Solvatochromic Fluorescent Probe for Visualizing Protein Aggregation via STED Imaging

Elucidating the structure of protein aggregates is vital for overcoming human diseases arising from protein misfolding. It is rarely reported that the conventional protein-amyloid fibrillation probe ThT can be used as a STED agent to visualize protein aggregation structures at the nanometer level, suffering from small Stokes' shift and photobleaching. Herein, we report a donor–acceptor (D–A)-type fluorophore, IAD2, with red emission and a large Stokes' shift (196 nm). It is demonstrated that IAD2 exhibited a 7.8-fold enhancement on the fluorescence intensity and an obvious 94 nm blue shift on the emission wavelength after binding with bovine insulin fibril, due to the alternation of the microenvironment. Besides, IAD2 has a relatively higher binding affinity to insulin fibrils in comparison to ThT. Molecular docking assay also verified the binding sites and interaction forces between IAD2 and insulin fibrils. Owing to its high depletion efficiency and good photostability, IAD2 was utilized to stain insulin fibrils to achieve STED imaging with a resolution of 101.2 nm. This study will shed light on the design of novel solvatochromic fluorescent probe for the super-resolution imaging of protein aggregate structure.