A general strategy to develop cell permeable and fluorogenic probes for multicolour nanoscopy

It is difficult to develop suitable fluorescent probes for live-cell nanoscopy, but a general strategy is now reported that can transform regular fluorophores into fluorogenic probes with excellent cell permeability and low unspecific background signals. Using this approach, probes in a variety of colours were developed for different cellular targets and used for wash-free, multicolour, live-cell confocal and STED microscopy. Live-cell fluorescence nanoscopy is a powerful tool to study cellular biology on a molecular scale, yet its use is held back by the paucity of suitable fluorescent probes. Fluorescent probes based on regular fluorophores usually suffer from a low cell permeability and an unspecific background signal. Here we report a general strategy to transform regular fluorophores into fluorogenic probes with an excellent cell permeability and a low unspecific background signal. Conversion of a carboxyl group found in rhodamines and related fluorophores into an electron-deficient amide does not affect the spectroscopic properties of the fluorophore, but allows us to rationally tune the dynamic equilibrium between two different forms: a fluorescent zwitterion and a non-fluorescent, cell-permeable spirolactam. Furthermore, the equilibrium generally shifts towards the fluorescent form when the probe binds to its cellular targets. The resulting increase in fluorescence can be up to 1,000-fold. Using this simple design principle, we created fluorogenic probes in various colours for different cellular targets for wash-free, multicolour, live-cell nanoscopy.