Transfer of Electronic Excitation Energy Between Thioflavin T and Its Styryl Derivatives Incorporated Into Amyloid Fibrils

The spectral and luminescent properties of the styryl derivative of thioflavin T (ThT) 2-{(1E,3E)-4-[4-(dimethylamino)-2,6-dimethylphenyl]buta-1,3-dien-1-yl}-3-ethyl-1,3-benzothiazolium-3 tosylate (Th-C23) were studied. The fluorescence decay kinetics of Th-C23 incorporated into amyloid fibrils (AF) were not exponential and could be represented as a unimodal distribution of fluorophores over duration α(t). ThT and Th-C23 were shown to form an effective donor–acceptor (D–A) pair when incorporated into the AF structure. The degree of polarization of the acceptor fluorescence was high (~0.4) within the entire spectrum, despite the energy transfer, and was explained by the special orientation of D–A pairs, parallel and antiparallel. The measured fluorescence spectrum of the donor (ThT) and absorption spectrum of the acceptor (Th-C23) were used to calculate the critical energy-transfer radius R₀, which could take values from 54 to 68 Å depending on the mutual orientation and quantum yield of the donor and acceptor.