Stsiapura Vitali I, Maskevich Alexander A, Kuzmitsky Valery A, Turoverov Konstantin K, Kuznetsova Irina M
Yanka Kupala State University, Physics Department, 22 Ozheshko Street, 230023 Grodno, Belarus.
J Phys Chem A. 2007 Jun 7;111(22):4829-35. doi: 10.1021/jp070590o. Epub 2007 May 11.
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 phi between the benzthiazole and the dimethylaminobenzene rings has been found to be approximately 37 degrees. The energy barriers of the intramolecular rotation appearing at phi = 0 and 90 degrees are quite low: semiempirical AM1 and PM3 methods predict values approximately 700 cm-1 and ab initio methods approximately 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 phi = 90 degrees and that the intramolecular charge-transfer takes place upon the ThT fragments' rotation from phi = 0 to 90 degrees. Ab initio CIS/RHF calculations were performed to find optimal geometries in the excited S1 state for a series of conformers having fixed phi values. The CIS calculations have predicted a minimum of the S1 state energy at phi approximately 21 degrees; however, the energy values are 1.5 times overestimated in comparison to experimental data. Excited state energy dependence on the torsion angle phi, obtained by the INDO/S method, reveals that ES1(fluor) is minimal at phi = approximately 80-100 degrees, and a plateau is clearly observed for torsion angles ranging from 20 to 50 degrees. 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 phi growth from 37 to 90 degrees. 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.
对硫黄素T（ThT）分子在基态S0和第一激发单重态S1下进行了量子化学计算。已确定基态的ThT具有明显的非平面构象：苯并噻唑环与二甲基氨基苯环之间的扭转角φ约为37度。在φ = 0和90度时出现的分子内旋转能垒相当低：半经验AM1和PM3方法预测的值约为700 cm-1，从头算方法预测的值约为1000 - 2000 cm(-1)。对S1(abs)激发态的垂直跃迁进行的INDO/S计算表明，对于φ = 90度的扭曲构象，能量ES1(abs)最小，并且在ThT片段从φ = 0旋转到90度时发生分子内电荷转移。进行了从头算CIS/RHF计算，以找到一系列具有固定φ值的构象异构体在激发态S1下的最佳几何结构。CIS计算预测在φ约为21度时S1态能量最低；然而，与实验数据相比，能量值被高估了1.5倍。通过INDO/S方法获得的激发态能量对扭转角φ的依赖性表明，ES1(fluor)在φ = 约80 - 100度时最小，并且在20至50度的扭转角范围内明显观察到一个平台。基于计算结果，提出了ThT在激发态S1下的以下光物理过程方案。根据该模型，激发单重态的ThT分子发生扭曲的内电荷转移（TICT）过程，导致从荧光局域激发（LE）态转变为非荧光TICT态，并伴随着扭转角φ从37度增加到90度。TICT过程有效地与LE态的辐射跃迁竞争，并导致低粘度溶剂中ThT荧光的显著猝灭。对于粘性溶剂或当ThT分子位于相当刚性的微环境中时，例如当它与淀粉样纤维结合时，由于空间位阻，染料分子内的旋转受阻，这导致LE→TICT猝灭过程受到抑制，并产生高荧光量子产率。
