A Comparison of Three Fluorophores for the Detection of Amyloid Fibers and Prefibrillar Oligomeric Assemblies. ThT (Thioflavin T); ANS (1-Anilinonaphthalene-8-sulfonic Acid); and bisANS (4,4'-Dianilino-1,1'-binaphthyl-5,5'-disulfonic Acid).

Amyloid fiber formation is a key event in many misfolding disorders. The ability to monitor the kinetics of fiber formation and other prefibrillar assemblies is therefore crucial for understanding these diseases. Here we compare three fluorescent probes for their ability to monitor fiber formation, ANS (1-anilinonaphthalene-8-sulfonic acid) and bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid) along with the more widely used thioflavin T (ThT). For this, we have used two highly amyloidogenic peptides: amyloid-β (Aβ) from Alzheimer's disease and islet amyloid polypeptide (IAPP) associated with type II diabetes. Using a well-plate reader, we show all three fluorophores can report the kinetics of fiber formation. Indeed, bis-ANS is markedly more sensitive to fiber detection than ThT and has a submicromolar affinity for Aβ fibers. Furthermore, we show that fluorescence detection is very sensitive to the presence of excess fluorophore. In particular, beyond a 1:1 stoichiometry these probes demonstrate marked fluorescence quenching, for both Aβ and IAPP. Indeed, the fiber-associated fluorescence signal is almost completely quenched in the presence of excess ThT. There is also intense interest in the detection of prefibrillar amyloid assemblies, as oligomers and protofibrils are believed to be highly cytotoxic. We generate stable, fiber-free, prefibrillar assemblies of Aβ and survey their fluorescence with ANS and bis-ANS. Fluorescence from ANS has often been used as a marker for oligomers; however, we show ANS can fluoresce more strongly in the presence of fibers and should therefore be used as a probe for oligomers with caution.