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 beta (A beta) 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 A beta(42) aggregates through switch-on, enhanced (similar to 30 fold) red fluorescence (E-max = 654 nm) and characteristic colorimetric (light red to purple) optical outputs. Interestingly, TC exhibits selectivity towards A beta(42) fibrils compared to other abnormal protein aggregates. TC probe show nanomolar binding affinity (K-alpha = 1.72 x 10(7) M-1) towards A beta(42) aggregates and also displace ThT bound to A beta(42) fibrils due to its high binding affinity. The A beta(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 A beta(42) fibrils have been further validated by molecular docking and time dependent density functional theory studies.