Ultrafast bond twisting dynamics in amyloid fibril sensor.

The fundamental process of bond twisting that is responsible for the fluorescence sensing activity of the most extensively used amyloid fibril sensor, Thioflavin T, has been revealed using ultrafast time-resolved fluorescence spectroscopy. From the wavelength-dependent fluorescence decay kinetics and the subsequently constructed time-resolved emission spectra (TRES), the dynamic Stokes shift and the change in the spectral width were observed. These results are rationalized on the basis of the proposition that, following photoexcitation, Thioflavin T undergoes ultrafast bond twisting to form a twisted intramolecular charge-transfer state that is weakly emissive in nature. Formation of the twisted state from the local excited state was found to occur in the subpicosecond time domain (time constant approximately = 570 fs). Quantum chemical calculations support the proposition of the bond twisting process in the photoexcited Thioflavin T and suggest that the twisting around the central C-C single bond, rather than the C-N single bond, of the Thioflavin T molecule is mainly responsible for the observed ultrafast dynamics in the excited state. Detailed time-resolved fluorescence studies of Thioflavin T incorporated in amyloid fibril show substantial retardation in the bond twisting dynamics, suggesting the involvement of this process in the sensor activity of the dye.