Comparative analysis of the absorption and fluorescence spectra and fluorescence excitation spectra ofthioflavin T (ThT) in various solvents and in the compositionofamyloid fibrils has shown that ThT, when ex-cited in the region of the long-wavelength absorption band, fluoresces in the spectral region with a maximumat 478–484 nm. The appearance in aqueous and alcohol solutions of a fluorescence band with a maximumnear 440 nm has been attributed to the presence in the composition of the ThT preparations of an impuritywith an absorption band in the 340–350-nm range. The literature data showing that in glycerol ThT has awide fluorescence spectrum with two maxima are due to the artifact connected with the use of a highconcen-tration of the dye. It has been suggested that the cause of the lowquantumyield of ThTaqueousand alcoholsolutions is the breakage of the system of conjugated bonds due to the reorientation of the benzothiozole andbenzaminic rings of ThT in the excited state with respect to one another. The main factor determining thehighquantumyield of fluorescence of ThT incorporated in fibrils is the steric restriction of the rotation of therings about one another under these conditions. The suggestions made have been verified by the quantum-chemical calculation of the ThT molecule geometry in the ground and excited states.
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