A High Affinity Red Fluorescence and Colorimetric Probe for Amyloid β Aggregates

A major challenge in the Alzheimer’s disease (AD) is its timely diagnosis. Amyloid β (Aβ) aggregates have been proposed as the most viable biomarker for the diagnosis of AD. Here, we demonstrate hemicyanine-based benzothiazole-coumarin (TC) as a potential probe for the detection of highly toxic Aβ(42) aggregates through switch-on, enhanced (~30 fold) red fluorescence (E(max) = 654 nm) and characteristic colorimetric (light red to purple) optical outputs. Interestingly, TC exhibits selectivity towards Aβ(42) fibrils compared to other abnormal protein aggregates. TC probe show nanomolar binding affinity (K(a) = 1.72 × 10(7) M(−1)) towards Aβ(42) aggregates and also displace ThT bound to Aβ(42) fibrils due to its high binding affinity. The Aβ(42) fibril-specific red-shift in the absorption spectra of TC responsible for the observed colorimetric optical output has been attributed to micro-environment change around the probe from hydrophilic-like to hydrophobic-like nature. The binding site, binding energy and changes in optical properties observed for TC upon interaction with Aβ(42) fibrils have been further validated by molecular docking and time dependent density functional theory studies.