Computational Study of Thioflavin T Torsional Relaxation in the Excited State

Abstract

Quantum-chemical calculations of the Thioflavin T (ThT) molecule in the ground S0 and first excited singlet S1 states were carried out. It has been established that ThT in the ground state has a noticeable nonplanar conformation:  the torsion angle φ between the benzthiazole and the dimethylaminobenzene rings has been found to be ∼37°. The energy barriers of the intramolecular rotation appearing at φ = 0 and 90° are quite low:  semiempirical AM1 and PM3 methods predict values ∼700 cm-1 and ab initio methods ∼1000−2000 cm-1. The INDO/S calculations of vertical transitions to the S1(abs) excited state have revealed that energy ES1(abs) is minimal for the twisted conformation with φ = 90° and that the intramolecular charge-transfer takes place upon the ThT fragments' rotation from φ = 0 to 90°. Ab initio CIS/RHF calculations were performed to find optimal geometries in the excited S1 state for a series of conformers having fixed φ values. The CIS calculations have predicted a minimum of the S1 state energy at φ ∼21°; however, the energy values are 1.5 times overestimated in comparison to experimental data. Excited state energy dependence on the torsion angle φ, obtained by the INDO/S method, reveals that ES1(fluor) is minimal at φ = ∼80−100°, and a plateau is clearly observed for torsion angles ranging from 20 to 50°. On the basis of the calculation results, the following scheme of photophysical processes in the excited S1 state of the ThT is suggested. According to the model, a twisted internal charge-transfer (TICT) process takes place for the ThT molecule in the excited singlet state, resulting in a transition from the fluorescent locally excited (LE) state to the nonfluorescent TICT state, accompanied by torsion angle φ growth from 37 to 90°. The TICT process effectively competes with radiative transition from the LE state and is responsible for significant quenching of the ThT fluorescence in low-viscosity solvents. For viscous solvents or when the ThT molecule is located in a rather rigid microenvironment, for example, when it is bound to amyloid fibrils, internal rotation in the dye molecule is blocked due to steric hindrance, which results in suppression of the LE → TICT quenching process and in a high quantum yield of fluorescence