A fluorescent bis-styryl-benzothiadiazole (BTD) with carboxylic acid functional groups (X-34/Congo red analogue) showed lower binding affinity toward Aβ1-42 and Aβ1-40 fibrils than its neutral analogue. Hence, variable patterns of neutral OH-substituted bis-styryl-BTDs were generated. All bis-styryl-BTDs showed higher binding affinity to Aβ1-42 fibrils than to Aβ1-40 fibrils. The para-OH on the phenyl rings was beneficial for binding affinity while a meta-OH decreased the affinity. Differential staining of transgenic mouse Aβ amyloid plaque cores compared to peripheral coronas using neutral compared to anionic bis-styryl ligands indicate differential recognition of amyloid polymorphs. Hyperspectral imaging of transgenic mouse Aβ plaque stained with uncharged para-hydroxyl substituted bis-styryl-BTD implicated differences in binding site polarity of polymorphic amyloid plaque. Most properties of the corresponding bis-styryl-BTD were retained with a rigid alkyne linker rendering a probe insensitive to cis−trans isomerization. These new BTDbased ligands are promising probes for spectral imaging of different Aβ fibril polymorphs.

We revisited the Congo red analogue 2,5-bis(4'-hydroxy-3'-carboxy-styryl)benzene (X-34) to develop this highly fluorescent amyloid dye for imaging Alzheimer's disease (AD) pathology comprising Aβ and Tau fibrils. A selection of ligands with distinct optical properties were synthesized by replacing the central benzene unit of X-34, with other heterocyclic moieties. Full photophysical characterization was performed, including recording absorbance and fluorescence spectra, Stokes shift, quantum yield and fluorescence lifetimes. All ligands displayed high affinity towards recombinant amyloid fibrils of Aβ1-42 (13-300 nm K ) and Tau (16-200 nm K ) as well as selectivity towards the corresponding disease-associated protein aggregates in AD tissue. We observed that these ligands efficiently displaced X-34, but not Pittsburgh compound B (PiB) from recombinant Aβ1-42 amyloid fibrils, arguing for retained targeting of the Congo red type binding site. We foresee that the X-34 scaffold offers the possibility to develop novel high-affinity ligands for Aβ pathology found in human AD brain in a different mode compared with PiB, potentially recognizing different polymorphs of Aβ fibrils.