Dyeing Activity
As suggested by its intense red color, congo red has important spectrophotometric properties. Indeed, its UV-visible absorption spectrum shows a characteristic, intense peak around 498 nm in aqueous solution, at low dye concentration. Congo red molar extinction coefficient is about 45000 /. in these conditions. Aggregation of the dye and binding to amyloid fibrils tends to red-shift the absorption spectrum, whereas binding to cellulose fibers has the opposite effect. Congo red also shows a fluorescent activity when bound to amyloid fibrils, which tends to be used as a sensitive diagnosis tool for amyloidosis, instead of the traditional histological birefringence test.
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