Charge Transfer Process Determines Ultrafast Excited State Deactivation of Thioflavin T in Low-Viscosity Solvents

Abstract

Here we provide first direct experimental results about photoinduced TICT-state formation for Thioflavin T (ThT). In this work, femtosecond transient absorption spectra dynamics for ThT, dissolved in low-viscosity solvents (water, ethanol, 2-propanol, butanol) was investigated. It was found that decay lifetime of fluorescent LE-state for ThT in low-viscous solvents does not exceed 12 ps, and its value correlates well with rising time of the absorption band at 470 nm. It indicates that LE-state of ThT initially formed upon photoexcitation is quite rapidly converted to a transient state characterized by absorption at 470 nm. We associate this emerging intermediate state with nonfluorescent TICT-state of the dye. Rate of LE --> TICT process significantly depends on viscosity and is comparable to the rate of solvent relaxation resulting in time-dependent Stokes shift of ThT stimulated emission band. TICT-state deactivation was found to be also viscosity dependent and its lifetime changed from 3.8 +/- 0.1 ps (in H(2)O) to 360 +/- 60 ps (in butanol). It was proposed that a nonradiative deactivation process proceeds through a conical intersection between TICT(S(1)') and S(0) energy levels. The results obtained confirm the earlier proposed model that twisted internal charge transfer process takes place in the excited state of the dye and that ThT behaves as a molecular rotor (Stsiapura, V. I.; Maskevich, A. A.; Kuzmitsky, V. A.; Uversky, V. N.; Kuznetsova, I. M.; Turoverov, K. K. J. Phys. Chem. B 2008, 112, 15893-15902).

Ultrafast Twisting Dynamics of Thioflavin‐T: Spectroscopy of the Twisted Intramolecular Charge‐Transfer State

“…Stsiapura et al reported the time evolution of the TA spectrum in the l = 520-600 nm region, which could reveal the evolution of the ESA band only, but missed the SE band in the l = 700-950 nm region, which is an important feature of the relaxed excited state of ThT. [33] This SE band has been observed for the first time in this work.…”

“…Earlier, Stsiapura et al also reported that bleach recovery was not complete in the 20 ps time domain. [33] They attributed the incomplete recovery of the ground state of ThT to the photochemical ring opening of the benzothiazole moiety of the molecule. However, the proposition of the photochemical reaction possibly can be ruled out on the basis of the fact that ring opening of ThT would lead to a loss of conjugation, and thus, absorption spectra of the resulting molecular fragments would appear only in the UV region.…”

“…[32] Despite the reason behind the amyloid fibril sensitivity of ThT being largely understood, several intricate points regarding the dynamics of the excited state of ThT have been disputed recently. [24,33] Nath et al observed a temporary isoemissive point in the time-resolved area-normalized emission spectra (TRANES); this was attributed to the 1:1 conversion of the local excited (LE) state to the twisted intramolecular charge-transfer (TICT) state, taking place with a time constant of about 540 fs. [24] Moreover, they concluded that the TICT state was a weakly emissive state with the emission maximum at l % 650 nm.…”

Caracterização de lipoproteína de baixa densidade (LDL) por meios espectroscópicos

“…(1) -Um anel Benzotiazol Os três grupos que formam a estrutura da Tioflavina T são muito rígidos e tanto a sua estrutura quanto o comportamento fotofísico são determinados majoritariamente pelas suas orientações espaciais [88,89].…”

“…Estudos recentes demonstraram que a ThT comporta-se com um Rotor Molecular [88], [89], [90]. Rotores moleculares são moléculas fluorescentes caracterizados pela capacidade de formar estados torcidos através da rotação de um dos seus fragmentos com relação ao restante da molécula [91].…”