Specific in situ discrimination of amyloid fibrils versus α-helical fibres by the fluorophore NIAD-4

A wide range of human pathologies, including neurodegenerative diseases and Other forms of amyloidosis, are associated with the formation of insoluble fibrillar protein aggregates known as amyloids. To gain insights into this process analytical methods are needed, which give quantitative data on the molecular events that are taking place. The dye Thioflavin T (ThT) is widely used for the spectroscopic determination of amyloid fibril formation. Different binding affinities to amyloids at neutral and acidic pH and the frequently observed poor binding at acidic pH are problematic in the use of the cationic ThT. The uncharged fluorescence probe [[5'-(4-hydroxyphenyl)[2,2'-bithiophen]-5-yl]methylene]-propanedinitrile (NIAD-4) has been recently designed by Swager and coworkers, in order to eliminate some of the limitations of ThT. Here we have used this novel dye for in vitro monitoring of the amyloid formation processes of de novo designed model peptides. Amyloid structures were successfully detected by NIAD-4 at neutral as well as acidic pH and no significant fluorescence was detectable in the presence of α-helical fibres. Thus, NIAD-4 proved to be a valuable alternative to ThT for spectroscopic studies on amyloid structures over a broad pH range.