonstrate that sensors based on this platform behave as a directly linked donor-acceptor system that operates through an intramolecular acceptor-excited photoinduced electron transfer (a-PET) mechanism. To evaluate the PET process, a series of benzene- and thiophene-substituted derivatives were prepared and the photophysical properties, binding affinities, and fluorescence responses toward glutamate, norepinephrine, and dopamine were determined. The calculated energy of the highest occupied molecular orbital (EHOMO ) of the pendant aryl substituents, along with oxidation and reduction potential values derived from the calculated molecular orbital energy values of the platform components, allowed for calculation of the fluorescence properties of the benzene sensor series. Interestingly, the thiophene derivatives did not fit the typical PET model, highlighting the limitations of the method. A new sensor, NeuroSensor 539, displayed enhanced photophysical properties aptly suited for biological imaging. NeuroSensor 539 was validated by selectively labeling and imaging norepinephrine in secretory vesicles of live chromaffin cells.