Basic Orange 21: A molecular rotor probe for fluorescence turn-on sensing of amyloid fibrils

Abstract

Developing label-free fluorescence sensor for amyloid fibrils is a very important and active field of research. Currently, there is a surge in efforts towards developing fluorescence sensor probes for amyloid fibrils which operates via fluorescence turn-on mechanism. Herein, we have identified a new molecular rotor probe, Basic Orange 21 (BO21), which displays a very large fluorescence turn-on emission selectively in response to insulin fibrils in the green region of the visible spectrum. The ground-state absorption, steady–state and time-resolved emission measurements have been extensively employed to characterize the interaction of BO21 with the insulin fibril matrix. The ionic strength dependent results indicate that BO21 binds to the fibril mostly through hydrophobic interactions which has been further corroborated by molecular docking calculations. Moreover, BO21 can also monitor the kinetic progression of the fibrillation process of insulin protein without interfering with the fibrillation process itself. Compared to extensively employed amyloid marker, Thioflavin-T, this new probe, BO21, works with comparatively longer excitation and emission wavelength which is desired for an amyloid marker, and additionally, the sensing performance of BO21 is immune to the ionic strength of the medium which is expected to significantly fuel the prospect of this new probe in various amyloid investigations.