Thioflavin

Thioflavin can refer to either of two dyes used for histology staining.

Thioflavin T (Basic Yellow 1 or CI 49005) is a benzothiazole salt obtained by the methylation of dehydrothiotoluidine with methanol in the presence of hydrochloric acid. The dye is used to visualize plaques composed of amyloid beta found in the brains of Alzheimer's disease patients as well as other amyloid proteins. When it binds to beta sheets, such as those in amyloid oligomers, the dye undergoes a characteristic 115 nm red shift of its excitation spectrum that may be selectively excited at 442 nm, resulting in a fluorescence signal at 482 nm. This red shift is only observed if amyloid fibrils are present. It will not undergo this red shift upon binding to precursor monomers or small oligomers, or if there is a high beta sheet content. If no amyloid fibrils are present in solution, excitation and emission occur at 342 and 430 nm repectivly.

Thioflavin S is a homogenous mixture of compounds that results from the methylation of dehydrothiotoluidine with sulfonic acid. It is also used to stain Alzheimer's plaques. Like Thioflavin T it binds to amyloid fibrils but not monomers and gives a distinct spectral shift upon binding. However the dye is unable to be used in quantitative measurements of fibril solutions due to its high background fluorescence .