A SNAP-tag fluorogenic probe mimicking the chromophore of the red fluorescent protein Kaede

Self-labelling protein tags with fluorogenic probes serve as great fluorescence imaging tools to understand key questions of protein dynamics and functions in living cells. In the present study, we report a SNAP-tag fluorogenic probe 4c mimicking the chromophore of the red fluorescent protein Kaede. The molecular rotor properties of 4c were utilized as a fluorogenic probe for SNAP-tag, such that conjugation with SNAPf protein leads to inhibition of twisted intramolecular charge transfer, triggering fluorogenecity. Upon conjugation with SNAPf, 4c exhibited approximately a 90-fold enhancement in fluorescence intensity with fast labelling kinetics (k ₂ = 15000 M ^-1 s ^-1 ). Biochemical and spectroscopic studies confirmed that fluorescence requires formation of folded SNAPf·4c covalent conjugate between Cys 145 and 4c. Confocal microscopy and flow cytometry showed that 4c is capable of detecting SNAPf proteins or SNAPf fused with a protein of interest in living cells. This work provides a framework to develop the large family of FP chromophores into fluorogenic probes for self-labelling protein tags.